1. Department of Geology and Geological Engineering Van Tuyl Lecture Series- Spring 2016 4:00-5:00 p.m. in Berthoud Hall Room 241 Thursday, February 11, 2016 J. Rick Sarg S. Huang, M. Swierenga, K. Logan, C. Symcox, K. Tanavsuu-Milkeviciene, and J. Boak Colorado School of Mines, V. P. Wright, Cardiff University and BG Group “Lacustrine Carbonates – Beaches, Microbes, and Chemical Precipitates – A Tale of Two Lakes, Green River and South Atlantic Pre-Salt” Abstract: Carbonate deposits in ancient lake systems are products of the biotic and chemical environments in these lakes and can comprise significant hydrocarbon reservoirs within organic-rich lacustrine systems. The Green River (Eocene) carbonates are excellent process analogues for the pre-Salt reservoirs (Lower Cretaceous) in the South Atlantic region. Despite being deposited in very different tectonic regimes – post-orogenic foreland (Green River) and syn-rift (South Atlantic), the lake depositional processes and chemical controls on lake carbonates are similar. The Green River Formation of Colorado, Utah, and Wyoming provides insight into the South Atlantic lake basins, the most significant new hydrocarbon province discovered in the last decade. Both the Green River and the South Atlantic lakes are organic-rich, alkaline lakes with pH values thought to be as high as 9-10. Co-variance of C and O stable isotopes in the Green River and lack of spring deposits (i.e., tufa and travertine) indicates a closed lake system dominated by surface inflow. The Green River lake is rich in Na, Ca, Mg, and HCO 3 resulting in precipitation of nahcolite, trona, and calcite and early replacement ferroan dolomite. The South Atlantic rift lakes are thought to be closed systems as well, and are rich in Ca, Mg and Si resulting in formation of stevensite, calcite spherulites, and replacement dolomite. The formation of dolomite in both lake systems contributes to enhanced reservoir quality. Carbonate sedimentary environments common to both lake systems include beach and shallow littoral bioclastic coquinas and ooid grainstones; and littoral to sublittoral microbial deposits. The Green River shoreline deposits range from dolomitic intraclastic packstone to oolitic and ostracod grainstones deposited in meter-scale shallowing upward cycles. The South Atlantic shorelines are characterized by m to 10’s of m thick, molluscan lime rudstones and grainstones also deposited in shallowing upward cycles. Interparticle porosity dominates in both lake shorelines. Microbial deposits are common in both systems and comprise stromatolitic and thrombolitic biostromes and bioherms. The Green River microbial-rich units are characterized by 1-3 m thick, deepening upward cycles that commonly begin with shallow littoral intraclastic rudstone/grainstone and/or oolitic wackestones to grainstones. These are overlain by dendrolitic and agglutinated stromatolites and thrombolites. Cycles are capped by fine-grained laminated stromatolites and transition upward into oil shale. These shallow littoral microbial deposits transition offshore into lower sublittoral to upper profundal fine- grained, laminated stromatolites. Intra-particle, interparticle, fenestral, and vuggy pore types are common in microbial and associated deposits resulting in excellent reservoir quality. In contrast to the Green River, late syn-rift South Atlantic carbonates are dominated by meters to 10’s of m thick dolomitic spherulite grainstones. Spherulitic deposits have high porosity and permeability and are excellent reservoirs. Porosity is pseudo-fenestral and is interpreted to result from stevensite dissolution.