Controls on Oil Shale Deposition in the Green River Formation

The interaction of organic matter and clay mineral surfaces is a well-documented preservational mechanism shown to lead to hydrocarbon enrichment in marine source rocks. However, the role that clay minerals play in the preservation of organic carbon has not been thoroughly investigated in lacustrine settings, despite the fact these systems contain significant reserves of hydrocarbons. Here we investigate the mechanisms controlling organic enrichment in oil shales of the Eocene Green River Formation. We focused on an interval of systematically alternating oil shale and argillaceous carbonates, that form meter-scale cycles in marginal lacustrine deposits of the Parachute Creek Member in the Uinta Basin, Utah, to identify the dominant mechanism of enrichment. We combine high-resolution, quantitative clay mineralogical analyses with stable isotope measurements of carbon and oxygen from carbonate minerals to determine the relative importance of environmental and/or mineralogical influence on organic carbon depositional patterns. These results show systematic increases in the trioctahedral smectite content of the clay mineral fraction of the most organically enriched oil shale intervals, whereas organic lean intervals contain a lesser proportion of trioctahedral smectite relative to discreet illite. Since mineral surface area scales with organic matter preservation in many marine sediments and soils, parallel trends in smectite and organic matter content within the Green River Formation support the hypothesis that mineral surfaces also played an important role in organic matter preservation within these oil shales.