Climate, chemistry, and microbial influence on mineralogy and diagenesis in an ancient lake, Green River Formation (Eocene), Utah and Colorado

The Piceance and Uinta lake basins formed between ca 54–48 Ma years ago, in early/middle Eocene, and contain the largest known oil shale resources in the world. Integrated analysis of world-class outcrops, core, logs, seismic, and stable isotopes has delineated eccentricity-driven composite sequences forming cycles of lake expansion and contraction; longer-term, climate driven changes in lake chemistry and the resulting mineral composition. Analysis of this lake system has enhanced prediction of reservoirs, organic richness, and minerals that determine rock physics.

The Green River Formation represents mixed siliciclastic–carbonate–evaporite and organic-rich lake deposits deposited during the Eocene Climate Optimum (ECO). Depositional cycles occur under longer-term changing lake chemistry from initial fresh conditions (Stage 1) to highly fluctuating alkaline/saline conditions at the peak of the ECO (Stages 2/3), to increased runoff, reduced alkalinity/salinity, and maximum lake expansion (Stages 4/5). Profundal mineral phases parallel lake stages. Illite, quartz, and detrital feldspar characterize Stage 1; quartz, authigenic feldspars, dolomite, dawsonite, nahcolite, and halite characterize Stages 2 and 3; and dolomite, calcite, clay, and analcime characterize Stages 4 and 5. Facilitated by microbial decay of organic matter (OM), increasing alkalinity in Stages 2 and 3 results in clay conversion to authigenic albite, and precipitation of dawsonite and nahcolite.

Dolomite formed in littoral environments associated with microbialites by recrystallization of precursor carbonate and in profundal environments, by direct precipitation or possibly by recrystallization of metastable precursor carbonate to ordered dolomite (Figure). Microbial decay of OM in profundal low-oxygen environments produced alkaline lake waters through methanogenesis, denitrification, and bacterial sulfate reduction. This favored precipitation of dolomite, and Na-carbonates, authigenic feldspars, and analcime from lake water and phreatic pore water. Extracellular polymeric substances (EPS) excreted by microbial communities provided nucleation sites for Mg-carbonate. Fe-dolomite overgrowths precipitated where microbial Fe reduction occurred in stagnant, oxygen-depleted, alkaline pore waters.