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Sedimentology and the petroleum industry
Sedimentary Geology - ElsevierPublishingCompany, Amsterdam - Printed in The Netherlands
SEDIMENTOLOGY AND THE PETROLEUM INDUSTRY L. K~JLBL
Osterreiehisehe MineralOlverwaltungA.G., Vienna (Austria)
(ReceivedApril 24, 1967)
SUMMARY
A consideration of the actual state of relations between sedimentology and the petroleum industry shows that between the rapid development of sedimentology on the one hand, and the application of the results to concepts of the petroleum industry on the other hand, a striking discrepancy exists. The essential reasons for this surprising fact may be seen from the opinions occasionally expressed by important petroleum geologists at congresses and other meetings. The concepts of the petroleum industry, and especially those of exploration activity, are necessarily influenced by hypotheses which, in the course of their application, often become more and more dogmatic. As a result, the value of the concepts used is decreased. It is now an important task of applied sedimentology continually" to confront the results of new sedimentological research with the hypotheses which are common in use in petroleum geology, and eventually to enforce a re-examination of these ideas. Important results of sedimentological investigations which should influence current ideas concerning source beds, primary distribution of organic matter in sediments, occurrence and distribution of hydrocarbons, primary migration and reservoir traps are briefly outlined.
INTRODUCTION
When we are willing to understand from the general term "sedimentology" not only a new descriptive branch of the science of geology but, in accordance with BROUWER(1962), "a high degree of integration of research results from many branches of geology, obtained by a wide variety of techniques", then the relationship between sedimentology and the petroleum industry'is clearly recognizable. The focal point of the petroleum industry represents the most important mobile organic phase in sediments-petroleum. Every stage of its fascinating history--formation, migration, accumulation, entrapment, and finally the discovery of new oil pools--is to be understood only in connection with a better understanding of the accompanying sediments. Therefore it is easy to understand that the rapid development of sedimentology Sediment. GeoL, 1 (1969') 223-234
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during the last decades has been partly due to the many inducements and the financial support that sedimentological investigations have received from the petroleum industry. But between the rapid development of sedimentology and its application to problems concerning the petroleum industry there exists a remarkable discrepancy. One may recognize this fact by studying thoroughly the excellent bibliography compiled by VATAN (1960). Listed in this bibliography are 2,225 papers published between 1955 and 1959, but less than 1 ~ deals with the application of sedimentology to problems of the petroleum industry. Therefore, it is by no means surprising to find PASSVGA(1960) stating that sedimentology does not have the place it deserves in petroleum exploration. "Exploration remains an advanced structural study of poorly known sediments." PASSmA (1960) believes that two principal reasons may be responsible for this lack. First, structural theories are one-sided preferences in petroleum exploration and second, ancient sediments are often merely compared with recent sediments but seldom directly studied. It is important to the concepts of petroleum exploration to determine, by direct study of ancient sediments, sedimentation factors and particularly the depth of ancient seas. Knowledge of the deposition of ancient sediments, especially their bathymetric distributions, plays an important part in several concepts related to petroleum exploration. In these considerations PASSEGA (1960) agrees with BROVWER (1962), who referred to the geological fact that crustal movements have markedly affected the history of sedimentology. "This should be kept in mind when applying results of studies on recent sediments to ancient ones" (BROUW~R,1962). But when we are trying to get to the roots of the above-mentioned problem it appears that still other reasons need to be considered. Such a need was clearly expressed by HUBBERT(1965) at the 15th Annual Convention of the Gulf Coast Association of Geological Societies, Houston, 1965. H ubbert pointed out that the decline of our ability to find new oil fields may also in part be the result "of our inflexibility with regard to our geological ideas of where oil ought to be found." It may even be necessary, he pointed out, to re-examine our premises concerning oil exploration from the point o1 view that "oil geology pertains to oil, rocks, and water and is not limited to over-simple ideas concerning structural and stratigraphic traps." These important considerations by Hubbert imply fundamental problems between the relationship of sedimentology to the petroleum industry. Concepts of exploration activities are necessarily influenced by hypotheses underlying basic problems of petroleum geology. In the course of application, many of these lose their flexibility and become increasingly dogmatic. Consequently, th~se concepts are changing for the worse. It is now a fundamental task of sedimentelogy to overcome these inflexibilities and to be helpful in the re-examination of our premises concerning oil exploration by confronting the results of research in sedimentology with the basic problems of Sediment. Geol.. 1 (1967) 223-234
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petroleum geology. In doing so, sedimentology will soon reach the place it truly deserves in petroleum geology and in the petroleum industry. It is not possible to treat exhaustively here all the branches of interest which are included in sedimentology; furthermore, there are excellent compilations available (references). Therefore this discussion will be limited to some fundamental questions of petroleum geology which demonstrate particularly well the irtcreasing influence of sedimentology on the petroleum industry.
SOURCE BEDS
When the question of the origin of petroleum arises, it is immediately involved with the concept of source beds. This concept is commonly discussed, and especially with a view to oil exploration. It is generally assumed that the transformation of organic matter into oil is already completely accomplished in the source beds. Because the organic matter is usually finely disseminated in the pelitic sediments, it is assumed that the original tiny oil droplets must at first also be finely disseminated. But when we take into consideration recent progress in the various branches of sedimentology, it seems doubtful whether this simplified concept is on the whole correct. As this is a complex problem, it may be helpful to split the discussion into several stages: primary distribution of the organic matter, occurrence of hydrocarbons, primary migration, and reservoir traps.
PRIMARY DISTRIBUTION OF THE ORGANIC MATTER
Up to now, available observations confirm the theory that petroleum originated through a biochemical transformation of sedimentary organic matter in a reducing environment. Occurrence and distribution of organic matter in the sediments is therefore an important factor for all concepts of petroleum geology. In the course of several years, BITTERLI(1962a, b, 1963a, b, 1964) investigated about 1,590 samples of primary bituminous rocks, collected in Western Europe and ranging in age from Cambrian to Pliocene. This extensive research project was carried out by the Koninklijke/Shell Exploration in the Produktie Laboratorium, Rijswijk, The Netherlands. Bitterli determined that bituminous material is neither inherent in one particular lithofacies nor in one particular environmental facies. Instead, the bituminous materials are distributed predominantly in fine-grained and laminated rocks of different kind. For the concept of oil exploration it is remarkable that conditions favorable to the formation of bituminous sequences are related to paleogeographic turning points. Transgressions, regressions, and eustatic oscillations which are followed by stagnation and anaerobic conditions favour the formation of bituminous rocks. Sediment. GeoL, I (1967) 223-234
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The conditions of the environments of deposition were marine, brackish and limnic, and often a transitional or an alternating facies was observed. If we compare the observations of BITTERL~ (1962a, b, 1963a, b, 1964) with the commonly accepted theories about petroleum source beds, then we must acknowledge the importance of his conclusions. Neither the theory of petroleum formation only in special deposits of a sapropelitic facies, nor the theory that the genesis of oil is an ordinary case in the development of sediments can be dogmatically held in lace of the sedimentological results of his investigations. B1TTERLI(1962a, b, 1963a, b, 1964) states that organic matter is nearly always present in sufficient quantity, but oil in commercial quantities can be due only to a concurrence of a number of favorable factors (geochemical facies, pH, Eh, mechanism of migration, accumulation, entrapment, etc.). To date, we are still far from completely understanding the concurrence of these factors. Bitterli's observations are in full accord with the experiences of the oilindustry. Many oil pools (in Brasilia, Vienna Basin, China, etc.) have been found in formations whose oil yield is not explained by the common theories about petroleum source beds. The results of Bitterli's investigations increase the possibilities for exploration activity in areas which, prior to now, have not been regarded as especially promising.
OCCURRENCEOF HYDROCARBONS Recent development of methods of analyses have made it possible to obtain better knowledge of the composition and distribution of hydrocarbons which occur in both sediments and crude oil. SMITH (1954) and MEINSCnEIN (1959) pointed out that sediments and petroleum contain the same types of hydrocarbons. From this, one may conclude that petroleum is really derived from sedimentary organic matter. But Meinschein further emphasized that the distribution of these compounds is markedly different in sediments and in crude oil. From these observations it follows that the formation c! petroleum is not a simple combination ol hydrocarbons but rather a selective process. It therefore seems improbable that oil formation in finely disseminated droplets is accomplished in source beds. In assuming this, the selective process is missing. MEmSCHEIN (1959) believes that the selection occurs during the primary migration. To date, the mechanisms of hydrocarbon migration and accumulation have been to a great extent unknown. But the important investigations of BAKER (1959, 1960) undoubtedly point the way to a better understanding of these processes. Baker considers the problem of petroleum formation by selective accumulation of hydrocarbons from the viewpoint of colloid chemistry. Witherspoon demonstrated that petroleum contains colloidal particles (WnnERSPOON, 1958; ME1NHOLD,1964). BAKER(1959) found that two different kinds of colloidal particles are present; these he named, in accordance with the invesSediment. Geol., 1 (1967) 223-234
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tigations of MGBA1N and JENKINS(1922), ionic and neutral micelles. He emphasized that the hydrocarbons are selectively dissolved in the micelles. "The frequency distribution of hydrocarbons in crude oils reflects variations in the kind and size of the micelles in which the sediment hydrocarbons selectively d i s s o l v e " (BAKER, 1962). His results indicate that hydrocarbons, at first finely disseminated in the sediments, are selectively dissolved and transported out of the source beds in a colloidal solution of formation waters containing natural solubilizers (organic acid soaps). The release of the selectively solubilized hydrocarbons in suitable carrier beds is a further important stage of petroleum formation. This release is regarded as a colloid-chemical process which may be caused by variations of the physical and chemical conditions (change of the salt concentration, change of pressure, temperature, etc.). Probably at first an emulsion is formed, and then oil globules of different size, and finally a segregation of water, oil, and gas (secondary migration) may occur in the carrier beds. At present it is not yet possible to describe in detail all the stages of this process which finally result in the formation of crude oil. But it is probable that the investigations of BAKER (1962) suggest the most successful approach to a better understanding of the mystery of petroleum. In light of the considerations outlined above, the complete process of petroleum formation must involve selective solution, migration, accumulation and release of sediment hydrocarbons in formation waters containing natural solubilizers. "Until the process occurred, petroleum would not exist in the sediments" (BAKER, 1962). The results of Baker's investigations are eminently qualified to influence a re-examination of many hypotheses commonly in use in petroleum geology.
PRIMARY MIGRATION
The transportation of finely disseminated hydrocarbons from the sediment into the carrier beds is called primary migration. Although primary migration plays an important role in hypotheses concerning petroleum formation, this process is rarely examined thoroughly. "When primary migration is mentioned one usually must be content with the words squeezing or flushing" (BAKER, 1962). It is obvious that the motivator of the migration is the compaction of sediments. This process is carefully examined and described by HEDBERG (1936), TERZAGHI (1947), EMERYand RITTENBERG0952), WELLER(1959) and VON ENGELHARDT(1960). Compaction begins with the expulsion of water from the porous fine-grained sediment (which may be sedimented with a porosity of about 80 ~). With increased burdening by younger sediments, porosity decreases rather rapidly. A significant change in the mechanism of compaction when porosity reaches about 35 ~ was recognized by HEDBERG (1936). Until this porosity is reached the sediments are in the "dewatering stage", in which all particles are surrounded by water. In the subSediment. Geol., 1 (1967) 223-234
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sequent stage, the stage of "mechanical deformation", the interstitial water is almost completely squeezed out, and the mineral particles are in direct contact. This stage is reached at a depth of about 500 ft. When migration is considered from the point of view of the mechanism of compacting sediments, it is clear that the only possible stage with sufficient effect is the dewatering stage. The slowly decreasing porosity in the following stages of compaction has less influence on the migrating formation water. Sedimentological investigation about the compaction of sediments show that the squeezing out from the colloidal solutions of hydrocarbons in the formation water must essentially be accomplished during the dewatering stage of compaction. This conclusion leads to the question of the time span needed to originate the sediment hydrocarbons. Opinions on this subject are divergent. Some authors believe that the transformation of organic matter into sediment hydrocarbons takes place during a span of geological periods. Therefore it is assumed that the same source rock may be capable of yielding petroleum at intervals in geological time, depending on the change of pressure and teml:erature in connection with the subsidence of a sedimentary basin (WELTE 1964, 1965). But there exist observations which show that this assumption is probably not always right. SMITH (1954) examined sediments and cores from the Gulf of Mexico, and by means of the 14C-method he estimated the age of the organic matter to be about 30,000 years and the age of the sediment hydrocarbons to be l 1,800 -k 1,400 years. Kidwell and Hunt received similar results in recent sediments of Venezuela (source beds, 13,400 ~ 1,200 years; hydrocarbons, 14,000 years)(SMrrU, 1954; KIDWELLand HUNT, 1958a, b; MEINHOLO, 1962). These investigations show that the transformation of organic matter into hydrocarbons is a process which probably runs its course during the early diagenesis of sediments. This is in accord with the postulation that any migration caused by compacting sediments is effective only in the relatively short spaces of time during which dewatering takes place. The dewatering stage being finished, it is very difficult to extract hydrocarbons from the source beds. These difficulties increase enormously with increasing lithification of the sediments. The assumption held by some, that hydrocarbons migrate in a gaseous phase, is rot plausible (MvINSCHEIN, 1961); and the often-quoted theory of oil migration through joints and fissures is chiefly based on a confusion of primary migration with "remigration" (Gussow, 1955). (Remigration is migration of a pre-existing oil pool, caused by younger tectonic movements.) Many efforts have been made to establish the relationship between crude oil and the assumed source beds (BUCHTAet al., 1963; SXRAKnOV, 1957). But in the light of the above-mentioned sedimentological investigations, it seems highly unlikely that any relationship can be acknowledged (MEzNrfOLD, 1966). By selective solution and migration, the source beds have lost those compounds through which, after release in the carrier beds, crude oil has yet to be formed. The organic matter remainSediment. Geol., I (1967) 223-234
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ing in the source beds is rather comparable to the "pomace'" that stays behind in the press (KIDWELLand HUNT, 1958b). The results of BAKER'S(1959, 1960, 1962) investigations, and recent knowledge of the mechanism of primary migration by compacting sediments, necessarily influence concepts of petroleum exploration. A careful study of the paleogeographic develop; merit, and particularly of the hydrodynamic conditions of the area being explored is of basic importance. The drainage of a basin during periods of subsidence and deposition of sediments undoubtedly deserves more attention than the more commonly emphasized question of potential source rocks.
RESERVOIR TRAPS
The basic task of the petroleum industry is to recognize those locations in which oil in commercial quantities probably will be found. Valuable tools for the solution of this difficult problem are the results of sedimentological research. Customarily, structural and stratigraphic (lithologic) traps are distinguished. But MARTIN (1965) focused attention on a third kind of trap, one which up to now has received relativelylittle consideration. He termed this a "paleogeomorphical trap", because hydrocarbons are trapped in primarily paleogeomorphical phenomena. To date, most exploration has been directed toward a search for crude oil in structural traps. PASSEGA(1960) criticizes the idea that the occurrence of an anticline and a sequence of favorable-looking sediments is generally sufficient to localize an exploration well. But many of the largest oil and gas pools in the Western Hemisphere are trapped by non-structural phenomena. LEVORSEN(1965) states that "we have stumbled into many non-structural oil and gas pools while looking for purely structural traps". It is evident that structural traps which are obvious and not difficult to localize are the first to be tested. But now we know that many of the "purely" structural traps are in reality combined traps. Often it happens that primary entrapment is governed by lithological factors, and that this lithological trap is later transformed by tectonic movements (K6LBL, 1966). The one-sided direction of exploration--the search for purely structural traps-seems to be particularly doubtful in areas where geological units are not autochthonous. A careful analysis of the paleogeographical and sedimentological possibilities for the existence of an oil formation is, in such areas, an absolute necessity. LEVORSEN (1965) emphasizes that "the time seems to have arrived we must start looking directly for combination traps of all kinds involving different proportions of structure, stratigraphic change and fluid flow phenomena". In this start sedimentology will be an absolutely necessary guide. Geomorphological traps include all the phenomena of a buried paleo-relief (buried hills, ancient sandstone-filled valleys, fossil reefs, etc.). Many landscapes exposed for millions of years may be peneplains, but often they still have sufficient relief Sediment. Geol., 1 (1967) 223-234
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for the accumulation of hydrocarbons. The hydrocarbons may be trapped below or above the paleogeomorphical surface. An analysis of this type of trap must take into account all the geomorphological features, which includes both form and process. "The form creates the trap and tlze process shapes the form" (MARTIN, 1965). Paleogeomorphological traps are often to be expected in the basement of a subsided basin. A morphological map of the basement is then the first requisite for an exploration concept (KAvoUNEK et al., 1963). All features of the buried relief have to be carefully examinated and compared, with the various stages of sedimentation and. the drain age of the subsided basin. When we are studying paleogeomorphological traps (for example, buried hills) there often arises the question of where the oil comes from. An investigation of the origin and diager~esis of the subsurface waters which usually accompany crude oil may be helpful in recognizing the direction of migration; it is also an aid in deciding the question of whether the water comes by way of a vertical migration, from deeper parts of the basement, or by way of a lateral migration, from the sedimentary sequence which had covered and mantled the pre-existing relief (KOLBL, 1967).
FINAL REMARKS
In this brief review an attempt has been made to demonstrate that sedimentology is today, and shall be also in the future, of great importance to the development of the petroleum industry, chiefly by inflvencing every concept of petroleum exploration. But it is necessary to keep in mind that the results of sedimentological research still involve many hypothetical ideas which have yet to be confirmed by further sedimentological work. To avoid that an ir~fl.exibility of ideas comes into use, it is absolutely necessary to compare continually the results of sedimentological investigations with the information we receive from exploration and exploitation wells. This may be done in several ways. The Rocky Mountain Section of the Americain Association of Petroleum Geologists dedicated a full meeting (September 1965) to the single theme: "The truth about typical Rocky Mountains oil traps". Of additional special value for this purpose is a comparison of the results received by sedimentological, geophysical, etc., methods with those received by exploitation activity in a well-known area, as is done, for example, in the Gifthorn Trough in northwest Germany (HECr~T, 1959; PH~LIJ'~'et al., 1963a, b). "The careful examination of todays producing areas will point the way to tomorrows success (LEvORSEN, 1965).
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LEVORSEN, A. I., 1959. Outlook for petroleum exploration. World Petrol Congr., Proc., 5th, N.Y., 1959, 23 : 447 pp. LEVORSEN,A. I., 1960. Paleogeologie Maps. Freeman, San Francisco, Calif., 174 pp. LEVORSEN, A. I., 1965. The obscure and subtle trap. 15th annual meeting of the Rocky Mountain Section, AAPG, Sept. 1965. (Abstract). Bull. Am. Assoc. Petrol. Geologists, 49 : 1566. LOMBARD,A., 1956. G~ologie S~dimentaire. Masson, Paris, 722 pp. MARTIN, R., 1965. Application of paleomorphology to exploration for oil and gas. 15th annual meeting of the Rocky Mountain Section AAPG, Sept. 1965. (Abstract). Bull. Am. Assoc. Petrol. Geologists, 49 : 1567. Mc BARN, J. W. and JENKINS, R. W., 1922. The ultrafiltration of soap solutions: sodium oleate and potassium laurate. J. Chem. Soc., 12 : 23-27. MEINHOLD, R., 1962. ErdOlgeologie. Akademie Verlag, Berlin, 227 S. MEmHOLD, R., 1964. Gesetzm/issigkeiten der ErdOlakkumulation in den Sedimentationsbecken. Z. Angew. Geol., 10 : 118-126. MErNEIOLD,R., 1966. Zur Diagnose der Erd61muttergesteine. Z. Angew. GeoL, 12 : 628-630. MEINSCIJErN,W. G., 1959. Origin of petroleum. Bull. Am. Assoc. PetroL Geologists, 43 : 925-943. MEtNSCrtEIN, W. G., 1961. Significance of hydrocarbons in sediments and petroleum. Geochim. Cosmochim. Acta, 22 : 58-64. PASSEGA,R., •960. S6dimentologie et recherche de p6trole. Rev. Inst. Franc. P~trole Ann. Combust. Liquides, 15 (12) : 1731-1740. PETERSON, J. A. and OSMOND, J. C., 1961. Geometry of Sandstone Bodies. Am. Assoc. Petrol. Geologists, Tulsa, Okla., 240 pp. PHILIPP, W., DRONG, H. J., FOCHTBAUER,H., HADDENHORST,H. G. and JANKOWSKY,W., 1963a. The history of migration in the Gifthorn Trough (N-W-Germany). World Petrol. Congr., Proc., 6th, Frankfurt, 1963, 19 : 22 pp. PHILIPP, W., DRONG, H. J., FOCHTBAUER,H., HADDENHORST,H. G. and JANKOWSgY, W., 1963b. Zur Geschichte der Migration im Gifthorner Trog. Erdi~lKohle, 16 : 456-468. POTTER,P. E. and PETTIJOHN,F. J., 1963. Paleocurrents andBasin Analysis. Springer, Berlin-G6ttingenHeidelberg, 296 pp. R~ECKER,R. E., 1962. Hydrocarbon fluorescence and migration of petroleum. Bull. Am. Assoc. Petrol Geologists, 46 : 60-75. RITTENBERG,S. C., EMERY, K. O., HULSEMANN, J., DEGENS, E. T., FAY, R. C., REUTER, J. H., GRADY, J. R., RICHARDSON,S. H. and BRAY,E. E., 1963. Biogeochemistry of sediments in experimental Mohole. J. Sediment. PetroL, 33 : 140-172. RUDAKOW,G. W., 1965. Der Migrationsmechanismus und die Bildung yon Erd61-und Gasansammlungen. Z. Angew. Geol., 11 : 638-645. SIaEPARO, F. P., 1960. Recent sediment studies, 1955-1959. Symposium: Sedimentology and the oil industry. World Petrol. Congr., Proc., 5th, N.Y., 1959, pp.9-27. SHEPARD,F. P., PHLEGER,F. B and VAN ANDEL,T. H. (Editors), 1960. Recent Sediments, Northwest Gulf of Mexico. Am. Assoc. Petrol. Geologists, Tulsa, Okla., 394 pp. SIEVER, R., 1960. Recent trends in american sedimentology. Symposium: Sedimentology and the oil industry World PetroL Congr., Proc., 5th, N.Y., 1959, pp.28-36. SILVERMANN, S. R., 1965. Migration and segregation of oil and gas. 15th annual convention Gulf Coast Assoc. of Geol. Societies and Regional Meeting of AAPG. (Abstract.) Bull. Am. Assoc. Petrol Geologists, 49 : 1766. SLoss, L. L , 1955. Location of petroleum accumulation by facies studies. World Petrol. Congr., Proc., 4th, Rome, 1955, pp.315-335. SMrrrI JR., P. V., 1952. Preliminary note on origin of petroleum. Bull. Am. Assoc. Petrol. Geologists, 36 : 411-413. SMrrn JR., P. V., 1954. Studies on origin of petroleum: occurrence of hydrocarbons in recent sediments. Bull. Am. Assoc. PetroL Geologists, 38 : 377-4134. SNARSKY,A. N., 1962a. Zur Frage der Erd61entstehung. Freiberger Forschungsh., C., 123 : 35-54. SNARSKY,A. N., 1962b. Die prim/ire Migration des Erd61s. Freiberger Forschungsh., C., 123 : 63-73. SOKOLOV,V. A., ZHUSE,T. P., VASSOYEV1CH,N. B., ANTONOV,P. L., GRIGORIYEV,G. G. and KOZLOV, V. P., 1963. Migration processes of gas and oil, their intensity and directionality. World PetroL Congr., Proc., 6th, Frankfurt, 1963, 47 : 13 pp.
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STEVENS, N. P., BRAY, E. E. and EVANS, E. D., 1956. Hydrocarbons in sediments of the Gulf of Mexico. Bull. Am. Assoc. Petrol. Geologists, 40 : 975-983. STRA~:HOV, N. M., (Rgdacteur), 1957. M6thodes d'6tude des rocfles s6dimentaires. Moscou 1957. Traduction par J. Pietresson de Saint Aubin, A. Vatan. Ann. Serv. d'lnformation G6ol., 35: tome 1,447 pp.: tome 2, 509 pp. TERZA~ml, K., 1947, Discussion of the Compressibility ~2fclays by Skempton (1945). Quart. J. Geol. Soc. London, p. 203-206. VON ENGELHARDT, W., 1960. Der Porenraum der Sedimente. Springer, Berlin-GOttingen-Heidelberg, 207 pp. VAN S'rRAATEN,L. M. J. U., 1964. Deltaic and Shallow Marine Deposits. Elsevier, Amsterd~m, 464 pp. VATAN, A., 1960. Bibliographic des travaux r6cents de s6dimentologie. Worm Petrol. Congr., Proc., 5th, N.Y., 1959. pp.37-162. WELLEg, J. M., 1959. Compaction of sediments. Bull. Am. Assoc. Pettvl. Geologists, 43 : 273-310. W~LT~i, D. H,, 1964, Ueber die Beziehungen zwischen Erd61en und Erd61muttergesteinen. ErdOI Kohle, 17 : 417-429, WELTE, D. H., 1965. Relation between petroleum and source rock. Bull. Am. Assoc. Petrol. Geologists, 49 : 2246-2268. Wvc-rl~, D. H., 1967. Entwicklungsgeschichte von Erd61e~ auf Grtmd geochemisch-gcologischer Untersuchungen. Erd61Kohle, 20 : 65-77. WI~SENEDER, H,, 1964. Die ErdOl-Muttergesteinsfrage im Wiener Becken. ErdiiI-Erdgas Z., 80 : 479481. WITr~ERSPOON, P. A., 1958. Studies on petroleum with the ultracentrifuge. Ill. State Univ. Surv., Rept. Invest., 1958 : 206.
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SEDIMENTOLOGY AND THE PETROLEUM INDUSTRY L. K~JLBL
Osterreiehisehe MineralOlverwaltungA.G., Vienna (Austria)
(ReceivedApril 24, 1967)
SUMMARY
A consideration of the actual state of relations between sedimentology and the petroleum industry shows that between the rapid development of sedimentology on the one hand, and the application of the results to concepts of the petroleum industry on the other hand, a striking discrepancy exists. The essential reasons for this surprising fact may be seen from the opinions occasionally expressed by important petroleum geologists at congresses and other meetings. The concepts of the petroleum industry, and especially those of exploration activity, are necessarily influenced by hypotheses which, in the course of their application, often become more and more dogmatic. As a result, the value of the concepts used is decreased. It is now an important task of applied sedimentology continually" to confront the results of new sedimentological research with the hypotheses which are common in use in petroleum geology, and eventually to enforce a re-examination of these ideas. Important results of sedimentological investigations which should influence current ideas concerning source beds, primary distribution of organic matter in sediments, occurrence and distribution of hydrocarbons, primary migration and reservoir traps are briefly outlined.
INTRODUCTION
When we are willing to understand from the general term "sedimentology" not only a new descriptive branch of the science of geology but, in accordance with BROUWER(1962), "a high degree of integration of research results from many branches of geology, obtained by a wide variety of techniques", then the relationship between sedimentology and the petroleum industry'is clearly recognizable. The focal point of the petroleum industry represents the most important mobile organic phase in sediments-petroleum. Every stage of its fascinating history--formation, migration, accumulation, entrapment, and finally the discovery of new oil pools--is to be understood only in connection with a better understanding of the accompanying sediments. Therefore it is easy to understand that the rapid development of sedimentology Sediment. GeoL, 1 (1969') 223-234
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during the last decades has been partly due to the many inducements and the financial support that sedimentological investigations have received from the petroleum industry. But between the rapid development of sedimentology and its application to problems concerning the petroleum industry there exists a remarkable discrepancy. One may recognize this fact by studying thoroughly the excellent bibliography compiled by VATAN (1960). Listed in this bibliography are 2,225 papers published between 1955 and 1959, but less than 1 ~ deals with the application of sedimentology to problems of the petroleum industry. Therefore, it is by no means surprising to find PASSVGA(1960) stating that sedimentology does not have the place it deserves in petroleum exploration. "Exploration remains an advanced structural study of poorly known sediments." PASSmA (1960) believes that two principal reasons may be responsible for this lack. First, structural theories are one-sided preferences in petroleum exploration and second, ancient sediments are often merely compared with recent sediments but seldom directly studied. It is important to the concepts of petroleum exploration to determine, by direct study of ancient sediments, sedimentation factors and particularly the depth of ancient seas. Knowledge of the deposition of ancient sediments, especially their bathymetric distributions, plays an important part in several concepts related to petroleum exploration. In these considerations PASSEGA (1960) agrees with BROVWER (1962), who referred to the geological fact that crustal movements have markedly affected the history of sedimentology. "This should be kept in mind when applying results of studies on recent sediments to ancient ones" (BROUW~R,1962). But when we are trying to get to the roots of the above-mentioned problem it appears that still other reasons need to be considered. Such a need was clearly expressed by HUBBERT(1965) at the 15th Annual Convention of the Gulf Coast Association of Geological Societies, Houston, 1965. H ubbert pointed out that the decline of our ability to find new oil fields may also in part be the result "of our inflexibility with regard to our geological ideas of where oil ought to be found." It may even be necessary, he pointed out, to re-examine our premises concerning oil exploration from the point o1 view that "oil geology pertains to oil, rocks, and water and is not limited to over-simple ideas concerning structural and stratigraphic traps." These important considerations by Hubbert imply fundamental problems between the relationship of sedimentology to the petroleum industry. Concepts of exploration activities are necessarily influenced by hypotheses underlying basic problems of petroleum geology. In the course of application, many of these lose their flexibility and become increasingly dogmatic. Consequently, th~se concepts are changing for the worse. It is now a fundamental task of sedimentelogy to overcome these inflexibilities and to be helpful in the re-examination of our premises concerning oil exploration by confronting the results of research in sedimentology with the basic problems of Sediment. Geol.. 1 (1967) 223-234
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petroleum geology. In doing so, sedimentology will soon reach the place it truly deserves in petroleum geology and in the petroleum industry. It is not possible to treat exhaustively here all the branches of interest which are included in sedimentology; furthermore, there are excellent compilations available (references). Therefore this discussion will be limited to some fundamental questions of petroleum geology which demonstrate particularly well the irtcreasing influence of sedimentology on the petroleum industry.
SOURCE BEDS
When the question of the origin of petroleum arises, it is immediately involved with the concept of source beds. This concept is commonly discussed, and especially with a view to oil exploration. It is generally assumed that the transformation of organic matter into oil is already completely accomplished in the source beds. Because the organic matter is usually finely disseminated in the pelitic sediments, it is assumed that the original tiny oil droplets must at first also be finely disseminated. But when we take into consideration recent progress in the various branches of sedimentology, it seems doubtful whether this simplified concept is on the whole correct. As this is a complex problem, it may be helpful to split the discussion into several stages: primary distribution of the organic matter, occurrence of hydrocarbons, primary migration, and reservoir traps.
PRIMARY DISTRIBUTION OF THE ORGANIC MATTER
Up to now, available observations confirm the theory that petroleum originated through a biochemical transformation of sedimentary organic matter in a reducing environment. Occurrence and distribution of organic matter in the sediments is therefore an important factor for all concepts of petroleum geology. In the course of several years, BITTERLI(1962a, b, 1963a, b, 1964) investigated about 1,590 samples of primary bituminous rocks, collected in Western Europe and ranging in age from Cambrian to Pliocene. This extensive research project was carried out by the Koninklijke/Shell Exploration in the Produktie Laboratorium, Rijswijk, The Netherlands. Bitterli determined that bituminous material is neither inherent in one particular lithofacies nor in one particular environmental facies. Instead, the bituminous materials are distributed predominantly in fine-grained and laminated rocks of different kind. For the concept of oil exploration it is remarkable that conditions favorable to the formation of bituminous sequences are related to paleogeographic turning points. Transgressions, regressions, and eustatic oscillations which are followed by stagnation and anaerobic conditions favour the formation of bituminous rocks. Sediment. GeoL, I (1967) 223-234
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The conditions of the environments of deposition were marine, brackish and limnic, and often a transitional or an alternating facies was observed. If we compare the observations of BITTERL~ (1962a, b, 1963a, b, 1964) with the commonly accepted theories about petroleum source beds, then we must acknowledge the importance of his conclusions. Neither the theory of petroleum formation only in special deposits of a sapropelitic facies, nor the theory that the genesis of oil is an ordinary case in the development of sediments can be dogmatically held in lace of the sedimentological results of his investigations. B1TTERLI(1962a, b, 1963a, b, 1964) states that organic matter is nearly always present in sufficient quantity, but oil in commercial quantities can be due only to a concurrence of a number of favorable factors (geochemical facies, pH, Eh, mechanism of migration, accumulation, entrapment, etc.). To date, we are still far from completely understanding the concurrence of these factors. Bitterli's observations are in full accord with the experiences of the oilindustry. Many oil pools (in Brasilia, Vienna Basin, China, etc.) have been found in formations whose oil yield is not explained by the common theories about petroleum source beds. The results of Bitterli's investigations increase the possibilities for exploration activity in areas which, prior to now, have not been regarded as especially promising.
OCCURRENCEOF HYDROCARBONS Recent development of methods of analyses have made it possible to obtain better knowledge of the composition and distribution of hydrocarbons which occur in both sediments and crude oil. SMITH (1954) and MEINSCnEIN (1959) pointed out that sediments and petroleum contain the same types of hydrocarbons. From this, one may conclude that petroleum is really derived from sedimentary organic matter. But Meinschein further emphasized that the distribution of these compounds is markedly different in sediments and in crude oil. From these observations it follows that the formation c! petroleum is not a simple combination ol hydrocarbons but rather a selective process. It therefore seems improbable that oil formation in finely disseminated droplets is accomplished in source beds. In assuming this, the selective process is missing. MEmSCHEIN (1959) believes that the selection occurs during the primary migration. To date, the mechanisms of hydrocarbon migration and accumulation have been to a great extent unknown. But the important investigations of BAKER (1959, 1960) undoubtedly point the way to a better understanding of these processes. Baker considers the problem of petroleum formation by selective accumulation of hydrocarbons from the viewpoint of colloid chemistry. Witherspoon demonstrated that petroleum contains colloidal particles (WnnERSPOON, 1958; ME1NHOLD,1964). BAKER(1959) found that two different kinds of colloidal particles are present; these he named, in accordance with the invesSediment. Geol., 1 (1967) 223-234
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tigations of MGBA1N and JENKINS(1922), ionic and neutral micelles. He emphasized that the hydrocarbons are selectively dissolved in the micelles. "The frequency distribution of hydrocarbons in crude oils reflects variations in the kind and size of the micelles in which the sediment hydrocarbons selectively d i s s o l v e " (BAKER, 1962). His results indicate that hydrocarbons, at first finely disseminated in the sediments, are selectively dissolved and transported out of the source beds in a colloidal solution of formation waters containing natural solubilizers (organic acid soaps). The release of the selectively solubilized hydrocarbons in suitable carrier beds is a further important stage of petroleum formation. This release is regarded as a colloid-chemical process which may be caused by variations of the physical and chemical conditions (change of the salt concentration, change of pressure, temperature, etc.). Probably at first an emulsion is formed, and then oil globules of different size, and finally a segregation of water, oil, and gas (secondary migration) may occur in the carrier beds. At present it is not yet possible to describe in detail all the stages of this process which finally result in the formation of crude oil. But it is probable that the investigations of BAKER (1962) suggest the most successful approach to a better understanding of the mystery of petroleum. In light of the considerations outlined above, the complete process of petroleum formation must involve selective solution, migration, accumulation and release of sediment hydrocarbons in formation waters containing natural solubilizers. "Until the process occurred, petroleum would not exist in the sediments" (BAKER, 1962). The results of Baker's investigations are eminently qualified to influence a re-examination of many hypotheses commonly in use in petroleum geology.
PRIMARY MIGRATION
The transportation of finely disseminated hydrocarbons from the sediment into the carrier beds is called primary migration. Although primary migration plays an important role in hypotheses concerning petroleum formation, this process is rarely examined thoroughly. "When primary migration is mentioned one usually must be content with the words squeezing or flushing" (BAKER, 1962). It is obvious that the motivator of the migration is the compaction of sediments. This process is carefully examined and described by HEDBERG (1936), TERZAGHI (1947), EMERYand RITTENBERG0952), WELLER(1959) and VON ENGELHARDT(1960). Compaction begins with the expulsion of water from the porous fine-grained sediment (which may be sedimented with a porosity of about 80 ~). With increased burdening by younger sediments, porosity decreases rather rapidly. A significant change in the mechanism of compaction when porosity reaches about 35 ~ was recognized by HEDBERG (1936). Until this porosity is reached the sediments are in the "dewatering stage", in which all particles are surrounded by water. In the subSediment. Geol., 1 (1967) 223-234
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sequent stage, the stage of "mechanical deformation", the interstitial water is almost completely squeezed out, and the mineral particles are in direct contact. This stage is reached at a depth of about 500 ft. When migration is considered from the point of view of the mechanism of compacting sediments, it is clear that the only possible stage with sufficient effect is the dewatering stage. The slowly decreasing porosity in the following stages of compaction has less influence on the migrating formation water. Sedimentological investigation about the compaction of sediments show that the squeezing out from the colloidal solutions of hydrocarbons in the formation water must essentially be accomplished during the dewatering stage of compaction. This conclusion leads to the question of the time span needed to originate the sediment hydrocarbons. Opinions on this subject are divergent. Some authors believe that the transformation of organic matter into sediment hydrocarbons takes place during a span of geological periods. Therefore it is assumed that the same source rock may be capable of yielding petroleum at intervals in geological time, depending on the change of pressure and teml:erature in connection with the subsidence of a sedimentary basin (WELTE 1964, 1965). But there exist observations which show that this assumption is probably not always right. SMITH (1954) examined sediments and cores from the Gulf of Mexico, and by means of the 14C-method he estimated the age of the organic matter to be about 30,000 years and the age of the sediment hydrocarbons to be l 1,800 -k 1,400 years. Kidwell and Hunt received similar results in recent sediments of Venezuela (source beds, 13,400 ~ 1,200 years; hydrocarbons, 14,000 years)(SMrrU, 1954; KIDWELLand HUNT, 1958a, b; MEINHOLO, 1962). These investigations show that the transformation of organic matter into hydrocarbons is a process which probably runs its course during the early diagenesis of sediments. This is in accord with the postulation that any migration caused by compacting sediments is effective only in the relatively short spaces of time during which dewatering takes place. The dewatering stage being finished, it is very difficult to extract hydrocarbons from the source beds. These difficulties increase enormously with increasing lithification of the sediments. The assumption held by some, that hydrocarbons migrate in a gaseous phase, is rot plausible (MvINSCHEIN, 1961); and the often-quoted theory of oil migration through joints and fissures is chiefly based on a confusion of primary migration with "remigration" (Gussow, 1955). (Remigration is migration of a pre-existing oil pool, caused by younger tectonic movements.) Many efforts have been made to establish the relationship between crude oil and the assumed source beds (BUCHTAet al., 1963; SXRAKnOV, 1957). But in the light of the above-mentioned sedimentological investigations, it seems highly unlikely that any relationship can be acknowledged (MEzNrfOLD, 1966). By selective solution and migration, the source beds have lost those compounds through which, after release in the carrier beds, crude oil has yet to be formed. The organic matter remainSediment. Geol., I (1967) 223-234
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ing in the source beds is rather comparable to the "pomace'" that stays behind in the press (KIDWELLand HUNT, 1958b). The results of BAKER'S(1959, 1960, 1962) investigations, and recent knowledge of the mechanism of primary migration by compacting sediments, necessarily influence concepts of petroleum exploration. A careful study of the paleogeographic develop; merit, and particularly of the hydrodynamic conditions of the area being explored is of basic importance. The drainage of a basin during periods of subsidence and deposition of sediments undoubtedly deserves more attention than the more commonly emphasized question of potential source rocks.
RESERVOIR TRAPS
The basic task of the petroleum industry is to recognize those locations in which oil in commercial quantities probably will be found. Valuable tools for the solution of this difficult problem are the results of sedimentological research. Customarily, structural and stratigraphic (lithologic) traps are distinguished. But MARTIN (1965) focused attention on a third kind of trap, one which up to now has received relativelylittle consideration. He termed this a "paleogeomorphical trap", because hydrocarbons are trapped in primarily paleogeomorphical phenomena. To date, most exploration has been directed toward a search for crude oil in structural traps. PASSEGA(1960) criticizes the idea that the occurrence of an anticline and a sequence of favorable-looking sediments is generally sufficient to localize an exploration well. But many of the largest oil and gas pools in the Western Hemisphere are trapped by non-structural phenomena. LEVORSEN(1965) states that "we have stumbled into many non-structural oil and gas pools while looking for purely structural traps". It is evident that structural traps which are obvious and not difficult to localize are the first to be tested. But now we know that many of the "purely" structural traps are in reality combined traps. Often it happens that primary entrapment is governed by lithological factors, and that this lithological trap is later transformed by tectonic movements (K6LBL, 1966). The one-sided direction of exploration--the search for purely structural traps-seems to be particularly doubtful in areas where geological units are not autochthonous. A careful analysis of the paleogeographical and sedimentological possibilities for the existence of an oil formation is, in such areas, an absolute necessity. LEVORSEN (1965) emphasizes that "the time seems to have arrived we must start looking directly for combination traps of all kinds involving different proportions of structure, stratigraphic change and fluid flow phenomena". In this start sedimentology will be an absolutely necessary guide. Geomorphological traps include all the phenomena of a buried paleo-relief (buried hills, ancient sandstone-filled valleys, fossil reefs, etc.). Many landscapes exposed for millions of years may be peneplains, but often they still have sufficient relief Sediment. Geol., 1 (1967) 223-234
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for the accumulation of hydrocarbons. The hydrocarbons may be trapped below or above the paleogeomorphical surface. An analysis of this type of trap must take into account all the geomorphological features, which includes both form and process. "The form creates the trap and tlze process shapes the form" (MARTIN, 1965). Paleogeomorphological traps are often to be expected in the basement of a subsided basin. A morphological map of the basement is then the first requisite for an exploration concept (KAvoUNEK et al., 1963). All features of the buried relief have to be carefully examinated and compared, with the various stages of sedimentation and. the drain age of the subsided basin. When we are studying paleogeomorphological traps (for example, buried hills) there often arises the question of where the oil comes from. An investigation of the origin and diager~esis of the subsurface waters which usually accompany crude oil may be helpful in recognizing the direction of migration; it is also an aid in deciding the question of whether the water comes by way of a vertical migration, from deeper parts of the basement, or by way of a lateral migration, from the sedimentary sequence which had covered and mantled the pre-existing relief (KOLBL, 1967).
FINAL REMARKS
In this brief review an attempt has been made to demonstrate that sedimentology is today, and shall be also in the future, of great importance to the development of the petroleum industry, chiefly by inflvencing every concept of petroleum exploration. But it is necessary to keep in mind that the results of sedimentological research still involve many hypothetical ideas which have yet to be confirmed by further sedimentological work. To avoid that an ir~fl.exibility of ideas comes into use, it is absolutely necessary to compare continually the results of sedimentological investigations with the information we receive from exploration and exploitation wells. This may be done in several ways. The Rocky Mountain Section of the Americain Association of Petroleum Geologists dedicated a full meeting (September 1965) to the single theme: "The truth about typical Rocky Mountains oil traps". Of additional special value for this purpose is a comparison of the results received by sedimentological, geophysical, etc., methods with those received by exploitation activity in a well-known area, as is done, for example, in the Gifthorn Trough in northwest Germany (HECr~T, 1959; PH~LIJ'~'et al., 1963a, b). "The careful examination of todays producing areas will point the way to tomorrows success (LEvORSEN, 1965).
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