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Abstract

AAPG Bulletin, V. 88, No. 8 (August 2004), P. 1163-1184.

Copyright copy2004. The American Association of Petroleum Geologists. All rights reserved.

The character and origin of lacustrine source rocks in the Lower Cretaceous synrift section, Congo Basin, west Africa

Nicholas B. Harris,1 Katherine H. Freeman,2 Richard D. Pancost,3 Timothy S. White,4 Gareth D. Mitchell5

1Department of Geosciences, 503 Deike Building, Pennsylvania State University, University Park, Pennsylvania; [email protected]
2Department of Geosciences, 503 Deike Building, Pennsylvania State University, University Park, Pennsylvania; [email protected]
3Organic Geochemistry Unit, Biogeochemistry Research Centre, School of Chemistry, University of Bristol, Cantock's Close, Bristol, United Kingdom; [email protected]
4Environmental Institute, College of Earth and Mineral Sciences, Pennsylvania State University, University Park, Pennsylvania; [email protected]
5Coal and Organic Petrology Laboratory, 105 Academic Projects Building, Pennsylvania State University, University Park, Pennsylvania; [email protected]

AUTHORS

Nick Harris has been a senior scientist in the Department of Geosciences at Pennsylvania State University since 1994. He previously worked as a research and exploration geologist for Conoco. He received his B.A. degree from Amherst College and his M.S. degree and his Ph.D. from Stanford University. His research focuses on source rocks, diagenesis, and the geology of the west African margin.

Katherine H. Freeman is a professor in the Department of Geosciences at Pennsylvania State University, where she has taught since 1991. She received her B.A. degree from Wellesley College and her M.S. degree and her Ph.D. from Indiana University. Her research interests include molecular and isotopic indicators of biogeochemical processes and analytical methods in organic and isotope geochemistry.

Richard D. Pancost is a lecturer in the Biogeochemistry Research Centre (Organic Geochemistry Unit) of the School of Chemistry, University of Bristol. He received his B.S. degree (1992) from Case Western Reserve University and his Ph.D. (1998) from Pennsylvania State University. His research interests include studies of organic matter preservation and the use of biomarkers in paleoclimate studies and geomicrobiology.

Tim White is a research geologist with the U.S. Geological Survey and an adjunct research associate in the Earth and Mineral Sciences Environment Institute at Pennsylvania State University. He received his B.A. degree from Washington and Lee University and his M.S. degree and his Ph.D. from Pennsylvania State University. His research interests include paleoclimatology, paleopedology, chemostratigraphy, and organic petrology.

Gareth D. Mitchell is a research associate and director of the Coal amp Organic Petrology Laboratories, the Energy Institute, at Pennsylvania State University. He holds two degrees in geology, a B.S. degree (1974) from the Southern Illinois University and an M.S. degree (1977) from Pennsylvania State University. Prior to working at Pennsylvania State University in 1986, he was employed as a research engineer by Bethlehem Steel Corporation.

ACKNOWLEDGMENTS

We thank members of the Penn State Congo Basin Consortium, including Anadarko, ARCO, Chevron, Conoco, Japan National Oil Corporation, Phillips, Mobil, Norsk Hydro, Occidental, Statoil, and Texaco, who have supported this research over the past several years. Rock-Eval pyrolysis, %TOC, and GC-MS analyses were made by Ted Sandomenico; Nikolai Pedentchouk analyzed samples for pristane and phytane. Discussions with M. A. Arthur, B. J. Katz, K. Kelts, E. Kirby, and M. Talbot have, wittingly or not, helped to shape our ideas. We thank reviewers Edward Purdy and Ralph Burwood for their extensive and constructive comments. Reviews by K. E. Peters and two anonymous reviewers of an earlier draft significantly improved this paper.

ABSTRACT

Lower Cretaceous synrift lacustrine shales from the Congo Basin, west Africa, have been analyzed with sedimentological and geochemical techniques to characterize source rock quality and identify triggers for deposition of intervals richest in organic carbon. The sequence includes a lower active rift section, deposited during active faulting and subsidence, overlain by an upper late rift section, deposited during reduced faulting and subsidence.

Total organic carbon (TOC) averages 2–3 wt.% throughout the active rift siliciclastic shale section, 6% in marls in the lower part of the late rift section, and 1–2% in deltaic shales in the upper part of the late rift section. Organic matter consists of mixed types I and III Previous HitkerogenNext Hit in the active rift shales, pure type I Previous HitkerogenTop in the late rift marls, and a type I and III mixture in the late rift deltaic shales.

Redox proxies indicate that the deep lake was relatively reducing throughout deposition of the active rift and lower late rift sections. Therefore, enhanced anoxia did not trigger deposition of the richest source rocks. Decreased sedimentation rates in the late rift do not account for the full increase in TOC nor the shift on organic matter type. The richest source rocks are associated with high rates of organic productivity and chemical sedimentation, indicating that flux of dissolved components to the rift lake, including nutrients for algae growth, was critical. We propose that reduced topography associated with the late rift was necessary for efficient cycling of plant-derived carbon into soil carbonate and ultimately the rift lake, and for enhancing chemical weathering and nutrient flux.

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