Lake-type controls on AUTHORS Alan R. Carroll ϳ Department of petroleum source rock potential Geological and Geophysical Sciences, University of Wisconsin, 1215 W. Dayton St., in nonmarine basins Madison, Wisconsin, 53706; [email protected] Alan R. Carroll and Kevin M. Bohacs Alan R. Carroll has been an assistant professor at the University of Wisconsin, Madison, since 1996, specializing in sedimentary basins in western China and the western United States. ABSTRACT He received geology degrees from Carleton College (B.A. degree, 1980), the University of Based on numerous empirical observations of lacustrine basin strata, Michigan, Ann Arbor (M.Sc. degree, 1983), we propose a three-fold classification of lacustrine facies associa- and Stanford University (Ph.D., 1991). He tions that accounts for the most important features of lacustrine worked as an exploration geologist for Sohio petroleum source rocks and provides a predictive framework for (1983–1986) and a research geologist for exploration in nonmarine basins where lacustrine facies are incom- Exxon Production Research Company (1991– pletely delineated. 1995). He is an associate editor of the AAPG (1) The fluvial-lacustrine facies association is characterized by Bulletin. freshwater lacustrine mudstones interbedded with fluvial-deltaic Kevin M. Bohacs ϳ ExxonMobil Upstream deposits, commonly including coal. Shoreline progradation domi- Research Co., 2189 Buffalo Speedway, nates basin fill, resulting in the stacking of indistinctly expressed Houston, Texas, 77252; cycles up to 10 m thick. In map view, the deposits may be regionally [email protected] widespread but laterally discontinuous and contain strong facies Kevin M. Bohacs is a sedimentologist and contrasts. Transported terrestrial organic matter contributes to stratigrapher with the Petroleum Geochemistry mixed type I–III kerogens that generate waxy oil (type I kerogen is section of ExxonMobil Upstream Research hydrogen rich and oil prone; type III kerogen is hydrogen poor and Company (URC) in Houston, Texas. He mainly gas prone). The Luman Tongue of the Green River For- received his B.Sc. (honors) degree in geology mation (Wyoming) and the Honyanchi Formation (Junggar basin, from the University of Connecticut in 1976 China) provide examples of this facies association, which is also and his Sc.D. degree in experimental present in the Songliao basin of northeastern China, the Central sedimentology from M.I.T. in 1981. At URC, he Sumatra basin, and the Cretaceous Doba/Doseo basins in west- leads the application of sequence stratigraphy central Africa. and sedimentology to organic-rich rocks from (2) The fluctuating profundal facies association represents a deep sea to swamps and lakes in basins combination of progradational and aggradational basin fill and in- around the world. As a research associate, his cludes some of the world’s richest source rocks. Deposits are re- primary focus is to keep the geo- in geochemistry, integrating field work, gionally extensive in map view, having relatively homogenous subsurface investigation, and laboratory source facies containing oil-prone, type I kerogen. Examples include analyses. He has written numerous articles on the Laney Member of the Green River Formation (Wyoming), the the stratigraphy and sedimentology of Lucaogou Formation (Junggar basin, China), the Bucomazi For- mudrocks, hydrocarbon source rocks, and mation (offshore west Africa), and the Lagoa Feia Formation (Cam- lake systems. He was co-recipient of the AAPG pos basin, Brazil). Jules Braunstein Memorial Award for best (3) The evaporative facies association represents dominantly poster session paper in 1995 for work on coal aggradational fill related to desiccation cycles in saline to hyper- sequence stratigraphy and the AAPG saline lakes and may include evaporite and eolianite deposits. Sub- International Paper Award in 1998 and was littoral organic-rich mudstone facies are relatively thin but may be an AAPG Distinguished Lecturer for 1999– 2000. Copyright ᭧2001. The American Association of Petroleum Geologists. All rights reserved. Manuscript received December 15, 1998; revised manuscript received June 28, 2000; final acceptance August 31, 2000. AAPG Bulletin, v. 85, no. 6 (June 2001), pp. 1033–1053 1033 quite rich and widespread. The highest organic enrichment co- ACKNOWLEDGEMENTS incides with the deepest lake stages. Low input of land plant organic matter results in minimal lateral contrasts in organic content. In This article benefited from discussions with some cases a distinctive type I-S (sulfur-rich) kerogen may generate many individuals, including S. C. Brassell, Y. Y. oil at thermal maturities as low as 0.45% vitrinite reflectance equiv- Chen, R. Cunningham, D. J. Curry, G. Genik, alent. Examples include the Wilkins Peak Member of the Green K. S. Glaser, G. J. Grabowski Jr., G. B. Hiesh- ima, G. H. Isaksen, B. J. Katz, M. R. Mello, K. River Formation (Wyoming), the Jingjingzigou Formation (Junggar Miskell-Gerhardt, J. E. Neal, P. Olsen, D. J. basin, China), the Jianghan and Qaidam basins (China), and the Reynolds, C. Scholz, and K. O. Stanley. G. J. Blanca Lila Formation (Argentina). Grabowski and H. B. Lo collected some of the data used in this article. M. Wartes provided useful comments on an early draft of the INTRODUCTION manuscript. Field studies in the Junggar basin in 1987, 1988, and 1992 were funded by the The deposits of nonmarine sedimentary basins account for a grow- Stanford-China Industrial Affiliates, a group of ing segment of current petroleum exploration and exploitation op- companies that included AGIP, Amoco, An- portunities, especially in areas of rapid market growth such as schutz, BHP Petroleum, Chevron, Conoco, Elf- China, southeast Asia, and western Africa. The specific techniques Aquitaine, Enterprise Oil, Exxon, Mobil, Occi- needed for locating, assessing, and developing hydrocarbon reserves dental, Pecten, Phillips, Sun, Texaco, Transworld Energy International, and Unocal. within lacustrine basins remain relatively undeveloped compared Conoco provided additional support for this to those for marine systems, and they constitute a significant source article. The Donors of The Petroleum Research of financial risk (cf. Sladen, 1994). This uncertainty stems not from Fund, administered by the American Chemical a lack of data on lacustrine systems, but instead from their great Society, also provided support for this re- sedimentologic complexity, as evident from numerous studies of search. We thank Exxon Production Research modern and ancient lakes. Furthermore, most lacustrine source rock Co. for permission to publish this article. J. A. models have focused on inferred climatic controls (e.g., Eugster and Curiale, B. J. Katz, B. Wiggins, and an anony- Kelts, 1983; Talbot, 1988), although paleoclimate modeling has mous reviewer all provided very helpful had only limited success in predicting the actual occurrence of reviews. organic-rich lacustrine facies (e.g., Barron, 1990). Fortunately, some of the complexity evident in modern lakes resolves itself in ancient lacustrine deposits, resulting in the ex- pression of three commonly recurring motifs in the lithology and stratigraphy of lacustrine deposits. For example, Bradley (1925) was able to generalize a trend within the heterogeneous Eocene Green River Formation (Utah, Colorado, and Wyoming) from “relatively shallow fresh-water lakes,” to “depositional basins that alternately flooded and evaporated either partially or completely,” and finally to a period when “the lakes became playa-like” and evaporites pre- cipitated. A similar division, based on geography rather than on geologic time, was proposed over 60 years later by Olsen (1990) for the widespread lacustrine deposits of the Triassic–Jurassic New- ark Supergroup (eastern United States). Olsen idealized lacustrine facies associations as either Richmond-type, Newark-type, or Fundy-type and proposed that these associations reflect differences in climatically controlled basin hydrology. Mello et al. (1988) de- veloped similar concepts involving organic facies, subdividing Bra- zilian source rocks into freshwater and brackish-saline lacustrine facies based on relative concentrations of biological marker com- pounds that are sensitive to lake salinity and organic matter input. Others have characterized biomarker distributions indicative of hypersaline lacustrine source rocks (e.g., Jiang and Fowler, 1986). Carroll and Bohacs (1995, 1999) proposed a classification of ancient 1034 Lacustrine Petroleum Source Rocks lake basins into three end-member types that corre- LACUSTRINE SEDIMENTARY FACIES spond to commonly recurring lacustrine facies associ- ASSOCIATIONS ations. The terms “overfilled,” “balanced-fill,” and “un- derfilled” lake basins refer to the balance between Lacustrine deposits may be characterized according to tectonically controlled potential accommodation and (1) their sedimentary facies, fauna, and flora, (2) their climatically controlled water plus sediment supply. A internal stratigraphic relations (such as parasequence unique advantage of this model is that it permits broad stacking), and (3) the character of associated deposits. characteristics of hydrocarbon source rock richness and In particular, evidence for open vs. closed basin hy- gas vs. oil generation to be predicted from limited geo- drology and the presence and nature of depositional logic observations of nonsource sedimentary facies cyclicity provide the fundamental bases for categoriz- (typically encountered