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An Integrated Sedimentological and Geochemical Analysis of a Lacustrine Sedimentary Cycle in the Triassic Cow Branch Formation of the Dan River Basin

Abstract

The North American Atlantic passive margin contains a number of early Mesozoic lacustrine basins that comprise the Newark Supergroup. These lacustrine basins record sedimentary cycles attributed to Milankovitch forcing and contain organic-rich facies that make them potential targets for energy production. The Virginia Solite Quarry in the Dan River Basin (DRB) exposes several complete cycles of laminated black shale and interfingering grey shale and siltstone, interpreted to record lake level rise and fall. The presence of organic-rich facies is a shared characteristic of the rift basins and many have been evaluated as hydrocarbon reservoirs containing both wet and dry gas with a mean of more than 3,800 BCF natural gas and 135 MMB natural gas liquids. The preservation of organic carbon and subsequently the reservoir quality of the DRB and other analogous rift basins depends heavily on depositional conditions, especially redox chemistry of the water column and pore fluids. Geochemical evidence, including total organic carbon (TOC), pyrite sulfur (Spy) and iron speciation data point to the presence of anoxic, ferruginous waters. Towards the center of the cycle, TOC content increases markedly, with values up to 3.52 wt% coinciding with the presence of darker, more laminated sedimentary lithofacies. The lightest d13Corg isotope values coincide with higher TOC values, suggesting a two-component mixing system dependent on lake level and sedimentary input or that the majority of preserved organic carbon is a result of primary production within the lake. At the interval of the highest TOC content, a spike in overall sulfur content occurs; likely the result of slowed sedimentation at lake transgression and high-stand. The consistently low pyrite sulfur content (generally less than 0.2 wt%), coupled with high Corg/Spy ratios suggest however, that deep waters never became euxinic (anoxic, H2S-rich). The iron speciation data indicate that an active iron cycle was present and that the studied portion of the Cow Branch Formation was deposited under intermittent to persistent anoxic conditions, preserving the signal throughout the cycle. The research presented here holds implications for production of these reservoirs, both for immediate and practical use, by helping to define better target intervals, as well as future use, by increasing understanding of organic carbon preservation in other East coast rift basins and analogous lacustrine environments.