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GEOLOGY OF PETROLEUM

A Series of Books' in Geology Geology of India &; Burma, 6th ed. Principle of Petrology Palaeontology Invertebrate, 8th ed. Sedimentary Rocks, 3rd ed. Rutley's Elements of Mineralogy. 26th ed. The Study of Thin Rocks Petrography Mineralogy, 2nd ed.

M.S. J(rlshnan

Tyrell Woods F. J. Pettijohn Read Moorhouse Turner Barry & Mason

TEXTBOOK OF PETROLOGY 3 Vo'. SERIES Petrology of the Igneous Rocks Hatch & Wells Petrology of the Metamorphic Rocks Roger Mason Petrology of the Sedimentary Rocks, 6th ed. Greensmith Levorsen Geology of Petroleum, 2nd ed. Raup Principles of Paleontology, 2nd ed. Petrology (Igneous, Sedimentary and Ehlers/Blalt Metamorphic) David Clark Plane Surveying - Vol. I Higher Surveying - Vol II David Clark Hydro-Electric Engineering J. Guthrie Brown Practice (in 3 Vo/s.) Vol I Civil Engineering Vol If Mechanical & Electrical Engineering Vol III Economics, Operations & Maintenance

SECOND EDITION

GEOLOGY OF PETROLEUM A. I. LEVORSEN Sections on Hydrodynamics and Capillary PreMure revised and edited by

FREDER1CK A. F. BERRY Umversity of Callforma, Berkeley

~

CBS

CBS Publishers & Distributors 4596/1 ~A, 11 Darya Ganj, New Delhi ~ 110 002 (India)

ISBN: 81-239-0931-4

First Indian Edition: 1985 Reprint: 2001 Reprint: 2003 Reprint : 2004 This edition has been published in India by arrangement with W.H. Freeman and Company, New York All rights reserved. No part of thIs book may be reproduced or transmitted in any fonn or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system without permission, in writing, from the publisher. Published by: Satish Kumar Jain for CBS Publishers & Distributors, 4596/1-A, 11 Darya Ganj, New Delhi - 110 002 (India)

For sale in India only Printed al,' Nazia Printers, Delhi - 110006

FOREWORD

A. I. LEVORSEN died on July 16, 1965. Shortly before his death he had ..,~~\. ..,~~~~\.\'\,~ndon (1922), Vol. I, pp. 208-212. A. B. Cook and Clare Despard, "Historical Records Relating to Oil," Jour. Inst. Petrol. Technol., Vol. 13 (1927), pp. 124-134. R. J. Forbes, Bitumt!1Z and Petroleum in Antiquity. E. J. Brill. Leiden (1936), 109 pages. Laurence Lockhart, "Iranian Petroleum in Ancient and Medieval Times," Jour. Inst. Petrol. Technol.. Vol. 25 (January 1939), pp. 1-18. 8. W. F. Foran, Oil from the Garden of Eden," Petrol. Engr., Vol. 14 (October 1942), p. 85. 9. R. J. Forbes, "Petroleum and Bitumen in Antiquity," Jour. Inst. Petrol. Technol., Vol. 25 (1939), pp. 19-23. 10. E. H. Cunningham Craig, op. cit. (note 4), p. 144. G. M. Lees, "Pliocene Oil Seepages in Persia," Jour. Inst. Petrol. Techno!., Vol. 13 (1927), pp. 321-324. Contains map showing seepages. G. M. Lees, 'The Geology of the Oilfield Belt of lran and Iraq," in The Science of Petroleum, Oxford University Press, London and New York (1938), Vol. I, pp. 140148. Bibliog. 17 items. 11. Frederick G. Clapp, "Geology and Bitumens of the Dead Sea Area. Palestine and Transjordan," Bull. Amer. Assoc. Petrol. Geol., Vol. 20 (July 1936), pp. 881-909. 12. Sir Boverton Redwood, op. cit. (note 7), Vol. 1, pp. 1-31 and 201-208. 13. Col. Yule (ed.) , Tile Book of Ser Marco Polo tile Venetian, London (1871). i, 4. 14. Laurence Lockhart, op. cit. (note 7), pp. 3-14. A. Beeby Thomp;on. op. cit. in note 2, Vol. 2. p. 547. 15. U.S. Coast and Geodetic Survey, Map. No. 5116, San Miguel Passage.

44

INTRODUCTION

16. A. Beeby Thompson, op. cit. in note 2, Vol. 1. pp. 222-223 and 235-236. 17. George Shepard. "Observations on the Geology of the Santa Elena Peninsula. Ecuador. SA.," Jour. Inst. Petro!. Technol.. Vol. 13 (1927), pp. 424-461. Bibliog. 31 items. 18. H. G. Kugler, "Nature and Significance of Sedimentary Volcanism," in The Science 01 Petroleum, Oxford University Press, London and New York (1938), Vol. 1, pp.297-299. • H. G. Kugler, "Contribution to the Knowledge of Sedimentary Volcanism in Trinidad," Jour. Inst. Petrol. Technol., Vol. 19 (1943), llP' 743-759. Discussion to p. 772. Bibliog. 26 items. 19. E. H. Cunningham Craig, op. cit. (note 4), p. 160. 20. A Becby Thompson,op. cit. in note 2, Vol. 1, pp. 228-229. 21. V. A Gorin, 'The Bibi·Eibat Tectonics and the Prospect of Development of the Lower Division," Azerbaijan Petroleum Industry (Baku), No. 11 and No. 12 (November and December 1933). I. M. Goubkin, "Tectonics of Southeastern Caucasus and Its Relation to the Produc· tive Oil Ficlds," Bull. Amer. Assoc. Petrol. Geo!., Vol. 18 (May 1934), pp. 603-671. Bibliog. 41 items (all in Russian). Mud volcanoes, pp. 663-{i70. I. M. Goubkin and S. F. Federov, "Mud Volcanoes of the Soviet Union and Their Connection with the Oil Deposits," XVIIth Int. Geo!. Cong., Moscow, USSR, Vol. 4 (1937), pp. 29-59. ~ H. O. Kugler, "A visit to Russian Oil Districts," Jour. Inst. Petrol. Technol., Vol. 2S (1939), pp. 81-82. Zl. H. L. Chhibber, The Geology of Burma, Macmillan &: Co., London (1934), Chap. 6 ("Mud Volcanoes"), pp. 79-86.17 references. 23. L Dudley Stamp, "Natural Gas Field of Burma," Bull. Amer. Assoc. Petrol. Geol., Vol. 18 (March 1934), pp. 315-326; mud volcanoes, pp. 323-325. 24. H. O. Kugler. "Contribution" et~. (IOC~~jIl note 18), 25. W. G. Weeks, "Notes on a New Mud Volcano in the Sea off the Coast of Trini· dad," Jour. Inst. Petrol. Technol., Vol. IS (1929), pp. 38S-391. 26. George H. Eldridge, "The Asphalt and Bituminous Rock Deposits of the United States," 22nd Ann. Rept. U.S. Geol. Surv., Govt. Printing Office, Washington, D.C., Part 1(1901), pp. 209-464. Herbert Abraham. Asphalts and Allied Substances, 6th cd., S vols., D. Van Nostrand, Princeton (1960--1963). Extensive bibliography and references. The most complete dis· cussion of asphalts and solid bitumens--worldwide occurrence, historical background, testing. uses. 27. L. L Hutchison, "Rock. Asphalt, and Asphaltite in Oklahoma," Ilull. 2, Okla. Geol. Surv. (March 1911), pp. 1-93. Z8. L C. Snider, "Rock Asphalts of Oklahoma and Their Use in Paving," Petroleum, Vol. 9 (1914). p. 974. 29. M. A. Carrigy. "Geology of the McMurray Formation," Memoir No.1, Research Council of Alberta, Geological Division (1959). Contains extensive list of references. M. A. Canigy, "Effect of Texture on the Distribution of Oil in the Athabaska Oil Sands. Alberta, Canada," Jour. Sed. Petrol., Vol. 32 (June 1962). pp. 312-325. S. C. Ells, Bituminous Sa,.'" "f Northern Alberta. Canada Dept. Mines, Branch No. 632 (1926), 244 pages. J. C. Sproule, "Origin of McMunay Oil Sands, Alberta," Bull. Amer. Assoc. Petrol Geol.. Vol. 22 (September 1938), pp. 1133-1149. Discussion to p. 1152. 43 references cited. Alberta Society of Petroleum Geologists, "Northern Alberta Oil Sands," Bull. Amer. Assoc. Petrol. Geol., Vol. 35 (February 1951), pp. 181-184.6 references. Theo. A. link, "Source of Oll in Tar Sands' of Athabaska River, Alberta, Canada," Bull. Amer. Assoc. Petrol. Geol., Vol. 35 (Apri11951). pp. 854-862. 7 references.

THE OCCURRENCE OF PETROLEUM [CRAPTER

2]

45

30. Research Council of Alberta, Edmonton, Canada, "Athabaska Oil Sands," in K. A. Clark Volume, Inf. Series No. S (1963), M. A. Carrigy (ed.), 18 articles, 241 pages. Thco. A Link, op. cit. (note 29), p. 854. 31. W. V. Howard. '~ithification Processes and Early Oil Formation in Sediment," O. & O. Jour., Vol. 42 (June 17, 1943), reviewed in Jour. Inst. Petrol. TechnC'l. Vol. 29 (1943),p. 324A. 32. T. Sterry Hunt, "Contributions to the Chemical and Geological History of Bitumen, and of Pyroschists or Bituminous Shales," Amer. Jour. Sci. &: Arts, 2nd series, Vol. 35 (1863), pp. 157-171. S. F. Peckham, Petroleum and Its Products, Dept. ()f Interior, Census Offke, U.S. Geol. Ptg. Office (1885), p. 63, quotilJg James M. Stafford. F. C. Phillips, "On the Occurrence of Petroleum in the Cavities of Fossil~.·· Proc. Amer. Phil. Soc., Vol. 36 (1897), pp. 121-126. John R. Rceves, "An Inclusion of Petroleum in a Fossil Cast" (Bloomington, Indiana), Bull. Amer. Assoc~ Petrol. Geol., Vol. 9 (May-June 1925), p. 667. 33. George Homans Eldrige. "The Santa Clara Valley Oil District, Southern Califomia," Bun. 309, u.s. Geol. Surv. (1907), pp. 22 and 77. 34. F. G. Clapp et al., Petroleum and Natural Gas Resources of Canada, Dept. of Mines (1915), Vol. 2, p.262. 35. A. 1Jee1Jy 1homprorr, up. cit. (aotc 2}, VoL 1, pp.248-249. 36. Josiah Edward Spurr, The Ore Magmas, McGraw-Hill Book Co., New York (1923), Vol. 2, pp. 655-663. 37. L. P. Stockman, "Mercury in Three Wens at Cvmric," Petrol. World, February 1947, p. 37. 38. H. Andrew Ireland, "Petroliierous Iron Ore of Pennsylvanian Age in Eastern Ohio," Bull. Amer. Assoc. Petrol. Geo!., Vol. 28 (1944), pp. lOS 1-1056. 39. Charles O. Carlson. "Bitumen in Nonesuch Formation of Keweenawan Series of Northern Michigan," Bull. Amer. Assoc. Petrol. Geol., Vol. 16 (August 1932), pp. 737-740. 40. Nils Sundius, "Om oljesldffer och skifferolje-industrien i vart land" (an account of the oil shales and the shale-oil industry of Sweden). Ymer (Svenska Sallslmpet Antrop. ochGeography),Argang 63 (1943),kep l,pp. 1-16. 41. Dean E. Wiuchester, "Oil Shale of the Rocky MQuntain Region," Bull. 729, U.S. Geol. Sun. (1923),204 pages. Extensive bibliography, pp. 143-202. M. J. Gavin. "Oil Shale: m..torical, Technical and Economics Study," Bull. 210, U.S. Bur. Mines (1924). Ralph H. McKee, Shale Oil (Mon. 25, Amer. Chern. Soc.), Reinhold Publishing Co., New York (1925),326 pages. The Science of Petroleum, Oxford University Press, London and New York (1938), Vol. 4, Part V. Section 43 ("Oil Shales, Torbanites, Cannels, etc.") and Section 44 ("Shale Oils and Tar Oils") contain several authoritative articles on "oil shales," the Scottish shaJeo()i1 industry, the Estonian shaJe-oil industry, and the occurrence and geol~ 081 of "oil shales." K. C. Heald and Eugene Ayres, "Our Reserves of Coal and Shale," in Leonard M. Fanning (ed.), Our Oil Resources, McGraw-Hill BOOk Co., New York (1945), PIl. IS7-209. Bibliog. 17 items. 4l. U.S. Bur. Mines. "Composition ()f Oil Shales" (Geological Note), Bull. Amer. Assoc. Petr()1. Geol., Vol. 7 (1923), pp. 296-297. 43. A. L. Down and G. W. Himus, ''The Classification of Oil Shales and Cannel Coals," Jour. Inst. Petrol. Technol., Vol. 26 (July 1940), pp. 329-333. 44. lames M. Schopb, "Cannel, Eoghead, Torbanite, Oil Shale," Econ. Geol., Vol. 44 (January·February 1949), pp. 68-71. 45. E. G. Woodruff, "Petroliferous Provinces," Eull. 150, Amer. Insl Min. Met. Engrs. (1919), pp. 907-912; Trans., Vol. 65 (1921), pp. 122-204. Discussion by Charles Schuchert and others, pp. 204-216.

46

INTRODUCTION

Frederic H tah!'!;!. "Classification of Exploratory Drilling and Statistics for 1943," Bun. ArneI'. A'\sc>;;. Petrol. Geol, Vol. 28 (June 1944); Part II, definitions, pp.703-711. 46. Wallace Pratt, Oil in the Earth. University of Kansas Press. Lawrence, Kansas (3rd pig. 1944), 110 pages, pp. 31-33. 47. Wetem Hemistlhere Oil Study Committee, Independent Petroleum Association of America. Wilshington 6, D.C., Report of the Subcommittee on Government Policies and Laws (October 1952). Contains a summary of the laws of the countries in the Western Hemisphere pertaining to petroleum and petroleum exploration. 48. William H. Twenhofel, Treatise on Sedimentation, 2nd cd., Williams &; Willdns Co., Baltimore (1932),926 pages, p.858. 49. National Petroleum Council, Washington, D.C., Submerged Lands Productivity Capacity (May 28, 1953),43 pages, maps, and geologic sections. SO. American Commission on Stratigraphic Nomenclature, "Report No.2-Nature. Usage, and Nomenclature of Time-StratigrapJPc and Geologic-Time Units," Bull. Amer. Assoc. Petrol. Geol., VoL 36 (August 1952). pp. 1627-1638. Contains tlJe principles of modern rock classification. J. Laurence Kulp, "Geologic Time Scale," Science, Vol. 133 (April 14, 1961), pp. 1105-1114. 51. Robert F. Walters. "Oil Production from Fractured Pre·Cambrian Basem*nt Rocks in Central Kansas," Bull. Amer. Assoc. Petrol. Geol., Vol. 37 (February 1953), pp. 300313. 52. Kalervo Rankama, "New Evidence of the Origin of Pre-Cambrian Carbon," Bull. GeoL Soc. Amer., Vol. 59 (May 1948), pp. 389-416. 53. John W. Gruner, "AJgae, Believed to be Archean," Jour. Oeol., Vot, 31 (1923), pp. 146-148. 54. John W. Gruner, "Contributions to the Geology of the Mesabi Range," Bull. 19. Minn. Gcol. Surv. (1914), pp. 59-64.

PAR T TWO

The Reservoir 3. The Reservoir Rock 4. The Reservoir Pore Space (Porosity and Permeability) 5. Reservoir Fluids-Water, Oil, Gas 6. Reservoir Traps-GeneraZ and Stmctural 7. Reservoir Traps (continued)-Stratigraphic and Fluid 8. Reservoir 7raps (continued)-Combination and Salt Domes

I NTRODU eTlON TO PART TWO

THE PETROLEUM RESERVOIR is that portion of the rock that contains the pool of petroleum. The location of every oil and gas pool may be said to be the result of a complex of interrelated geologic conditions. Each reservoir is unique in its details, but general relations may be seen that permit broad classifications of the major elements that control a reservoir. The petroleum reservoir consists of four essential elements, each of widely variable development, each with many gradations, and each of varying importance in the location and size of the pool of petroleum. They are: 1. The reservoir rock, or containing material. The composition and texture of the reservoir rock, and its continuity or lack of continuity, are of prime interest in the geology of petroleum. The edges of the reservoir rock may coincide with the edges of the petroleum pool, as where a lens is filled with oil and gas; or the reservoir rock, though extending through a large region, may become a petroleum reservoir only at locally favorable areas. 2. The pore space, or void space, sometimes called the reservoir space, is expressed as a fraction or percentage of the total volume of the rock (for example, 0.23 or 23 %) and is called its porosity. The effective pore space is that portion of the reservoir rock that is available for the migration, accumulation, and storage of petroleum. The measure of the ease with which fluids may move through the interconnected pores of the rock is called its permeability. Porosity and permeability are properties that depend on the presence of pore space. They are of special interest because they determine the capacity of the reservoir rock both to hold and to yield petroleum. 3. The fluid content consists of the water, oil, and gas that occupy the effective pore space within the reservoir rock. Under favorable conditions the oil and gas are concentrated into pools, but most of the reservoir pore !.t>ace outside the pools contains only water or water with petroleum measurable

50

THE RESERVOIR

in parts per million. The petroleum, then, occurs within an aquifer-within a water environment. The fluids may be in a state of either static or dynamic equilibrium; that is, at rest or in motion. During their geologic life they undoubtedly have been in motion at some time or even continuously because of changes brought about by erosion, deposition, and deformation, and any other changes that upset the equilibria of the fluid pressure, temperature, density, volume, and chemical characteristics. These changes cause the fluids to move along gradients from areas of higher energy potential toward areas of lower energy potential. Although the movements of the fluids cannot be observed directly, the concentrations of oil and gas into pools and the widespread evidence of fluid pressure gradients is evidence of such movement. 4. The reservoir trap, or the trap, is the element that bolds the oil and gas in place in a pool. Most geologists think of the trap as the shape of the reservoir rock clement that permits a petroleum pool to accumulate underground. As we shall see later (pp. 340-343) the trap may actually be due in part to the fluid pressure gradients that exist in the reservoir fluids. As considered here, the trap is the shape of the reservoir rock together with its pore space. Rock traps are formed from a wide variety of combinations of structural and stratigraphic features of the reservoir rocks. A trap generally consists of an impervious cover-the roof rock-overlying and sealing a porous and permeable rock that contains the oil and gas. The upper boundary, as viewed from below, is concave; the top is generally arched, but it may form an angle or peak. In practice the term "trap" usually means any combination of rock structure and of permeable and impermeable rocks that will keep oil and gas from escaping, either vertically or laterally, because of differences in pressure or in specific gravity. Some petroleum reservoirs completely fill the trap, so that if any additional oil or gas were added it would spill out around the lower edges. Other reservoirs occupy merely a part of the apparent capacity of the trap. The lower boundary of the reservoir is, either wholly or partly, the plane of contact of the oil and gas with the underlying body of water upon which the pool rests. It is known as the oil-water contact or oil-water table. * The water fills all of the pore space of the reservoir rock below the oil-water contact, and that portion of the reservoir pore space that is not fill'ed with oil and gas. If the water is at rest, the contact plane is level or approximately level. But, if the water is in motion, because of a hydrodynamic fluid potential gradient parallel to the bedding and across the pool, the lower boundary of the reservoir may be an inclined plane, and the pool is said to have an inclined or tilted oil-water table. Occ3'.ionally the tilt of the oil-water table is enough to flush the oil and

* The m"l-water table should be distinguished from the water table. which has had a long priority of usage In the geology of ground water to denote the surface below which aU pores are filled with water. The ground-water table is, in effect, the ci,-·w(lter table. in contrast to the oil-water table of the petroleum geologist.

INTRODUCTION TO PART TWO

51

gas out of a potential trap. in which case the rock trap is not effective, and there is neither reservoir nor pool. These broad generalizations about the nature of a petroleum reservoir will apply to most of the known pools of petroleum in the world. The next five chapters are concerned with details of these elements as they combine in varying proportions to trap a pool of petroleum.

CHAPTER 3

J'he

~eservoir ~ock

Reservoir rock: classification - nomenclature - fragmental - chemical miscellaneous. Well lOgs. Marine and nonmarine reservoir rocks.

BROADLY SPEAKING. any rock that contains connected pores rnay become a reservoir rock. As a matter of fact, however, nearly all reservoirs are in unmetamorphosed sedimentary rocks, and most of them in sandstones, limestones, and dolomites. Shales, slates, and igneous rocks are known to be reservoir rocks under exceptional conditions, but these conditions are rare and anomalous. A reservoir rock may be limited to the area of the pool of petroleum, or it may persist. with uniform lithological and physical characteristics, far beyond the pool.

CLASSIFICATION Since nearly all petroleum reservoir rocks are of sedimentary origin, any classification of reservoir rocks is essentially a classification of sedimentary rocks. A number of classifications have been proposed,! some descriptive and some genetic, but most of them designed chiefly for the use of the specialist in sedimentary petrography. Classifications of petroleum reservoir rocks for practical use should be as simple and broad as possible. for the petrol~um geologist must keep his terminology understandable to the operator, driller, and engineer, who supply many of bis basic data and to whom he has to convey his own ideas. Many terms that are perfectly good scientific deSCriptions, and that have clear and exact meanings for geologists, do not find Dluch favor in the petroleum industrysuch terms, for example, as "arenaceous" for "sandy." "argillaceous" for "shaly." and "rudaceous" for "conglomeratic." We need terms that are generally understood yet sufficiently deiinite.

R.ESER.VOIR ROCK [CHAPTER

3]

53

A simple, broad, primary classification of reservoir rocks, based largely on the origin of the rock, divides them into three groups: (1) fragmental (clastic); (2) chemical and biochemical (precipitated); (3) miscellaneous. This may oversimplify a complex and difficult pro"lem, but such a rough classification is useful in the geology of petroleum a)ld is readily understandable. It is the system used here. The chief difficulty in applying any rock classification is that there are many gradational types that are hard to classify. Reservoir rocks, like all sediments, commonly grade into one another. Complex reservoir rocks are named according to their dominant constituent or rock characteristic, with an adjective to indicate the minor constituent, as in "limy sand" and "sandy lime." It is sometimes useful to class a reservoir rock as of marine or nonmarine origin. This genetic classification may be combined with a lithologic classification, as in the terms "marine limestone," "continentjll sandstone," and "nonmarine conglomerate." It is often useful, also, to place the rock in the standard geologic time scale ~l\d Uienby c\aes. WateIs of primary alkalinity generally contain silica also. These waters are termed soit.

* The reaction '..lues of the various ions in oil·field water are not In prcportion to their various weight •. The reaction value for each IOn may be expres~ed tn mllligram~ per litH Of as a ~rceni:l.ge of the sum of all the reaction values In the analysis. The reaction value is found by lhe followiob formula: reactIOn value = amount by weight (mg/liter) X reaction coefficienl 21 =

valence amount by weight (mg/liter) X - - . - - - atomiC weight

= eqUivalents per mIllion (epm) ~

milliequlvalents per liter (meq/l)

When the reaction val ••~ of a water is reduced to percentages, the character of the water is indicated 'WIthout use of the concentration. The reaction coefficients for the ions usually determined in water analy,>is are: Sodium (Na+) Potassium (K+)

Calcium (Ca++) Magnesium (Mg+ +) Hydrogen (H+)

0.0434 0.0256 0.0499 0.821 0.992

Sulfate (SO.-) Chloride (Cn Nltra'e (NOn Carbonate (COJ-) Bicarbonate (HCO.-)

00208 0.0282 O. JC!61 0(,333 o (JIM

164

THB RESERVOIR

(4) Secondary alkalinity. Weak acids combined with secondary bases. Secondary alkalinity is characteristic of calcareous formations. It is also known as temporary hardness.

Of these properties, primary salinity and secondary alkalinity will always be present. If the strong acids exceed in amount the primary bases, the third property will be secondary salinity; if the strong acids do not exceed the primary bases in amount, the third property will be primary alkalinity. The ionic statement and the derivation of the Palmer properties, as applied to oil-field water analyses, are illustrated in Table 5-1. It may be noted that the weights of positive and negative ions do not ba1ance, but that the reaction TABLE 5-1 Ionic Statement and Reaction Properties

Milligrams

Equivalents per Million (epm)

Reaction Value

or Reaction Value

in Percentage

per liter

(Palmer)

Positive Ions Na+ and K+ (by difference as Na+) Ca++

17,000 2.960

765.0 1< 2

=

0.30

(Ca++ and Mg++ carbonates) Source: Data from L. C. Case, "Application of Oil Field Water to Geology and Production," Oil W".kly, October 29, 1945, pp. 48-54.

RESERVOIR FLUIDS-WATER, OIL, GAS [CHAPTER

5]

165

values of positive and negative ions are exactly equal. Comingling of waters may cause precipitation or scale formation in casing aod tubing. In addition to the common elements sodium, potassium, calcium, and magnesium, minor amounts or traces of various elements have been found when complete analyses are performed. These minor amounts are erratic in quantity :.utd are not ordinarily determined in the average water analysis. The minor elements include barium, strontium, iodine, bromine, boron, copper, manganese, silver, tin, vanadium, and iron. Barium, for example, seems to be found in many of the Paleozoic brines of the Appalachian region. It is thought that the barium appeared in the sediments originally as barite, which was precipitated in reactions induced by the meteoric waters entering the formations along their outcrop.2t' Radioactive salts have also been precipitated from oilfield brines. 29 A few representative examples of oil-field water analyses, together with an analysis of an average sea water, are given in Table 5~2. Most descriptions of oil fields contain analyses of the reservoir waters, and numerous lh.ts of oil-fIeld water analyses have been published. 30 When the water analyses are given in hypothetical combinations, it becomes necessary to reduce these to the ionic form. The factors used for calculating the amount of the positive radical in various salts are given in Table 5-3. Water analyses and total solids show considerable variation from sand to and even in the same well. Table 5-4 shows some of the differences in one well in Russell County, Kansas, in which Permian rocks at the surface are underlain by rocks of Pennsylvanian age, which rest unconformably on the Arbuckle limestone of Cambro-Ordovician age. A variety of diagrams have been devised by which the chemical analysis of an oil-field water may be plotted so that its character can be readily "isualized and the comparison with other waters easily seen. This is effective when plotted at the points of occurrence on stratigraphic and structural cross sections. Three such diagrams are shown in Figure 5-15. Part A shows the widely used Tickell method. 31 Its chief disadvantage is that the concentrations are not indicated. Part B shows the Parker method,32 which has the advantage of concentrating more detail into a small space than most diagrams. Part C shows the Stiff method,33 which has the advantage of showing the salt concentrations so that dilution effects are reduced, and of making a distinctive pattern by which the different water types can be readily distinguished. The unit milliequivalents per liter is used. Two chemical differences between ~ordinary sea wllter and oil-field brines are (1) the absence of the sulfate radical (SO.=) from some oil-field brines and its presence in sea water, and (2) the absence of alkaline earths (Ca and Mg) from some oil-field waters and their presence in sea water. s• The absence of sulfate from the waters of the San Joaquin Valley, California, has been explained by its reduction to sulfide, with a corresponding formation of carbonate,35 and this is also believed to explain in part, at least, the sulfate-

Conroe sands

Oolitic limestone l. Cretaceous

Prue sand

Rodessa, Texas-La.

Davenport, Okla.

Bradford sand

SlmplO", sand Ordovician

Arbuckle limestone Ordovician

Garber, Okla.

Devonian

Oklahoma City, Okla.

Bradford, Penn.

Sandstone CretaceaUI

Burgan, Kuwait

Pennsylvan;an

Woodbine sand u. Cretaceous

East Texos

Eocene

Conroe, Texas

2,000-3,000 It

,

MIocene

I

Reservoir Rock, Age

Lagunilla., weslern Venezuela

Sea water, ppm Sea water, percent

Pool

139,496

184,387

77,340

119,855

140,063

95,275

40,598

47,100

43

352

122

73

360

18

387

288

5,263

HCO.-

268

730

132

284

198

259

42

120

0.2

7.7

89

150

2,690

CO.-

55.:1

50.-

19,:150

cr

TABLE 5-2 RepresentatiYe Oil-field Water Analyses (ppm)

+ K+

60.73~

91,603

32,600

62,724

61.538

46,191

24,653

27,620

2,003

31.7

11,000

Na+

21,453

18,753

13,260

9,977

20,917

10,158

1,432

1,865

10

1.2

420

Ca++

2,791

3,468

1,940

1,926

2,874

2,206

335

553

63

3.8

1,300

Mg++

224,868

298,497

125,870

194,736

225,749

154.388

68,964

77,468

7,548

35,000

Totol ppm

10

9

8

7

6

5

4

3

2

Referencel

810. Sun Com'Sny Stewarf No.3.

4. H. f. Minor and Marclll A. Hanna, "East Texas Oil field, Rusk, Cherokee, Smith, Gregg and Upshu( Countie" r .. "a,:' in Siro/iarapnic Type Oil Fie/dl, Amer. Assoc. Petrol. Geol., Tulscr, Oklc. (1941/. p. 639. Stanolind No. I Everett,. 5. 8y 'he courtesy 01 the Gulf Oil Corp. A rl!presenfafive analysis. 6. H. B. Hill and R.I(. Guthrie, EnginPering Study of the Rodessa 011 fields in (ouisiana, Texas, and Arkansas, RI 3715, U.S. 8"r. Mines (August 19431. p. 90. No. H. 7. Stanley B. While, "Davenport field, lincoln County, Oklahoma," in Stratigraphic Type Oi' Fie'cI.~Amer. Assoc. Petrol. Geol., Tul,a, Okla. (1941 I, p. 403. Tello' Co., PotterlOn No. I. . 8. Jerry 8. Newby el a/., "Bradford Oil Field, McLean County, Pennsylvania, and Cattaraugus Counly, New York," in Structure of Typical American Oil FieIJ" Amer. Anoc. Petrol. Geal., Tulsa, Okla., Val. 2 (1929), p. 435. Q, H. B. Hill, f. l. Rawlins and C. R. Sopp, "Engineering Report an Oklahoma City Field, Oklahoma," RI 3330, U.S. Bur. Mine. (January 19371, p. 214, Analysis J. Carler Oil Co. Dunniven No. I, at 6,454 feet. 10. Wesley G. Gi,h and Roymond M. Carr, "Garber Field, Garfleld County, Oklahoma," in Structure of Typical American Oil Field., Amer. Assoc. Petrol. Geo!., Tulsa, Okla., Vol. I (1929), p, 191, Cesden-Marland No. 41, at 4,383 feet, flowing approximately 10,000 barrel. of waler per day.

p.

1. W. Dittmar, "Reporl on Resear,hes ;nlo Ihe Composition of Ocean Wafer,~Collecfed by H. M. S. Chellenger," Cholien;". RepO,ls, Vol. 1 Phr'ic. and Chemi.try (18841, pp. 1-251. AVerage of 77 water samples representative of all oceans. 2. Staff of Caribbean Pelralellm Company, "Oil Field. of Roya' Outch-Shell Group in Wesle,n Ven.,zuela," Bull. Arne•. Assoc. Pelrol. Geol.. Vol. 32 {April 19481. p.557. . 3. Frank W. 1i;;/laux. Jr., and E. O. Buck, "Conroe Oil Field, Montgomery Counly, Texas," in Gulf Coost O,{ f/ftkf., Amer. Auoe. Petrol, Geol., Tulsa, Oklo. (1936).

168

THE RESERVOIR

TABLE 5-3 Factors Used to Convert Hypothetical Combinations to Ionic Form (Water Analyses) Given

To Find

Given

Fodor

To Find

Factor

KCI

K

0.524

CoSO,

Co

0.294

NoC' CoC.!

No

0.394

MgSO.

Mg

0.202

Co

0.361

,

"

\~ ~y

,oO/"o tne production of a well or group of wells during a considerable time and extrapolates the production curve into the future. The curves may be plotted on rectangular, semi-log, or log-log paper; the log papers have the advantage of permitting the curves and their extensions to be projected as straight lines. The estimated future production of all the wells for the anticipated economic life of the property may then be added together to give an estimate of the total recoverable oil left in the reseTVoir. This is the most reliable method of estimating recoverable reserves when a property has had a few years of experience upon which to base the curve. It is generally unsatisfactory, however, where the production has been prorated or artificially curtailed.

Chemical Properties of Crude Oil Crude oil and natural gas, when underground and in their natural state, are at higher temperatures and under greater pressures than at the surface. All crude oils have some natural gas dissolved in them, and if there is more than enough gas to saturate the crude oil at the pressure and temperature that exist in the reservoir, the excess free gas accumulate, as a free gas cap. (See p. 463.) The changes in pressure and temperature that occur during the production or chemical analysis of oil vaporize, release, or break down some of the hydrocarbons. It is therefore difficult or even impossible to obtain an accurate analysis of the thousands of compounds that are found in a crude oil as it exists under ground. The original cornpositiol1 of a crude oil can at

* Reservoir energy is the energy in the reservoir that causes the oil and gas to move into the well. See also pp. 458-459.

5]

RESERVOIR FL urns -WATER, OIL, GAS [CHAPTER

177

best be determined only approximately. The difficulty in separating the indivIdual hydrocarbons in a crude oil may be seen in the fact that it has taken thIrty-seven years of study to isolate and analyze 234 compounds. 51 The tremendous advances that are being made in hydrocarbon anlaysis-gas chromatography, mass spectrometry techniques, and hydrocarbon isotope geochemlstrY--I11dke possible the rapid analysis of hydrocarbon molecules and a much more accurate and precIse understandmg of the compositions of many of the petroleum fractIons. The geologIst IS chIefly interested in the chemical and physical properties of petroleum substances as they occur underground: their chemical nature, and the changes III their composition that may result from the repeated changes in temperature and pressure which have been occuring throughout geologic time and whIch bear on their origlO, migration, and accumulation. The refiner, on the other hand, is more interested in the numerous commercially valuable compounds that can be formed artifiCially in the refinery. Many, pO~~lb!y rno

"0 '"~60

"

~

S40 E :>

Para ffi ns

o

~ 20

__ Norma I ParaffIns -

o FJGURE

5-22 The pelCenfa*ge compoli/lOn by volume of fhe chief products obtained from United .)tales crude oils. [Redrawn from Shafjer and Rossilli Proc. Amer. Petrol. Inst., Vol. 32 (1952), p. 64.]

RESERVOIR FLUIDS-WATER, OIL, GAS [CHAPTER

5]

183

Most petroleum oils consist of hundreds or even thousands of members of a few hom*ologous series· of hydrocarbons. They also contain large numbers of hydrocarbon compounds containing sulfur, nitrogen, or oxygen, and these compounds are found to occur in each fraction of the whole. Since the molecu~r weights of these hydrocarbon compounds are large, the presence of small volumes, 1 percent or less, may indicate that a large weight percentage of a crude oil is made up of compounds containing these elements.

Paraffin (Alkane) Series. The paraffin series of hydrotarbons is a saturated, straight-chain (aliphatic) series having the general composition C n H 2n + 2• It is a hom*ologous series, progressing by a CH 2 increment from its simplest member, methane (eIL), up to complex molecules with over 60 carbon atoms. Isomers with branched-chain structure occur in increasing numbers in addition to the normal, or n, members, beginning with butane (C 4 H lO ), until isomers with large numbers of carbon. atoms are theoretically possible; however, only a few isomers of the paraffin members higher than the octanes have been identified. The commoner paraffin hydrocarbons are shown in Table 5-6. The paraffin hydrocarbons-sometimes ca!led the methane series-are chemically inactive; in fact, the name "paraffin" means "having little affinity." Methane (CH 4 ) is the simplest of all the hydrocarbons and is also the most able. It forms in swamps from decaying vegetable matter, as "marsh gas," and is the chief constituent of natural gas. Members of the paraffin series are generally the most abundant hydrocarbons present in both gaseous and liquid petroleums. All the members below pentane (C 5H,2) are gaseous at ordinary temperatures and are the chief constituents of natural gas. Paraffins between pentane and pentadecane (C15H3~) are liquid and are the chief constituents of straight-run (uncracked) gasoline. The higher members of the paraffin series are waxy solids. Gasoline is generally composed of the hydrocarbons that boil within the temperature range from 40° to 200°C (from approx. 100° to 400 0 P), and its composition varies with the crude oil trom which it is obtained. In addition to the large number of paraffin hydrocarb0l1s found in cmde oil, others are formed during the l'igh-ternperature cracking that also form~ gasolines. The relative amounts of the different hydrol..arbcn series in different straight-run (uncracked) gasolines aT'! s!J0\Nn ill T'lble 5-7. SOlilC nude oils cOr]talfl paraffin waxes, both amorphous and microcrystalllne, which .. rc obtained from the higher boiling-point fractions.r.4 Many wells g!\I;: twuole when pardffin wax is deposited on the walls of the tubing as the oil cools i., ri,;ng to the surface. Paraffin wax way also precipitate and clog • A hom*ol()g,)u~ :,erie, is one in \\hi.:h each member differs trom the next member r-y tht :;ame increment. Thus, in the paraffin series, each member differs from the next ;)' !he Increment of CH", The members of a hom*ologous series are said to be hOl1l%,;,\ of .one dnothf'r. Thus methane (CH.) is a hom*olog of all compounds in the paraffin Sns. op. cit. (reference nO'e 5~). p. 452. olefin, may be formed by minor cracking e!fecls during distillation.

1 The

the pores at the face of the reservoir when the expanding gas cools as it enters the well. Some wells produce a Vaseline-like substance that can be shoveled except during the hot summer months when the temperature is above the melting point. A still more nearly solid phase of the paraffin series occurring naturally is ozokerite, a plastic, wax-like, paraffin vein material found in Utah and near Boryslaw, Poland. Naphthene (Cycloparaffin) Series. The naphthene series of hydrocarbons, also known as the cycloparaffin series, is a saturated (single covalent bonds), hom*ologous, closed-ring series, the members of which have the general formula C"H 2n • It is isomerous with the olefin (alkane) series of the same composition, but members of the olefin series are structurally open-chain and unsaturated. For example, the cyclopropane member (C3H 6 ) of the naphthene series has the structural formula H

I 1/1 H-C-C-H H

C-H

~

CH 2CH2CH 2

Mol. wt. 42.08 B.P. -34.4°C

whereas its counterpart (isomer) among the olefins, propylene (also CaRs), has the structural formula

H I

H 1

C=C-C-H

I I H H

\

H

CH 2 : CH· CHa Mol. wt. 42.08 B.P. -47.0°C

and is an open-chain, unsaturated compound. There is some doubt whether the olelins are present in crude petroleums underground, but they are present in petroleums under ordinary surface conditions and are common in the corn-

186

THE RESERVOIR

pounds produced in refinery operations. The naphthenes resemble the paraffins in physical and chemical characteristics, but are more stable than their isomers, the olefins. Cyclopentane (C:;H10 ) and cyclohexane (C 6H I2 )* are the chief members of the naphthene series found in petroleum,55 although many naphthenes with from three to more than thirty carbon atoms in the rings are known. The naphthenes cyclopropane (CaH6) and methyl cyclopropane (C4H R) are gase: at ordinary temperatures and pressures, but all the other monocyclic naph· thenes are liquids. As seen in Table 5·7 (p. 185), Figure 5-22 (p. 182), and Figure 5-19 (p. 181), the naphthenes (cycloparaffins) constitute an important portion of petroleums as well as of most products, ranging between 7 and 31 percent in the straight-run gasolines from the fields listed. Crude oils with high percentages of naphthenic members are also called "asphalt-base crudes" because they include not only the simple naphthenic members but also many comple, asphaltic members from the higher boiling-point ranges. Aromatic (Benzene) Series.

The aromatic (or benzene) series of hydrocarbons, so named because many of its members have a strong or aromatic odor, is an unsaturated, closed-ring (carbocyclic) series, having the general formula C.H 2B _ 6 • Benzene (C 6H 6 ) , a colorless, volatile liquid, is the parent and most common member of the series found in petroleums. Other members commonly found in petroleum are toluene (methylbenzene, C 6H 5CH 3 ) anlf xylene (dimethylbenzene, C6 H 1CH 3 CHl\). While aromatics are present in all petroleums, the percentage is generally small, as seen in Figure 5·19, page 181, ranging from about 10 percent in the Pennsylvania crude oils up to an exceptionally high 39 percent in the Borneo crudes. Benzene and its derivatives also occur extensively in the light oil fractions of tars obtained from the dry distillation of coals at temperatures above 1,OOO"C. Crude oils high in aromatics yield fractions of high octane rating. • Cyclopentane, C,RIO CH1 CH.CH,CH,CH. Mol. wt. 70.13 B.P.49.5"C

Cyclohexane, C,Hu CH.CH2 CH,CH,CH.CH Mol. wt.84.16 B.P.81.4°C

H

H

\../ C

H, / H-C

"'- /1 C-IJ

I

I

u/" / 'H H-C

C-l1

C

/ .....

11

E

RESERVOIR FLUIDS-WATER, OIL, GAS [CHAPTER

5]

187

H

I

H C H "'-/"'\,/

c c II I c c /'\/''''H C H

Benzene, CeHe Mol wt. 78.11 B.P.80°C

I

H

The structure of benzene, called a Kekule structure, * is a ring with alternating single and double bonds between the six carbon atoms, and with each carbon atom linked to one hydrogen atom. The benzene structure has the hexagonal pattern shown above. Derivatives call be obtained by replacing one or more H atoms with methyl (CHs ) or some similar group.

Other Constituents Asphalt. Asphalt is a brown-ta-black, solid-to-semisolid mixture of highboiling point and high-molecular-weight hydrocarbon compounds that occurs either naturally or as a residue in the refining of some petroleums. It generally contains appreciable atnounts of ~ulfur, oxygen, and nitrogen together with varying amounts of inert matter. The asphalts are c10sely associated with the naphthenes (cycloparaffins). See pages 185, 186.

Sulfur. Sulfur occurs to some extent (0.1-5.5% by weight) in practically all crude oils 56 and in each of the fractions that make up the oil. It may be in anyone or mOTe of the fo}]owing forms: (1) free sulfur (S); (2) hydrogen sulfide (H 2 S); (3) organic su!fur compounds, such as thiols, or mercaptans, which contain the SH group (an example is propanethiol, or propyl mercaptan, CsHsS), and the disulfides, which contain S2 (an example is 2,3-dithiabutane, C2H6~). Many sulfur hydrocarbons are found in cracked distillates, but it is not known whether they are formed during the high-temperature distillation or whether they were originally present in the crude oil. No compounds carrying more than one atom of sulfur, except the disulfides, have been isolated from crude oil. Sulfur hydrocarbons form polar compounds, and they play an important role in boundary tensions, as discussed on pages 443-445. The presence of sulfur and sulfur compounds in gasoline causes corrosion, bad odor, and poor explosion. Before the development of modem cracking processes by refineries, the presence of sulfur made petroleum less desirable and consequently worth Jess per barrel Since sulfur can now be removed from oil, this price differential has been largely eliminated, and sulfur-bearing crude oils are nearly equal in \Talue to nonsulfur crudes.

* Named after the Gennan chemist Friedrich August Kekule, who first visualized the molecule as a group of little balls (atoms) joined by sticks. The benzene ring was discovered by Kekule after a dream of a monkey chasing its tail.

188

THE RESERVOIR.

Crude oils of low API (American petroleum Institute) gravity, or high specific gravity, generally contain more sulfur than others. Sulfur content has a wide range: at the low extreme are the high-gravity Pennsylvania crude oils carrying 0.07 or 0.08 percent sulfur, and at th~ high extreme are some heavy Mexican crude oils carrying from 3 to 5 percent sulfur. Many asphalt and bitum~n seepages and oil shales have a high sulfur content. The Mexican heavy-oil seepages, locally called "chapopotes," contain from 6.15 to 10.75 percent sulfur. Crude oils carrying less than 0.5 percent sulfur are called "lowsulfur crudes," whereas those carrying more than 0.5 percent are called "highsulfur crudes." Forty-two percent of the crude oil produced in the United States in 1946 was low-sulfur, and 58 percent was high-sulfur. 57 The sulfur content of crude oils may vary greatly even within the same producing region. The sulfur content of each boiling range varies, moreover, for each kind of oil. 'The gasolines of western Texas, for example, are high in sulfur, whereas the gasolines from other high-sulfur crude oils, such as those of the Middle East, contain very little sulfur, the sulfur being concentrated in the residues. It has been found in Wyoming, for example, that the high-suI. fur, low-gasoline, aromatic-naphthene-base crude oils are likely to be associ. ated with the limestone and dolomite reservoir rocks, and that the low-sulfur, high-gasoline, paraffin-based oils are found in the sand reservoirs. 58 The approximate average sulfur content of crude oils of various gravities is shown in Figure 5-23. The chart shows the general increase in sulfur content with de· crease in API gravity (increase in specific gravity).

r-INTER~EDIATE

West Texu>

~~UI"11:•

9. 33 0

FIGURE

20 40 60 80 100 120 140 160 Temperature • d.~re.. Fahrenheit

10. 11. 12. 13.

Walters. Kans. Healdton, Okla. Vaughn, Kans. Cress, Kans. Bemis, Kans. Sullivan, Kans. Burrton, Kans. Aylesworth, Okla. Aylesworth, Ok/a. Fargo, Okla. Bloomer, Kans. Silica, Kans.

293 32.1 34.4 36J) 37.7 34.5 37.4 36.5 41.0 41.6 44.2

5-33 Relation of viscosity to temperature in .different crude oils of Oklahoma and Kanms. The crude oils and their API gravities are listed heside the figure. [Redrawn from Nelson, O. &: G. Jour .• January 5. 1946. p. 70. Fig. 2.]

RESERVOIR FLUIDS-WATER, OIL, GAS [CHAPTER

5]

205

there is DO increase in the intermo1ecular distances, causes an increase in the frequency of the colli~ions between molecules and thus an increase in friction. As more and more gas is dissolved in crude oil, the viscosity of the oil i~ progressively reduced. This is one of the most important effects of the presence of dissolved gas in oil. As more gas goes into solution it! a crude oil, the API p-avity of the oil increases and the specific graVity decreases. The effect ot dissolved gas on both the viscosity and the gravity of a crude oil is shown iL Figure 5-34. The viscosity of an oil is at a minimum at the saturation pressure -that is, the pressure at which the oil contains in solution all the gas it is able to hold and the excess gas is first released from solution; the "bubble-point pressure" (see pp. 437-438).82 As the pressure is reduced, an additional amount of gas is released from solutjon. and the viscosity of the residual oil is increased. The increase in viscosity, due to release of gas, is greater than the normal decrease in viscosity that a decline in pressure causes in a gas-free oil or in an oil containing gas at pressures below the bubble point. The net result is an increase in the viscosity of the oil as the pressure is reduced below the saturation pressure and gas is released from solution. The importance of the viscosity of oil is evident when we remember that, jf the viscosity should be reduced by half, either twice as much oil would flow through the same sand or it would requite only half as much pressure to force an equal amount through. The change in viscosity of a specific oil, that of the West Edmond field, Oklahoma, as the pressure changes and in relation to the shrinkage factor and the solubility of gas in solution, is given in Figure 5-35. Viscosities also vary directly with the densities of oils, and the densities vary with the composition. Thus, the greater the number of carbon atoms in a member of a hydrocarbon .series, the greater will be its viscosity as well as its density. Some heavy crudes require heating to make them flow through the Density at 60 o F_ (in grams per milliliter) 57

45

40 11>

~

"-

35

Ih

~30

C

.

~ 25

>~ 2.0 0

~

FIOURE

5-34

Tlte effect of dissolved gar on the viscosity Ilnd gravity of a crude oil. [Redrllwn from O. &: G. rour., January 13, 1944, p.37.1

>

54

""

1.5 1.0

IS 0

/

/ V 0

L

Viscosity

'"

/

/

51

48

"'-.

GrlMly

r-

39 ~6

i~

200

Olssolved gos -

-

.;

42

33 100

45 ~

:X

/

'0. G

500 400 cu. ft. per bbl. 300

~

C)

206

THE RESERVOIR Sam Z770psio

I

Reservoir FIGURE

1600

pressure - lb. per sq. in. absolute

5-35 The effect of gas in solution and of a drop in pressure on oil from the West Edmond oil field, OklahcJlna. The graph is based on a hottomhole sample analysis. (Redrawn from Littlefield, Gray. and Godbold, Trans. Amer. Inst. Min. Met. Engrs., VoL 174 (1948), p. 147, Fig. 8.)

pipe lines. The combined relation of temperature and gravity to the viscosities of different crude oils is shown in Figure 5-36 for a group of Oklahoma oils. Viscositie!. are measured by viscosimeters, of which a number have been developed. Each of the cornman cODlIIlercial types-the SayboJt Universal, Sayholt Furol, Redwood No.1, Redwood No.2, and Engier-is calibrated with its own

~300~~~-+--4-~~~--~~

~~~~~~--4-~~~--~~

~

0~1~--1~~~~~~~-+___~_~1

...or

~ &OI,--k~"k--~~.J..;;

~ ~,cr-+'-;;-1~4.i:-~~d--+-I

Z ;:) ~~__+-~~~~~~~&--+~ !:i

.'«--m.~~.

~

3$,1----4--1----1

~ ),:

~~ •• :;

FIGURE

_J--

(I

ZIJ

40

iiO- 40

100-'20

TEMP£RAT\J~e:,

OEGREES rAHRE..H£IT

5-36

The progrersiv~ dc-ctease in viscosity with an increase in API gravity and an increase in temperature for a group oj typical Oklahorrta crud!' fjiis. [Redrawn from Nelson, O. & G./our., Janual'Y 5, 1946, p. 70.J

RESERVOIR FLUIDS-WATER, OIL, GAS [CHAPTER

5]

2()7

scale, and the scales may be converted to poises or stokes" by the use of convc;sion tables (see the Appendix) .80 The Saybolt Universal Viscosimeter is commonly used in the United States, whereas the Redwood and Engler instruments. which are similar to it, are used in Europe. The measurement is purely arbitrary, the reading being the number of seconds (Saybolt Universal, or S.U., seconds) required for a definite quantity of petroleum under controlled temperature and pressure to flow thnlUgh a 'special tube. It is desirable to obtain the viscosity of oj] under reservoir conditions of temperature and pressure and with varying amounts of gas in solution. The roIling-bail type of viscosimeter, designed fer this purpose, has a steel ball that rolls through an accurately bored barrel filled with oil and standing at an angle. The instrument is sealed at the temperature ar,d pressure desired, and the time the ball takes to roll the length of the tube IS electrically timed and calibrated to centistokes.

Refractive Index. The absolute refractive index (Rl) of a substance is the inverse ratio of the speed of light in that substance to its speed in a vacuum. The absolute refractive index can be obtained by conversion from the RI measured in air. The refractive index is defined as the ratio of the sine of the angle of incidence to the sine of the angle of refraction, both angles being determined with respect to a normal to the surface. When a ray of light passes from a lighter to a denser substance, it is bent toward the normal, owing t() dpcrease in velocity; when passing from a denser to a lighter medium, it is bent away from the normal. The range of refractive indices for petroleum is from 1.39 to 1.49. It is readily determined with an Abbe refractometer. The measurement offers a quick and fairly accurate method of determining the character of the oil from minute amounts that may be extracted from cores or drill cuttings. 83 The refractive index is also widely used in refinery operations to determine the character of petroleum fractions. Since the refractive index is dependent on the density of the oil, ~he heavier (lower API gravity) oils have the higher indices. Table 5-17 shows some representative relations between density and refractive index. The indices of a group of Venezuelan crude oils of varying API gravity are shown in Figllfe 5-37.

All oils exhibit morc or less fluorescence, also called "bloom" (see p. 84), the aromatic oils being the most fluorescent. The fluorescent colors of crude oils range continuously from yellow through green to blue. This Fluorescence.

The CGS unit of viscosity is the poise, and the cf'ntipoise (cn equal,> 1/100 poise. A fluid has a Viscosity of one poise when a tangential fo . . ce of one d)nc causef, a plane surface of one ~quare centimeter area, spaced one centimeter from a stationa!"y plane surface, to move with a constant velocity of one centime"." per second, the spac~ between the planes being filled with the vi~cous fluid (API Bull. 228, 1941). Air has a viscosity of 1.8 X 10-" water of 1 X 10-2, and gasoline of 0,6 X 10-' poise. The absolute, or kinematic, viscosity, which is the ratio of the viscosity i.n poises to the density of the fluid, is expressed in stokes and centislokes and is used for precise enginet:ring calculations.

208

THE RESE RVOIR

1.35

..

...c

0 0

c: •• 40

u

~ 1.4~ 4J

ol.~O

x

: ..

..

~- ~:.-

:s:

•

A

4J "0 ~ I.~~

~z

•

•

0:=

j A...~:s:

oto '!C g ~ ~ ... 0

••

.9

c

2c

~.

••

,.se o•

,0•

20

30•

~.

40

~o

•

•

60

70•

80"

90•

A. P.1. Degrees FIGURE

5·37 The relation 1. 30, No. '} \ 19~ 8/. p. 1630;li. S. &.. 1:, A",.ritan Petroleum Rtfining. 3rd .d., D. Van No.1."",, Co.,

New Y"rk 119451.? 45.

RESI RVOIR FLUIDS-WATER, OIL, GAS [CHAPTER

5]

211

cmde oils as 0.000817. In general, the heavier crude oils (low API gravity) have the lower coefficients of expansion, and the lighter crude oils (high API gravity) have the higber coefficients. (See also Table 5-16, p. 200.) (.'alorific Value. A calorie is the quantity of heat that will change the temperature of one gram of water from 3.5°C to 4.5°C. This unit is sometimes called the small calorie, and the large calorie equals 1,000 small calories. The British thermal unit (B.T.U.) is the amount of heat necessary to raise the temperature of one pound of water 1°F and is equal to 252 small calories. The calorific value of crude {lil decreases as the specific gravity increases (or as API gravity decreal>cs). A rough summary of the relation between the gravities and calorific values of crude oils is given in Table 5-18. The B.T.U. value per pound of crude oil is about 18,300-19,500, compared with 10,200-14,600 per pOUlnl of bituminous coal. The pllysicul characteristics of some representative crude oils are summarizeJ in Table 5-19.

I'JATURAL GAS The natural gas of a petroleum reservoir consists of the low-boiling-point hydrocarbon gases and may range from minute quantities dissolved in the oil up to toO percent of the petroleum content. In addition to the hydrocarbon gases there are gascou!.> impurities in varying amounts and consisting of hydrogen sulfide, nitrogen, and carbon dioxide. None of these gases is of much commercial value, but helium, which forms as much as 8 percent of some reservoir gases, may oe commercially important. Nat-Ilral gas may be broadly classified as associt.ted when it occurs with oil, and as nonassocillted when it occurs alone. The natural gas in a petroleum rcservoir may occur dS free gas, as gas dissolved in oil, as gas dissolved in water, or as liquefied gas. Free Gas. Free gas, when present, occupies the upper part of the reservoir and may be underlain either by oil (associated gas) or by water (nonassociated gas).* Gas Dissolved in Vil. When oil and gas are in intimate contact, a certain amount of gas dissolve's in t1le oil. The amount of gas in solution depends on the physical characteristics of both the gas and the oil, and on the pressure and temperature in thc reservoir. With few exceptions, all oil occurring in pools contains some gas in ':.olution, ranging from a few cubic feet up to thousands of cubic feet per barrel. Where the amount of gas is small, it is permitted to

* Natural gas is nearly everywhere con~ldeled to be the gaseous phase of petroleum. In Canada, however, the traditional usage has tJ~en legally changed, and nO'lassociated natural ga, is considered not petroleum but a separate sub~tance, whereas associated gas is considered as a petroleum substance. (See Borys v. Can. Pac. Ry and ImFerial Oil Ltd., Jud. Comm. Privy Council Jud~ment given Januar) 1953.)

THE RESERVOIR

212

TABLE 5-19 Physical Cltaracteristics of Some Typical Crude Oils Viscosity. Soybolt

Gravity (Average) Field. Location, Producing formation

Spec. Gr.

°APt

Univena'. Seconds.

Pour

Refer.

100°f

Point

encyibe'.> '1';\1'10\1'1> ~tlv.W.'I> fo>;' detenni.ni.ng liquid saturation of cores. OIL William A. Gruse and Donald R. Stevens, The Chemical Technology of Petroleum, 2nd ed., McGraw·Hill Book Co., New York (1942), 733 pages. A standard reference work. H. S. Bell, American Petroleum Refining, 3rd ed., D. Van Nostrand Co., New York (1945), 619 pages. A standard reference work. C. M. McKinney and O. C. Blade, Analyses of Crude Oil from 283 Important Oil Fields in the United States, &14289, U.S. Bur. Mines (May 1948). O. C. Blade, E. L. Garton, and C. M. McKiTmey, Analyses of Some Crude Oils from the Middle Easl, South America, and Canada, RI 4657, U.S. Bur. Mines (1948), 45 pages. Benjamin T. Brooks and A. E. Dunstan (editors), "Crude Oils, Chemical and Physical Properties," in The Science of Petroleum, Oxford University Press, London and New York (1950), Vol. 5, Part I, 200 pages. Many analyses and descriptive articles on crude oil. Emil J. Burcik, Properties of Petroleum Reservoir Fluids, John Wiley & Sons, New York and London (1957), 190 pages. Frederick D. Rossini, "Hydrocarbons in Petroleum," Jour. Chern. Education, Vol. 37, No. 11 (November 19~~O), pp. 554-561.

GAS Henry A. Ley (editor), Geology of Natural Gas, Amer. Assoc. Petrol. Geo!.. Tulsa, Okla. (1935), 1,227 pages. 38 articles by 47 authors. A standard reference work. C. E. Dobbin, "Geology of Natural Gases Rich in Helium, Nitro&en. Carbon

RESERVOIR FLUIDS-WATER, OIL, GAS [CHAPTER

5]

225

Dioxide, and Hydrogen Sulphide." in Geology ot Natural Gas, Amer. Assoc. Petrol. Geo!., Tulsa, Okla. (1935). pp. 1053-1072.

Norman 1. Clark. "It Pays to Know Your Petroleum," World Oil: Part I, March 1953, pp_ 165-172; Part II, April 1953, pp. 208-213. A clear explanation 01 the effects of changing reservoir conditions on oil and gas.

Reference Notes 1. Peter Grandone and Aiton B. Cook, Collecting and Examining Subsurface Samples of Petroleum, Tech. Paper 629, U.S. Bur. Mines (1941),67 pages. 2. Howard C. Pyle and John E. Sherborne, "Core Analysis," Tech. Pub. 1024, Petrol. TechnoI. (February 1939), apd Trans., Amer. Inst. Min. Met. Engrs., Vol. 132 (1939), pp. 33-61. 15 references. T. A. PoIlard and Paul P. Reichertz, "Core-Analysis Practices-Basic Methods and New Developments," Bull. Amer. Assoc. Petrol. Geo!., Vol. 36 (February 1952), pp. 230-252. 31 references. 3. Ralph Alexander Liddle, The Van Oil Field, Van Zandt County, Texas, Bull. 3601, University of Texas (January 1936),82 pages, 26 maps and sections. 4. Cecil Q. Cupps, Philip H. Lipstate, Jr., and Joseph Fry, Variance in Characteristics of the Oil in the Weber Sandstone Reservoir, Rangely Field, Colorado, RI 4761, U.S. Bur. Mines (April 1951),68 pages. Ralph H. Espach and Josepb Fry. Variable Characteristics oj the Oil in the Tensleep Sands/one Reservoir, Elk Basin Field, Wyoming and Montana, RJ 4768, U.S. Bur. Mines (April 1951), 24 pages. S. 1. C. Case et al., "Selected Annotated Bibliography on Oil· Field Waters," BUll. Amer. Assoc. Petrol. Geol., Vol. 26 (May 1942), pp. 865-881. 6. Theron Wasson and Isabel B. Wasson, "Cabin Creek Field, West Virginia," Bull. Amer. Assoc. Petrol. Geol., Vol. 11 (1927), pp. 705-719. 7. David B. Reger, "The Copley Oil Pool of West Virginia," Bull. Amer. Assoc. Petrol. Geo!., VallI (1927), pp. 581-599. 8_ Ralph E. Davis and Eugene A. Stephenson, "Synclinal Oil Fields in Southern West Virginia," Structure of Typical American Oil Fields, Amer. Assoc. Petrol. Geo!., Tulsa, Okla., Vol. 2 (1929), pp. 571-576. 9. James C. Crawford. "Waters of Producing Fields in the Rocky Mountain Region," Tech. Pub. 2383, Trans. Amer. Inst. Min. Met. Engrs., Vol. 179 (1949), pp. 264285. Bibliog. 12 items. 10_ L. C. Case, "The Contrast in Initial and Present Application of the Term 'Connate' Water," Jour. Petrol. Technol., Vol. 8, No.4 (April 1956). p. 12. 11. Charles R. Fettke, "Core Studies of the Second Sand of the Venango Group from Oil City, Pa.," Trans. Amer. Inst. Min. Met. EDgrs. (1927), pp. 219-230. Fettke was the first person to point out, in 1926, tbat an oil sand was not originally saturated with oil but contained appreciable quantities of water. 12. W. A. Bruce and H. J. Welge, 'The Restored-state Method for Determination of Oil in Place and Connate Water," in Production Practice and Technology. Amer. Petrol.Inst. (1947), pp. 166-174. 13. Parker D. Trask, "Compaction of Sediments," Bull. Amer. Assoc. Petrol. Geo!., Vol. IS (1931), pp. 271-276. 14. Cleo Griffith Rail and D: B. Taliaferro, A Method for Determining Simultaneously the Oil and Water Saturations of Oil Sands, RI 4004, U.S. Bur. Mines (December 1946). 16 pages. Bibliog. 34 items.

226

THE RESERVOIR

John G. Caran, "Core Analysis-an Aid to Profitable Completions," Mines Magazine (February 1947), pp. 19-24. 15 references. T. A. Pollard and Pau\. P. Reichertz, op. cit. (note 2). Discmsion of several methods of determining water saturation of reservoir rocks. 15. G. E. Archie, "The Electrical Resistivity Log as an Aid in Determining Some Reservoir Characteristics," Trans. Amer. lnst. Min. Met. Engrs., Vol. 146 (1942), pp. 54-62. G. E. Archie, "Introduction to Petrophysics of Reservoir Rocks," Bull. Amer. Assoc. Petrol. Geol., Vol. 34 (May 1950), pp. 943-961. W. O. Winsauer, H. M. Shearin, P. H. Masson, and M. Williams, "Resistivity of Brine-Saturated Sands in Relation to Pore Geometry," Bull. Amer. Assoc. Petro!. Geo!., Vol. 36 (February 1952), pp. 253-277. 14 references. R. G. Hamilton, "Exploitation of Geologic, Lithologic, Reservoir and Drilling Data to Improve Electric Log Interpretation," Tulsa Geo!. Soc. Digest, Vol. 21 (1953), pp. 34-36. 16. G. E. Archie, "Classification of Carbonate Reservoir Rocks and Petrophysical Considerations," Bull. Amer. Assoc. Petrol. Gee!., Vol. 36 (February 1952), p. 290. 17. M. P. Tixier, "Electric Log Analysis in the Rocky Mountains," O. & G. Jour., Vol. 48 (June 23,1949), pp. 143 ft. H. F. Dunlap and R. R. Hawthorne; "The Calculation of Water Resistivities from Chemical Analyses" (Tech. Note 67), Jour. Petrol. Techno!., March 1951, Sec. 1, p. 71 L. A. Puzin, "Connate Water Resistivity in Oklahoma," Petrol. Engr., August 1952, pp. B-67-78. Marion L. Ayres, Rollie P. Dobyns, and Robert Q. Bussell, "Resistivity of Water from Subsurface Formations," Petrol. Engr., December 1952, pp. B·36-48. Murphy E. Hawkins and J. L. Moore, "Electrical Resistivities of Oil Field Brines in South Arkansas and North Louisiana," Petrol. Engr., Vol. 28 (July-August 1956). Murphy E. Hawkins, "Electrical Resistivities of Oil Field Brines in Northeast Texas," Petrol. Engr. (July 1957), pp. B-52, B-68. 18. -L. C. Case, "Exceptional Silurian Brine near Bay City, Michigan" (Geo!. Note). Bull. Amer. Assoc. ~etrol. Geol .• Vol. 29 (1945), pp. 567-570. 19. Wayne F. Meents et al., l/linois Oi/-Field Brines, Ill. Petrol. No. 66, III. Geol, Surv. (1952), 38 pages, p. 7. 20. J. H. Reagan, "Notes on the Quirequire Oil Field, District of Pian, State of Monagas, Venezuela," Bol~tin de geologia y mineria, Torno 11, Nos. 2, 3, 4 (1938). 21. Staff of Caribbean Petroleum Company, "Oil Fields of Royal Dutch Shell Group in Western Venezuela," Bull. Amer. Assoc. Petrol. Geol., Vol. 32 (April 1948), p. 622. 22. James G. Crawford, "Oil-Field Waters of Wyoming and Their Relation to Geological Formations," Bull. Amer. Assoc. Petrol. Geol., Vol. 24 (July 1940), pp. 12141329. 23. Wayne F. Meents et 01., op. cit. (note 19). Numerous analyses and ten isocon maps of formation waters from various oil-producing formations. 24. Norbert T. Lindtrop, "Outline of Water Problems in New Grozny Oil Field, Russia," Bull. Amer. Assoc. Petrol. Geo!., Vol. 11 (October 1927), p. 1037. 25. C. E. Reistle, Identification 0/ 0,[ Field Waters by Chemical Analysis, Tech. Paper 404, U.S. Bur. Mines (1927), 25 pages. C. E. Wood, "Methods of Analysis of Oil Field Waters," in The Science oj Petroleum, Oxford University Press, London and New York, Vol. 1 (1938), pp. 646-652. Bibliog. lIS items. A general discussion of water analyses. L. C. Case and D. M. Riggen, "Analysis of Mineral Waters," O. &. O. Jour., Part I. December 23, 1948, pp. 74-75, and Part II, January 6, 1949, pp. 72-88. Details of the procedure used in modern oil-field water analyses. 26. Chase Palmer,· T/ip Geochemical Interpretation oj Water A/lrdyses, Bull. 479, U.S. Gem. Surv. (1911),31 pages. 27. E. M. Parks, "Water Analyses in Oil Production and Some An:al)'~ from Poison

RESERVOIR FLUIDS-WATER, OIL, GAS [CHAPTER

5]

227

Spider, Wyoming," Eult .-\mer. Assoc. Petrol. Geo!., Vol. 9 (September 1925), pp. 921-946. 28. E. T. lIeck. "Barium in Appalachian Salt Brines," Bull. Amer. Assoc. Petrol. Geol., Vol. 24 (1940), Pl'. 486-493. 29. Garland B. Gott and James W. Hill, Radioactivity in Some Oil Fields of Southel!stern K"nsas, Bull. 988·E, U.S. Geo!. Surv. (1953), 122 pages. 3e. Joseph IenSl."n, "C~1ifornia Oil·field Waters," in Problems of Petroleum Geology, Amer. A~IDC. PetroL Geol.. Tulsa, Okla. (1934), pp. 953-985. 123 analyses. L C. Case, "SL'bsurface Water Characteristics in Oklahoma and Kansas." op. cit., pp. 855-f68. 64 analyses. Walter R. Berger ll!'!rl Ralph H. Fash, "Relation (if Water Analyses to Structure and Porosity in the We,t Texas Permian Basin," op. cit., pp. 869-889. 42 analyses. JamesO. Crawford, op. cit. in note 21.512 analyses. James G. Crawford, "'Oil Field Waters of Montana Plains," Bull. Amer. Assoc. Petrol. 0001., Vol. 76 (August 1942), pp. 1317-1374. 279 analyses. Wayne F. Meents el d., op. cit. (note 19). 150 analyses and discussion of method of analysis. elm G. Ral1 and Jack Wright, Analysis of Formation Brines in Kansas, RI 4973, U.S. Bur. Mines CMay 1953). 600 analyses and discussion of method of analysis. 31. F. G. Tiekell, ' and permeability, is more difficult; in fact, these characteristics cannot be fully determined Without adequate subsurface data, which are obtained only from well records. Such data are based on studies of well logs, well cuttings, and cores, and on cross sections and subSUrface maps that show the distribution of rocks, their correlations, and their llnconformities. Several kinds of maps are needed to show the data upon which to base predictions of the position of favorable reservoir characteristics; these include areal-geology maps, structure maps, sand-distribution maps, formation-thickness (isopach) maps, subcrop and paleogeologic maps, prOductivity maps, isopotential maps, lithofacies maps, and other maps .. fhey are discussed in more detail in the section on subsurface mapping, pages 591-618. The simplest and COlI\monest way for a permeable underground fonnation to become a trap is to be folded into an anticline. An anticline is the most readily mapped of the ~ommon traps and can frequently be mapped at th\ surface. The close asso~iation of oil and gas pools with anticlinal folds was noted early and led to wbat has long been known as the anticlinal theory of oil and gas occurrence. Ge"

- ___ J

B

C ..:>

PIOll

.

'--

I

I>

I>

it

-~

::=I>

Plan

SectIOnS

FIGURE .6-23

ldedized diagrams showing characteristic traps formed chiefly by normal faulting, coupled with regional hom*oclinal dip: A, a trap !orm"d by a Single curved fault: B, a trap formed by two intersecting faults; C. a trap formed by several intersectIng faults. Arrows show dip.

256

THE RESERVOIR

FIGURE

,(

s~ec"on ~ .1('

r L-/ ,,-"-• .-.L...._

_

6-24

A, trap formed by intersection of a low fold with a normal fault; B, trap formed by intersection of a normal fault with a more acute lold (Arrows show direction of dip.)

shift than would normally be calculated from either one, or they may nullify each other. These two effects are shown diagrammatically in Figure 6-15. Shallow and Surface Weathering Phenomena. Wherever highly soluble formations arc exposed by erosion or come within the zone of circulating ground water, the resultant surface structure is often completely at variance with the deeper structure. 21 Solution of salt and other evaporites may throw the rocks at the surface into a jumbled mass of high dips, irregular folds, and erratic structures that have no meaning in relation to the deeper structure. Several large areas of salt solution, slumping, and collapse, for example, occur in western Texas, and the shallow structure is entirely unrelated to that of the underlying rocks. A section through the collapse over the Hendrick pool is shown in Figure 6-16. Because of the wide distribution of evaporitesp [o~sil slumps may occur in some areas, and some of the thinning, now ascribed to original causes, of the salt beJs found at the edges of depositional basins may well be due to sol'ltion by surface waters circulating during the time interval represented. The swelling of bentonitic and montmorillonite clays also may give rise to misleading surface folds. Surface dips in caliche have been mistaken for true formation dips and have caused structures to be mapped-and drilled·-that do not affect the deeper formations.:!ll At times the fold that carries down may be distinguished from the superficial fold by plotting a profile. The true fold, ABC in Figure 6-17,; rises above the regional dip' the supcrficidl folds, DEF and FGR, do not-they are, in fact, more like Tt'sithlal folds betwecn two synclines. Folding such as CEG! is sometimes spo~,en of as "pan.of-biscuit" fOlding.

TRAPS-GENERAL AND STRUCTURAL [CHAPTER

6]

257

Pre-unconformity Deformation. Folding and faulting that occur below buried unconformities are frequently not indicated at the surface, as the idealized section in Figure 6-18 shows. That is so over the Apache pool in Caddo County, Oklahoma, for example, and the reason is apparent from a structural section through the pool. (See Fig. 6-19.) A structural map of the overturned fold in the producing formation is shown in Figure 6-5, page 243, and a paleogeOlogic map of the pre-Pontotoc surface of unconformity in Figure 13-]2, page 605. In the Eakring field of England, also, several traps containing oil and gas pools are concealed below an unconformity surface. (See Fig. 6-20.) Overriding by Thrust Faults. Thrust faults may obscure the underlying structure, and a number of pools have been trapped in structures concealed by overriding sediments. An example is the Agha Jari anticline in Iran, shown in Figure 6-21, where the almost hom*oclinal dip above the overthrust fault gives no evidence of the underlying anticline. The Russell Ranch field, in the Cuyama VaHey of California, is another example. (See Fig. 6-22.) The prethrust, normal faulting and folding, which localize both the trap and the pool, are completely hidden by the overridhg sediments.

5

8

N

I 17

FIGURE

24

6-25

Structural mal' of the Vedder sand (Lower Miocetle) of the Round Moulltain field, Kern County, California. This is an example of a trap formed by a curved fault intersecting a hom*oclinai dip. [Redrawll from Brooks, AAPG Guidebook (1952), Los Angeiej, p. 148.]

-

~

"

R28E

If

~

R 29E

30

25 ~

,

"

258

THE RESERVOIR

16

.+ N

I

t+---One

mile--~

27 FIGURE

6-26

Structural map 0/ a phanlom horizon in the Chanac formation (PlioMiocene) of the West Edison oil field, Kem County. California. Tili.' is an example, Iypical of man)' pools in !Tie region, 0/ /rap1 formed chiefly by inlenecting faults superimposed on a hom*oclinal dip. [Redrawn from Sulfwold. Bull. Amer. Assoc. Pelro/. Geol., Vol. 37 (1953), pp. 802, 803.]

T. 30 S.

R. 29 E.

Displaced Pool. In most traps, if a pool of oil Or gas is present, it will occupy lile structurally hlghe~l po,ition in the reservoir rock, as the CI~t of a fold Gr the peak of a fault trap. There are some exceptions. however, where the pool is displaced for varying distances down one side of the trap. In romt

TRAPS-GENERAL AND STRUCTURAL

[CHAPTER

6]

259

I..L

CD

I•

I• I

FIGURE

\

\

I II 0;:

I I

o

If) (II

,CD 0 C\I

i

If)'" •

~:t]

w

6-27

Map of the top of the producmg Woodbine sand (Cretaceous) in the Richland pool, Navarro County, Texas The trace of the fault at the surface IS at EF, on top of the Austin chalk (Cretaceous); the fault trace overlymg the Woodbme sand IS at CD, and on top of the Woodbine sand it is at AB A great many pools are trapped In low folds cut by normal faults, such as this pool, and It may be considered tYPIcal. The sectIOn through XY is shown at the TIght. [Map redrawn from Lahee, in The Structure of Typical American 011 Fields, Vol. 1 (1929). p. 348, Fig. 25; section redrawn from Starnes, University of Texas Pub. 5116, p. 33].]

cases the crest of the structure will still be productive, and drilling into the highest point of the trap will make a discovery, but occasionally the displacement is enough to kave the crest of the structure barren. Displacement of pools is generally due to fluid potential gradients that result in the movement of water through the reservOir rock; if this condition is suspected, the fluid potential gradients of the area and the denSities of the Weiter and the expected oil should be studied, and the test well should be dnlled where the shape of the trap indicates the pool will be trapped under these conditions. The subject is consIdered in more detatl in Chapter 12.

260

THE RESERVOIR

w

--- ---- -Jurassic

-1\-----It

A

Salt 1\

/I

~

-,

1\

/I 1\

fI

-

-,

--

"

fI

/I

1\

"

1\ 1\

1\

/I

1\

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II

1\

1\

1\

1\

fI

/I

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fI

1\ 1\

fI

Water:

flleocO/n'

---_.lOll

1\ /I

Oil:

t

----4e.1iO/l

1\

/I

1\

1\

1\ fI

A fI

FIGURE

6-28

SectiOn through thl! Don'or oil pool

ill

the Emba province of the

USSR. Petroleum is trapped in a faulted Jurassic sand above a thick. folded salt mass. There is no evidence of ilJlrusion of the saIL. One well, at a

depth of 750 feet, produced over 75,000 barrels 0/ oil in 30 hours in 1911 [Redrawn from T_ Jeremenko, Neftyonoye Khozyaystro (Petroleum Econ omy), Moscow (1939), and C. W. Sanders, Bull. Amer. A,noc. Petrol. Geo/., Vol. 23 (1939), p. 505.]

Traps Caused by Faulting Normal, or gravity, faults and reverse and thrust faults 30 in the reservoir rock have wholly or partly formed the trap for many oil and gas pools; most pools found in str~ural traps, in fact, are modified by faults. Faulting may be the sole cause of the formation of a trap, but more commonly faults form traps in combination with other structural features, such as folding, tilting, and arching of the strata, or with variations-in the stratigraphy or permeability. Faulting has been a minor trap-forming element in many pools, where it modifies the trap and causes local variations in the production characteristics. Seep"~&e!: of oil ap.d gas are often associated with fault outcrops. Thus faults are commonly thought of as vertical channels permitting migration between reservoirs and to the surface. The presence of seepages at the surface suggests that the potentiometric surface of the aquifer is above the level of the ground. Lack of seepage, on the other hand, may indicate that the potentiometric surface is below the level of the ground. Many faults form the boundary plane of a pool of oil and gas, and this may be due to the fact that the fault is tightly sealed and holds the petroleum from further migration; or it probably more commonly is due to higher fluid potentials within the fault

TRAPS--GENERAL AND STRUCTURAL [CHAPTER 6]

261

channels and up-dip across the fault which act as an added barrier to the up-dip movement of petroleum. The combination of the fault and the hydrodynamic conditions forms a trap that holds a pool. Normal Faulting. Normal, or gravity, faulting, combined with a regional hom*oclinal dip, may form traps. There may be a single curved fault, as in Figure 6-23 (A), the intersection of two faults (B), or a combination of several faults (C). Normal faulting, combined with low folding, forms many pools, as in Figure 6-24 (A). As the folding becomes more acute, the trap becomes more definite (B); traps such as these are common on many elongated anticlines and domes. The Round Mountain field, Kern County, California, contains pools trapped by the intersection of curved faults with a hom*oclinal regional dip. (See Fig. 6-25.) The West Edison field, nearby, contains traps formed by the intersection of normal faults with a hom*oclinal regional dip. (See Fig. 6-26.)

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cr Wash*ta

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Trinity

FIGURE

.

1000 I

2000 ft.

Vert. and Horlz.

•

6·29 Section across the top of the Quitman field, Wood County. Texas. An example of a complex fault pattern developed along the crest of an elongate anticline. The Eagle Ford and Paluxy sands are productive in the high portion of nearly every fault block. The Woodbine sand is productive in one fault block. The oil-water contact is 104 feet higher in one Paluxy producing fault block than in the others, suggesting some post-accumulation faulting. [Redrawn from Scott, in The Structure of Typical American Oil Fields, Vol. 3, pp. 426-427, and Smith, University of Texas Pub. 5116, p. 318.1

262

THE RESERVOIR

~ ~tcrop

2

6po Fte'

FIGURB

6-30 Structure of the Kala oil and gas field, AZI1.bekovo Oil Trust, Baku region. Apsheron Peninsula, USSR. (See Fig. 2·3, p. 21, for location.) The contour interval is 10 meters (33 feet). The distribution of the oil and gas in the Sabunchi series (Productive formation, Pliocene) is shown. The cross faults form separate pools along the axis at the fold. The Kala fold is one of the simpler folds of the great Baku prodUCing province. [Redrawn from "Oil Deposits of Azerbaidjan," XJlllth Int. Geol. Cong., Moscow (1937), Vol. 4, p. 116.]

Many'pools in this part of California are trapped, in large part, by normal faults. ' The Richland pool, in Navarro County, Texas, is one of a great many pools trapped by a normal fault cutting across an arch or low fold in the reservoir rock.:u A structural map and section of the reservoir are seen in Figure 6-27, whIch shows not only the trace of the' fault at the surface but its intersection with the (producing sand. In the Dossor oil field, in the Emba province of the USSR,S2 shown in Figure 6-28, production is obtained from Jurassic sanm

TRAPS--GENERAL AND STRUCTURAL [CHAPTER 6]

263

that are arched and faulted; here, as in the Richland pool, the fault plane forms one side of the reservoir. Faulting often breaks up a field into separate pools; where that happens, the fault planes may become the boundary of a pool and tightly seal it off. The Quitman field, in Wood County, Texas, is one among many in which an anticlinal dome fold is broken into a number of fault blocks, each containing a trap in which oil and gas have accumulated. A section across the field is shown in Figure 6-29. Another is the Kala field, in the Baku district of the USSR, a structural map of which forms Figure 6-30. This combination of folding and faulting is common to many anticlinal traps. In some the transverse faults are not large enough to separate the pools; but in others, as in the Kala fold, many of the faults form boundaries of separate pools, distinguished by their differing oil-water contacts. Thrust faulting combines with anticlinal folding to form the traps in some of the prolific Cretaceous limestone reservoirs of western Venezuela. A section across the Mara field is shown in Figure 6-31 and shows the thrust horst along the axial part of the fold, a characteristic of several of these fields. The Inglewood field, in California, is an example of a trap composed of a dome that has been modified by normal faulting and forming several pools. A structural map of the rese1"loir and two sections across it are shown in Figure 6-32. The Creole field, in the Gulf of Mexico off the coast of Louisiana, consists of four separate pools trapped by normal faults associated with a dome fold caused by a deep-seated salt plug. The field is also of interest for another reason: it was all drilled from a single off-shore platform by means of directed drilling}3 The traces of the drill holes and their relations to the faults are Sea live

Met". 1000

2000

3000

+ +

+

+

+

+,

+

+

+

+ 6-31 Section across the Mara field, we~tern Venezuela. Large production OcellTS from the Cretaceous limestone and from the fractured granites of the basem*nt complex. Length of section about six miles. [Redrawn from Mencher et aI., Bull. Amer. Assoc. Petrol. Geol., Vol. 37, p. 725. to

FIGURE

+

Fig. 3.]

.

264

THE RESERVOIR

500r....

8'

!!lo..,.

... FIGURE

L-

6·32 Structure of the Inglewood field, Los Angeles basin, California, contoured on top oj the Gyroidina zone, and sections along AN and BH'. Movement along 'the main fault is chiefly horizontal, the movement being south along the east side (right lateral). The distribution of tlte oil is shown -as hatching in the sections. [Redrawn from Driver. Bull. 118, Calif, Div. Mines (1943), p. 307,]

shown in Figure 6·33. The distribution of the gas, oil, and water in the deeper pools is shown in Figure 6-34, and a section through the pool in Figure 6-35. The structure of the producing Woodbine formation (Upper Cretaceous), in the Van field, northeastern Texas, is shown in Figure 6-36. Two separate pools, A and B, are trapped by a fault superimposed on a dome fold. A section from northeast to southwest across the field is shown in Figure 5-4, page 150, where an analysis is made of the time the accumulation occurred. Minor faulting may in some cases follow incipient fracturing and the shallow effect of subsurface stresses related to the folding; as erosion and removal

TRAPS-GENERAL AND STRUCTURAL [CHAPTER

6J

265

200 ft FIGURE

Horizon/Ql traces 01 nineteen holes drilled from one offshore drilling platform in the Creole oil field, Louisiana, Gull 01 Mexico. The structural conditions encountered are shown in Figures 6-34 and 6·35. [Redrawn from Warson, in The Structure of Typical American Oil Fields, Vol. 3, A mer. A.ssoc. Petrol. Geol. (1948), p. 288, Fig. 5.] 6-33

THE RES ER VOIR

266

FIGURE

6-34

Structure and distribution of gas, oil, and water in the chief producing sand, the Gulf sand (Miocene), of the Creole od field, found at about 6,700 feet. [Redrawn jrom Wasson, in "Ihe Structure of Typical American Oil Fields, Vol. 3 (1948), p. 291, Fig. 7.]

of the overburden bring these stressed rocks closer to the surface, the stress caused by the folding may be relieved by minor faulting. Thus many fold traps are accompanied by minor faults, which mayor may not reach down to the reservoir, and faults that do reach the reservoir merely change the outline of the pool without affecting the water-oil contact, which remains planar throughout the pool. Such a pool is in the Petr6lea field, in the Barco region of Colombia,34 shown in Figure 6-37. The minor shallow faulting is both of a normal and a thrust type. Pools trapped by normal faulting are almost always on the upper side of the fault. One might expect, on looking at a cross section, that the lower side of a fault would form a trap, but it seldom does so, apparently because the oil and gas escape up-dip around the ends of the fault. Pools in which petroleum is trapped on the lower side of a fault are exceptional and are generally to be explained by a combination of minor faulting, permeability variations, hydrodynamic forces directed down-dip folding along the lower side, and truncation of the reservoir by the lower side of the fault. Many of the pools along the Gulf Coast of southern Texas and Louisiana, however. lie on the lower side of normal faults. These pools are found immediately to the south of a series-of

TRAPS-GENERAL AND STRUCTURAL [CHAPTER

NW -1000--- - - - - - - - - - -:--

267

6]

SE --------~-------

Feet

·2000---59e of porosity and permeability and undoubtedly is an accessory cause in many more pools.41 (See pp. 119-125.) It may alsO be regarded as the chICf cause of traps in a few instances where special condItions prevail. In the Florence field, in Colorado,4z

TRAPS-GENERAL AND STRUCTURAL [CHAPTER

279

6]

j{

·1000--- - - - - - - - \ - - -_ _ __

------- ------------~~---------------

A

~---

A S,m.

Sr';;--

FIGURE

6-50

--~---\~------------SL

I

-....

./

"-

---Krg

Structural map and sectlollS of the Achl-SU field, southeast of Grozny, m the northeastern Caucasus, USSR. The chief accumulatIOn IS trapped In an elongated, closed, overthrust fold, the reverse fault formmg the edge of the trap in some sands. Smaller accl/mulatlons are found helow the faliit In the truncated formatIOns Th,s type of trap IS charaetensllc 0/ a number of pools In the Old Grozny region of the eastern Cauca.~us. [Redlawn from I 0 BtOd, XVllth Jnl Geol. Congo (Mo~eow, 1937), Vol 4, p. 28, Figs. 3 alld 4]

"-

K'Q

"

s'

B Srm

Srm 2

---- ......

/./

Srm. _ _ _ _ -" KrQ

/

/

----

Tsc:h

which was one of th:: first oIl pools discovered in the Umted States, the od IS trapped wlthm the fractured portIOn of the PIerre shale (Cretaceous), which is nearly flat-Iymg and of umform texture over a wIde area beyond the field Apparently fracturing alone IS responsible for localizing the trap, for there IS no folding or stratigraphic change associated WIth the pool, and where the fracturing plays out there is no otl accumulation. A sectIon through the pool IS shown 10 Figure 6-53. ProductIon is erratIc, and one well is reported to have produced nearly a mIllIon and a half barrels of oi1. 43 Similar traps contain the gas found in the Cherokee shales (Pennsylvanian) of cae general conditions are found. Primary stratigraphic traps may be divided into two general groups: (1) lenses and facies of clastic and igneous rocks; (2) lenses and facies of chemical rocks, including biostromes, organic reefs, and bioherms.

Lenses and Facies in Clastic Rocks Some reservoirs are in thin lenticular bodies of porous and permeable clastic rock enclosed in impermeable sediments. Their areal extent generally does

.

8

'"

••

A~b'"

. . ....

~:.

:=" ' . ""

Impermeable

'

0.

.....

•

~

.

,0

- .........

.. ..... ~ ••• 0" ••

• • 0·0

...

..... .

,

•

: . Permeable ':. . ::. ...:: . ...

..

"

..

X

L_ Plan

Plan

~iI

ij=-=~--- --=--=-= ':..::.-.

~

X

-_~~ -:

water-

-

-~

.section

FIGURE.

X'

x-.:::::::--.. . .,:-.. . _---.:::.. .... ~_ ~./

-. . ;:

~

011_::::--

~;::"...

Water

........- .

-::-----........:

~echO(l~--:::-: ",,'. ;:,.....~--- _

~

•

7-1 Sketches showing (A) typical lens-type traps completely surrounded by impermeable rocks, and (B) an irregular up-dip edge of permeability

on

Q

hom*oclinal

re~ional

dip. Arrow shows direction of dip.

288

THE. R ESERVGIR

not ex':erd a few square miles, :llthuugh there 8.rc a number of exceptionally large ones. M,)st cOmmOnly the lens consists of dastic material--sandstones, ltrkos.:fmeability may mark the critical edge of a single trap or of a grrWF of traps. Tbe oil and gas pool m"3y completely fill the porous part of a sand lens; it m3Y occupy only the high portion of th:: lens; or, if the regional structure is rnonociinal, it may accumuiate in irregularities along the regional up-dip rage vf permcnbility. One t!ling to remember in exploration is that, where one Sllch primary s:ratigraphic trap is found to contai!1 oil, there may be others like it neaIby, for the conditions that determine the presence of facies and lens traps 3re commonly or regional extent, and the local phenomena are likely to be repeated over wide areas_ In many regions where petroleum is trapped in the~e sands, the pattern of pools reflect:; a compktely random distribution of sand patches, lenses, sandy zones, bars, and channels. An example lS the Third Stray sand 0f the Venango Group (Devonian), which is in northwestan Pennsylvania near the site of !he histrInQI G/In Dtan 000

HordonJllurt

GoIc_

-Jac:bon- "

"llanow"

100

Cypr... Upper POInI u Pam c:r.. s. Iltlhol

Rtnoun AlIt Vote. I,

St. Glrtwleve

....f.,.

o

Sha..

FIGURE

•

LUt\nton.

E=B ~

SlltstOftl

7-3 Strattgraplllc leetlon, using the Barlow-Beech Cree"- lllnestone as datum horizon, showing the relatIOns of the Chester (Upper MissiSSIppian) strata across the southern end of the Illinois basin. The section extends for seventy miles from the Poole 011 field In northwestern Kentucky '{I the outcrop of the jormatlOns in southern lndtana Note the great vertIcal exaggeratIOn in scale. The extreme lateral variableness In the section helps explam many of the 011 pools found In these rocks ill the lllinois basill. AI some place or other nearly every sand and ltmestone in the column has been jound productIve. [From Swann and Alher/on, Jour. GeoJ., Vol. 56, oppO!lte p. 272, Fig. 6.]

most of them are associated with folding, but many are lImited on one or more sides by the edges of permeability.5 A great many pools in sand lenses and patches are found throughout the Pennsylvanian rocks extending from Pennsylvania to Texas. One example is illustrated in Figure 7-4, an isometric diagram of the Dora pool, in Seminole County, Oklahoma. The sand lens, which is almost completely surrounded by impervious shales, IS expected to yield an ultimate total of about 10 million barrels of oil. 6 A somewhat different type of patchy sand reservoir is shown in Figure 7-5, a sectIOn through the Hull-Silk field in Archer County, Texas. The interbedded Pennsylvanian sandstones and limestones found there are characteristic of many pools in the Mid·ContlOent region. Regional facies changes from permeable to impermeable rocks determine the location of the edges of a great many 011 and gas pools, which sometimes are called strandline pools when they are associated with shore phenomena. Some patchy and lenticular sand formations may trap series or groups of pools

TRAPS-STRATIGRAPHIC AND FLUID [CHAPTER

,

FIGURE

.,

291

7]

,•

7-4 Isometric block diagram of the Dora sand (Pennsylvanian) in the Dora pool oj Seminole County, Oklahoma. The shape of the sand body suggests an origin associated with offshore phenomena. The sand is composed of well-sorted quartz grains ana varies up to 100 feet in thickness. The trap is an example of a sand lens or sand patch almost completely enclosed by shales. [Redrawn from Ingham. in Stratigraphic Type Oil Fields (1941), p. 418.]

located in long linear "trends_" Two outstanding examples are the YeguaJackson (Upper Eocene) trend (AA' in Fig. 7-6) and the Frio-Vicksburg trend (Oligocene) (BB'). The pools along these trends are trapped in various combinations of lenticular sands, up-dip pinch-outs, and local folding and ,2800

,3000

,32.00

,3400 ,:'.:,:',:':: ::':

=::::',:::.:,:::::: ~.,: ,~ .: ':'.~ ~~ :..:::':.'::~ .:;.::: ;~';:': '~. cdttercd chert nodules, which dlp away from the central mass at angles that arc commonly dS high as 40° and may even be as hIgh as 70°. The htgb dIpS are tpought to be the result of &Iumpmg and compactIOn of the hIDe mud caned "dreWlte.·' ~ These dolomite beds surrounding the bioherm commonly oveI1ap, wedge, and thicken toward the mIddle of the bioherm. An the eVIdence appears to md.-

320

FIGURE

THE~

RES ER VOIR

7-36 Section across a Niagaran (Middle Silurian) organic reef (bioherm) found in the well-known reef locality of the Wabash Valley in northern Indiana. The diameter of the reef shown is approximately 2,000 feet, and the height of the section approximately 145 fut. This section illustrates a reef that began to grow on a shale floor. into which it later settled as the overlying weight increased. [Redrawn from Cumings and Shrock, Bull. Geol. Soc. A mer., Vol. 39 (1928), p. 598, Fig. 7.]

cate that the Niagaran bioherms were built in fairly shallow water and stood as low mounds from a few feet to some tens of feet above a surrounqing sea bottom on which fine calcareous muds were being deposited. The organic reefs of the Niagaran epoch-in Illinois and Indiana, for example-consist of nearly pure carbonate, either limestone or dolomite, with higher electrical resistivity than the normal inter-reef rock; unlike many limestones, they contain no chert. They range from a few feet up to several miles in diameter, from a few feet up to a thousand feet in height; and from round through elliptical to ridge-like in shape. They may consist only of the massive core, in contact with the surrounding formations, or the core may' be flanked by bedded reef-derived detritus that dips away in all directions. Details of one of these reefs are shown in Figure 7-36.' An exposed· ancient reef that is analogous to present barrier reefs is the spectacular Capitan reef in the Permian Basin of western Texas and New Mexico,51 shown in Figure 7-37. This reef, which is partly above and partly below the surface, dwarfs all other known fossil reefs, having a thickness of 1,200 feet or more and extending for a distance of at least 400 miles. (See Fig. 3-10, p. 73.) Many oil and gas pools are found along much of its buried portion (BB'). The 10!cognition of this massive dolomitic limestone body, and of the equally tbick underlying goats*eep reefy mass, was long delayed-chiefly, it would seem, because of misinterpretation of the organic remains in the rocks, which presented a difficult problem. The Capitan and goats*eep reefs occur in the Guadalupe series of the Permian, and mark a transition from the deep-water clastic sandstones and interbedded limestones of the Permian Basin to the limestones in the shallow-water evaporite deposits of the back-reef or lagoonal areas. Another great organic reef development is the Abo-Wichita Albany (Permian) reef in West Texas and New Mexico. This was a narrow reef-tremlthat rimmed the Midland basin to the west, north and east. More

FIGURE

Thiele Mdd.d li"'tI'oft" b.d. 1$ 10 50 f•• , thick

~

one mile

Thln_b.llet.d, ligh. ;relY tifMslone, with It..... "-"d.to,. b.d_

~

•

'ilift.b.dded. tlark gray Ii"" Jlo ...

I[@I ......

(00,.. . .'01" . . . . . . . 1.......".

(80 ... Spti .., liMMfoAe)

Thift"'c&d.d. bklck II""..

~

'.lMh,e",.

'1.....ral ...4i. ,hl""'Mlle4I

r:-:-:l t..::.:..:.J

7-37 Section through the famous Guadalupe-Capitan .(Permian) reef in southeastern New Mexico. This reef extends for hundreds of miles to the east and southeast, where many oil pools are trapped in the porous Capitan reef limestone. [Redrawn from King, U.S. Oeol. Surv., Prof. Paper 2)5 (1948), Fig. 7.]

hundred ,"I ,hit"

Me .. iv. li".tlON, b.d. ""ftI'ol

~

4000

3000

2000

Feet

322

THE RESERVOIR INFERRED

CURRENT

DIRECTION

-_____ 0

_--_-_-_-_-~_-_-

___

- A,cent

---

-

member-_-_-_--'"'"_-_-_-_-

____

...... _-: -

--

.,.

.....

~

-=-c:>C'

•

_ A. blue-grey mart locI•• ::. - ::.

- !'''''dal sand facies - - - -k cherI limeslone focies: _ aonurus siltstone ocies -_-=---=--_-=--_-_-_-,:.CabGlitra

FlGUllE

formation

-_-=--_--'"""'_-_-_

----=---~--------=---------

7-38 Section through a bioherm, typical of many in the Alamogordo member of the Lake Valley formation (Mississippian), in the Sacramento Mountain region of southeastern New Mexico. It is believed that bioherms similar to these produce oil where buried in north-central Texas. [Redrawn from Laudon and Bowsher, Bull. Amer. Assoc. Petrol. Geol., Vol. 25 (1941), p. 2128. Fig. 10.1

than sixty oil pools have been discovered in locally developed reef mounds, and more discoveries are anticipated. 62 In the Sacramento Mountains of New Mexico~3 typical bioherms containing crinoidal remains are exposed in the Mississippian Lake Valley rocks. These bioherms are rounded, knob-like, fiat-bottomed masses of hard featureless limestone, a typical example being shown in Figure 7-38. They' are distinguished by their absence of bedding, particularly toward their centers. They vary in size, but some are as much as 200 feet thick and a mile across at the base. They have initial dips away from the center of as much as 40°. the only fossil remains are single, isolated, calcareous crinoid columns on the inferred lagoonal, or lee, side of the biohermal areas, where a loosely cemented, fragmental, and disintegrated crinoidal facies occurs, evidently a detrital deposit washed down from th.e growing reef. This is interbedded with the outer margins of the bioherms, from which it is separated by a definite lithologic break. Typical also of the lagool!al areas is a sand facies, and farther inward there is a calcareous fossiliferous marl; both of these are poorly developed, if at all, on the se~ward side. Because of the thickening toward the bioherm of the adjacent talus-like beds and the very abl11pt and pronounced thinning of the first overlying bed as it passes over the bioherms, Laudon and Bowsher believe that the individual bioherms stood as mounds above the sea floor. In the Borden, or Knobstone, group (Lowe! Mississippian) of Indiana bioherms occur as disconnected limestone masses composed chiefly of crinoids and containing much -chert; they are completely enclosed by clastic siltstones, fine-grained sandstones, and thin beds of shale.~' The Borden bioherms were built up cc."'1ltemporancomly with the enclosing c1a~tic sediments. A reef of organisms in Permian red beds, in South Park, Colorado, has been described by Johnson.!l~ The reef seems to have been built by one species

TRAPS-STRATIGRAPHIC AND FLUID [CHAPTER

7]

323

of lime-secreting alga, a large colony of which must have been growing here for a considerable time. It is approximately 1,100 feet long, 80 feet thick, and 300 feet wide. Its outlines are not symmetrical but show irregular tongues of algal limestone interfingering with the red beds. When material from this b!ohenn was examined microscopically, it was found that the organic structure had been partly destroyed by the crystallization of dolomite; algae ace nevertheless represented by numerous coupled and broken threads, tubes, and masses of cellular tiKsue. Many reefs are found in the Permian of the Perm Basin, USSR. They are productive in some places. 56 One that crops out as a hill is shown in the section through Tra-Tau in Figure 7-39. . Organic reefs in two Gulf Coast salt domes, Damon Mound and West Columbia, have been desc:ribed by Ellisor.67 She found that the foraminiferal assemblage within the limestone lens of Oligocene age differs from that in the surrounding shales of equivalent age and stratigraphic position, and that many of the limestone genera ate characteristic of warm, shallow seas around coral reefs. The diagnostic species Heterostegina antillea, which occurs throughout the limestone. is characteristic of the modern coral reefs of Antigua and the weSt Indies. Stratigraphic relations similar to those described may be identified around many fossil reefs, and they form a useful means of identifying a facies or its Osition with respect to other parts of the reef complex.1i8 The nomenclature is given on the idealized diagram through a reef in Figure 7-40. Many of these features may be correlated with the modern reef, as shown in Figure 7-30, page 314. The back-reef, or lagoonal, facies (A) consists of interbedded limestones, dolomites, sandstones, red shales, and evaporites such as anhydrites, which merge into the main reef (B) . The fore-reef, or seaward, side consists of gray-to-black bedded limestones and shales (D). which grade into the reef

FlGUIm

7-39 Section through Shikhan Tra·tau ("sl.ikhan" "7leans shllTp hill), a bioherm ,hat is pal"( of a chain of partially buried reefs. a few miles north of the lshimbcuvo oi; field. (See Fig. 7·51.) [Redrawn from Dunbar, Bull. Amer. A.lSOC. Petrol. Geol.• Vol. 24. p. 256, Fig. 7.J

324

THE RESERVOIR

D. Balin or Fore· Atef Faf ie .·pontic· faci., Alternating beddfd limestonlll and sand' stones stained brown and p,trolif._.

C.

Alternating bedd.d L.S. dolomite, r.d-b...., evaporites, shQle, sands, etc.

A.

.fo7~~::';§-~ :Z-f;@R'v

B.

Nassiylt. CC'ltrnolls, porous

tight· 41re, to whit. - weoth.rint

dolomites _ond limestones.

' V '

ft.

Dense, line·grained,b1adl.bituminOUI impeNiou. shole ond limeston... Giwes up oil

when

"eoted.

Little traci of oil in ovtcropl. FIGURE

7·40 Characteristic rock facies occurring in different parts of a reef. [Redrawn from Link, Bull. A mer. Assoc. Petrol. Geo!., Vol. 34 (1950), p. 2'&7, Fig. 1%.)

rock through a mixture of sand and reef debri~ (C). Coarse conglomerates with boulders up to 15 feet in diameter, and o~e transported block 900 feet long and 40 feet thick, have been observed as submarine landslide deposits that slid oft the steep talus slopes of organic reefs in Leonard-Capitan formation-, (Permian) of western Texas. 58 Fossil reefs occur in some regions at difterept elevations and at different stratigraphic positions within a thick sequence of rocks, thus forming a reef complex. possibly better described as a reefy or reef-like complex. Such conditions presumably result from alternating adv~nces and retreats of the sea across favorable reef.building territory. The advancing sea permits bioherms to grow progressively nearer the shore, as at A1' A 2, and Aa in Figure 7·41. Each successive reef is overlain by foredeep and basin facies, which indicate the transgressive nature of the deposits. Later, as the sea retreats, successive reef growths form along the retreating shore line, possibly as shown diagram· matically in Figure 7-42, where the reefs are represented by Rh R 2 , aHd Ra. FIGURE

A

Sea

Level

7-41

Idealized transgressive (A) and regressive (B) shor,iJ lines and the reef deposits formed under these condi· tions. Reels A·1 and B-2 are contemporaneous with the shore lines 1 and 2. etc. [Redrawn from Link, Bull. Amer. Assoc. Petrol. Geol., Vol. 34 (1950), p. 278. Fig. 13.]

TRAPS-STRATIGRAPHIC AND FLUID [CHAPTER FIGURE

7-42

Ideruiud

R-I

rombiMtion

R-2

325

7] R-3

0/ _

reefs formed under advancA ;, AA - I ' -'" ing shore line, A-I, A-2, and A-3, and reefs formed as the shnre line retreated, R-t, R-2, and R-3. Reefs A-3 and R-l correspond to the farthest shoreward advance of the sea. [Redrawn from Link, Bull. Amer. Assoc. Petro/. Geol., Vol. 34 (1950), p. 279, Fig. 15.J

The identifying characteristic of the regressive type of bioherm is the burial of the reef by lagoonal or back-reef material. In Figure 7-41 the bioherms As and Rl are the same and constitute the reef nearest the shore. 59 A transgressive ana regressive shore may also cause the reef core to slope as sh?WD in Figure 7-43. Productive Reefs. Organic reefs '1ntaining oil and gas are found at many levels within the geologic column; they are of widespread occunence. throughout the world, but appear to be especially common in North America-possibly beca. ,e of more drilling there. They range through all gradations and 'ombinations of atolls, table reefs, fringing reefs, barrier' reefs, biostromes,

A.

Regressive reef undergoing uplift and lor building out over its own . to I us - slopes - Former seo level - - -

~..

Seo level-

'\