Lacustrine Reconstruction of Green River Oil Shale Cyclicity: Characterization and Understanding Variations In Oil-Yields

In the spectrum of lacustrine facies in the Green River Formation, Wyoming, the Tipton Scheggs and Rife Beds represent freshwater, overfilled and brackish/saline, balanced-fill lake systems, respectively. In the lithologically uniform oil shales of the Tipton Member, a well-defined, continuous record of 6-10' cyclicity is identified from log signatures, Fischer assay oil-yields, and geochemical patterns. These cycles consist of an organic-rich base (high oil-yields) with a reduced but variable organic content in the remainder of the cycle. The cycle base contains an algal dominated hydrogen-rich kerogen that is sometimes depleted in 13C. Above the cycle base, small-moderate molecular differences in kerogen and extractable organic matter reflect a greater proportion of bacterial input and subtle increase in degradation. The cyclicity records a repetitive, two-stage perennial lake. During deposition of the cycle base, an increase in net moisture provided an expanded, nutrient-rich lake with higher productivity and elevated stratification. The second lake stage, which is related to lower net moisture, consisted of reduced lake levels and stratification, and resulted in relatively lower oil-yields. Time-series analysis of Tipton oil-yields defines the full suite of Milankovitch cyclicity. This well documented oil shale cyclicity is a product of orbital-driven changes in insolation rather than random deposition influenced by tectonic or other paleo-climatic variations. The sensitive response of the Tipton phase of paleo-lake Gosiute to environmental change provides geochemical signatures that help reconstruct changing limnology and identify lacustrine processes involved in the origin of these oil shales. The repetitive and predictable changes in these Green River lake systems provide a key to explaining variations in oil-yields, predicting oil shale properties and understanding resources.