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Organic Matter Preservation as Clay-Organic Nanocomposites in the Woodford Shale and its First Order Control on an Unconventional Play


The burial of marine sourced organic carbon (OC) in continental margin sediments is most commonly linked to oceanographic regulation of bottom-water oxygenation (anoxia) and/or biological productivity. Here we show that the dominant mode of organic carbon in the Devonian Woodford Shale occurs in nanometer intercalations of organic matter and illite that we term organo-mineral nanocomposites. High resolution transmission electron imaging provides direct evidence of this nano scale relation and indicates that organic carbon is constrained to clay-rich laminae. While discrete > micron scale organic particles such as Tasmanites are present in some lamina, a strong relation (r2= 0.94) between total organic carbon (TOC) and mineral surface area (MSA) over a range of 15% TOC indicate that the dominant association of organic carbon is with mineral surfaces and not as discrete pelagic grains. Where extensive bioturbation has occurred a second order modification of this relationship is expressed as a shift to lower TOC values and homogenization of MSA sample-to-sample variability. This situation reflects greater oxygen availability in sediments leading to oxidative burn down of OM or, more directly, stripping of organic carbon from minerals in animal guts. The TOC/MSA relationship extends across a range of burial depths and thermal maturities into the oil window and persists through illitization, which results in a loss of mineral surface and a greater TOC:MSA ratio. The Woodford Shale is a classic black shale commonly interpreted to result from an oceanographic anomaly of higher marine productivity and anoxia. These results point to the importance of high surface area detrital clay minerals as the key ingredient to organic carbon preservation. The flux of clay minerals to continental shelves and intracratonic basins is a function of continental processes and climate enhancing 2:1 clay mineral production. Preservation as organo mineral nanocomposites thus offer different paleoevironmental implications for this type of organic enrichment and has significant implications for rock properties governing unconventional hydrocarbon potential in economically important source rocks like the Woodford Shale.