ALBERT

Description: Zechstein 2 (Z2) carbonate microbialites flourished under arid paleoclimatic conditions in the Late Permian. Microbial carbonates from the Roker Formation outcrop in northeast England, with its subsurface equivalent being the Main Dolomite from northwest–central Poland. The Z2 carbonate deposits developed in supratidal through deep subtidal zones and consist of various stromatolites and thrombolites. Planar stromatolites and thrombolites characterize intertidal and supratidal facies, and biohermal stromatolites with oolitic grainstone and crinkled stromatolites typify shallow subtidal facies. The Z2 subtidal and/or intertidal microbialites with oolites form complexes more than 10 m (33 ft) thick and are important reservoir facies for hydrocarbons. Subtidal (slope) and intertidal (lagoonal) microbial mudstone and wackestone have poor reservoir properties but contain total organic carbon as much as 2 wt. % and are considered as potential source rocks. The thermal maturity assessed from C27 17α-trisnorhopane (Tm) and C27 18α-trisnorhopane (Ts) as the Ts/(Ts + Tm) ratio, C30 moretane/hopane ratio, sterane ratio expressed as 20S/(20S + 20R), and ββ/(ββ + αα) ratio shows to indicates a mature character of organic matter with respect to oil generation.

Description: 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 kerogen in the active rift shales, pure type I kerogen 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. 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 & 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.