Geochemical characteristics and AUTHORS Cari L. Johnson Department of Geological correlation of oil and nonmarine and Environmental Sciences, Building 320, Stanford University, Stanford, California, 94305-2115; current address: Department of Geology and Geophysics, source rocks from Mongolia University of Utah, 135 South 1460 East, Browning Building, Salt Lake City, Utah, 84112-0011; Cari L. Johnson, Todd J. Greene, David A. Zinniker, [email protected] J. Michael Moldowan, Marc S. Hendrix, and Cari Johnson is an assistant professor at the Alan R. Carroll University of Utah. She received geology degrees from Carleton College (B.A., 1996) and Stanford University (Ph.D., 2002), where she also completed postdoctoral research on sequence stratigraphy of ABSTRACT the San Joaquin basin. Her dissertation focused on the sedimentary record of Late Mezosoic extension New bulk and molecular organic geochemical analyses of source rock in the China–Mongolia border region. She continues and oil samples from Mongolia indicate the presence of lacustrine- to conduct research in basin analysis, sedimentation sourced petroleum systems in this frontier region. Samples of po- and tectonics, and petroleum geology in east-central tential source rocks include carbonate, coal, and lacustrine-mudstone Asia and western North America. lithologies that range from Paleozoic to Mesozoic in age, and rep- Todd J. Greene Department of Geological and resent a variety of tectonic settings and depositional environments. Environmental Sciences, Building 320, Stanford Rock-Eval and total organic carbon data from these samples reflect University, Stanford, California, 94305-2115; current generally high-quality source rocks, including both oil- and gas-prone address: Anadarko Petroleum Corporation, 1201 Lake Robbins Drive, The Woodlands, Texas, 77380 kerogen types, mainly in the early stages of generation. Bulk geo- chemical and biomarker data indicate that Lower Cretaceous lacus- Todd Greene attained a B.S. degree in earth sciences from the University of California at Santa trine mudstone found in core from the Zuunbayan field is the most Cruz (1994) and a Ph.D. in geological sciences at likely source facies for the East Gobi basin of southeastern Mon- Stanford University (2000). His dissertation focused golia. Oil and selected source rock samples from the Zuunbayan on tectonics, sedimentology, organic geochemistry, and Tsagan Els fields (both in the East Gobi basin) demonstrate geo- and petroleum systems of the Turpan-Hami basin of chemical characteristics consistent with nonmarine source environ- northwestern China. He is currently employed by Anadarko Petroleum in Houston, Texas, where he is ments and indicate strong evidence for algal input into fresh- to part of a regional studies team investigating basins brackish-water source facies, including elevated concentrations of and play types in the greater Rocky Mountains. unusual hexacyclic and heptacyclic polyprenoids. Despite similar- David A. Zinniker ities between Zuunbayan and Tsagan Els oil samples, biomarker Department of Geological and Environmental Sciences, Building 320, Stanford parameters suggest higher algal input in facies sourcing Zuunbayan University, Stanford, California, 94305-2115 oil compared to Tsagan Els oil. Tsagan Els oil samples are also gen- David A. Zinniker is a Ph.D. candidate in the Depart- erated by distinctly more mature source rocks than oil from the ment of Geological and Environmental Sciences at Zuunbayan field, based on sterane and hopane isomerization ratios. Stanford University. His research focuses on molec- ular fossils of plants and algae and their bearing upon ecology, evolution, depositional systems, and INTRODUCTION petroleum geology. His future projects include using molecular and macromolecular markers to study current ecological processes and events deep in Central and eastern China contain several petroleum-bearing, late geologic time. Mesozoic rift basins (e.g., Songliao and Erlian; Figure 1, inset). Var- ious studies address these Asian lacustrine systems from regional J. Michael Moldowan Department of Geolog- stratigraphic and source-geochemical perspectives (Yang, 1985; Fu ical and Environmental Sciences, Building 320, Stanford University, Stanford, California, 94305-2115 J. Michael Moldowan attained a B.S. degree in chemistry from Wayne State University, 1968, and a Ph.D. in chemistry from the University of Michi- Copyright #2003. The American Association of Petroleum Geologists. All rights reserved. gan in 1972. Following a postdoctoral fellowship in Manuscript received January 18, 2002; provisional acceptance August 20, 2002; revised manuscript marine natural products with Professor Carl Djerassi received November 25, 2002; final acceptance December 17, 2002. AAPG Bulletin, v. 87, no. 5 (May 2003), pp. 817–846 817 at Stanford University, he joined Chevron’s Biomarker and Sheng, 1992; Gao et al., 1997; Lin et al., 2001). Comparatively Group in 1974. Moldowan joined the Department of little is known about related Mesozoic rift basins in Mongolia (spe- Geological and Environmental Sciences of Stanford cifically, the Tamtsag and East Gobi basins; Figure 1), where pe- University as professor (research) in 1993. troleum systems remain poorly understood and underexplored Marc S. Hendrix Department of Geology, (Penttila, 1994; Sladen and Traynor, 2000). University of Montana, Missoula, Montana, 59812 Penttila (1994) estimated as much as 3–6 billion BOE recoverable Marc S. Hendrix received geology degrees from hydrocarbon resources in Mongolia, although reserve calculations are Wittenberg University (B.A., 1985), the University difficult to constrain in this poorly known petroleum province. Petro- of Wisconsin, Madison (M.S., 1987), and Stanford leum production in Mongolia is currently limited to the East Gobi University (Ph.D., 1992). He completed postdoctoral research at Stanford in 1994 and since has been a and Tamtsag basins (Figure 1), the latter hosting discoveries in 2001 professor of geology at the University of Montana, totaling an estimated 1.5 billion bbl of oil in place (Soco Inter- Missoula. His research interests include sedimentary national, 2001). Although historic discoveries in Mongolia generally basins and paleoclimate studies, particularly in have been modest, the potential for larger hydrocarbon accumula- western North America and central Asia. tions exists by analogy to similar basins in China. These include the Alan R. Carroll Department of Geology and largest producing field in China, Daqing (production 1.0 million Geophysics, University of Wisconsin, Madison, 1215 bbl/day; United States Department of Energy, 2001), in addition to W. Dayton St., Madison, Wisconsin, 53706 smaller accumulations in Erlian basin (e.g., Aershan field with esti- Alan Carroll conducts research on large lake basins mated 100 million bbl reserves; Sladen and Traynor, 2000). in Asia and the western United States, focusing on The East Gobi basin shares several characteristics with late their tectonic setting, sequence stratigraphy, and Mesozoic rift basins in China. These similarities include the age and petroleum potential. He worked for three years as an exploration geologist for Sohio, and five years for depositional style of nonmarine basin fill, and northeast-southwest Exxon Production Research. He is currently an asso- orientation of rift structures (Liu, 1986; Watson et al., 1987). Rifting ciate professor at the University of Wisconsin. began during the Late Jurassic and continued through the Early Cre- taceous, with widespread deposition of fluvial-lacustrine strata and periodic bimodal volcanic activity (Traynor and Sladen, 1995; Johnson ACKNOWLEDGEMENTS et al., 2001). Middle Cretaceous contraction and strike-slip faulting We thank D. Badamgarav, G. Badarch, R. Barsbold, inverted the East Gobi basin along its margin, forming a regional D. Janchiv, and our other colleagues at the Mongolian angular unconformity overlapped by an Upper Cretaceous postrift Academy of Sciences, Institute of Geology and Min- sequence (Figure 2) (Graham et al., 2001). The total original thick- eral Resources, the Petroleum Authority of Mongolia, ness of synrift strata is not known because of erosion during the and the Mongolian Paleontological Institute for their scientific and logistic support. Funding for this study basin-inversion event, but more than 2.5 km of sedimentary and was provided by Roc Oil, the Stanford Graduate volcanic graben fill is still preserved in the subsurface of the East Fellowship, the International Research and Exchange Gobi, based on proprietary seismic and well-log data (Johnson, 2002) Board, the Stanford-Mongolia Industrial Affiliates and correlative outcrop studies (Graham et al., 2001). program, and by National Science Foundation grants Synrift strata form source, reservoir, and seal units in south- EAR-9708207 and EAR-961455 to S. Graham and M. Hendrix, respectively. S. Graham was a principal eastern Mongolia, mainly in inversion-related structural traps (Fig- advisor on this and related projects in Mongolia and ure 2). In the East Gobi basin, the main source rocks appear to be offered much insight throughout this study. Col- lacustrine shales of the Lower Cretaceous Tsagantsav Formation leagues at various labs assisted with sample analysis, (synrift sequence 3 of Graham et al., 2001), based on previous geo- including K. K. Bissada (Houston Advanced Research chemical studies of source rocks from southeastern Mongolia (Yama- Center), H. Hada