Organic carbon distribution in submarine depositional systems of the Lewis Shale, greater Green River Basin, Wyoming

Understanding how organic matter is distributed in submarine environments is crucial for quantification of the global carbon cycle as well as the prediction of conventional source rock and unconventional reservoir presence. Turbidity currents efficiently deliver organic (and inorganic) carbon to the deep ocean, and the resultant distribution of carbon is affected by autogenic depositional processes and allogenic boundary conditions. The Upper Cretaceous Lewis Shale of the Green River Basin, Wyoming, represents a well-constrained shelf, slope, and basin-floor depositional system that is ideal for investigating the distribution of carbon in marine environments. This study evaluates pyrolysis data from 37 cored wells in the U.S. Geological Survey Core Research Center, analyzing total organic carbon (TOC), hydrogen index (HI), oxygen index (OI), and kerogen type. By characterizing the spatial and temporal distribution of organic matter, this study explores the controls on organic carbon transport and preservation in submarine depositional environments over geologic timescales.

The progradational evolution of the Lewis Shale results in coastal-plain, shelf, slope and basin-plain environments arranged into clinothems. TOC and kerogen type varies with these environments, with higher TOC and Type III and mixed Type II/III in the slope-to-basin-floor system, and lower TOC and Type IV in the northern coastal plain, reflecting oxidized and reworked organic matter. The spatial trends in TOC across the basin are complicated by the mixing of a northern and southern source area that are active at different times. Clinothem boundaries are marked by flooding surfaces with high TOC values with Type II/III kerogen, indicating continued terrestrial input during relative highstands. Ongoing work is focused on quantifying the distribution of organic carbon in sub-environments (e.g., channel, levee, lobe) to better delineate the controls on organic-matter distribution in the Lewis Shale and other submarine depositional systems around the world.