Climate History, and Lake Evolution Controls on Oil Shale Organic Richness — Green River Formation, Piceance Creek Basin

The Green River Fm. lake deposits (early-middle Eocene) in the Piceance Creek basin are largely composed of kerogen-rich and kerogen-poor mudstones (clay and carbonate). Lake evolution is defined by lake stages: S1-Fresh Lake, S2-Transitional Lake, S3-Rapidly Fluctuating Lake, S4-Rising Lake, S5-High Lake, and S6-Closing Lake. Lake stages correlate to the early to middle Eocene optimum. S1 appears to have formed during the warming phase of the climate optimum and represent the basin evolution from fresh to saline conditions. The lake changed during S1 from an open lake to a closed lake basin suggesting a change from abundant rainfall and high runoff, to more seasonal and dryer climate. Increased seasonality, and flashy runoff began during S2, indicating restricted lake conditions, and peaked during S3, at the maximum of the climate optimum, when arid conditions prevailed, and nahcolite and halite are abundant. The ensuing lake level rise (S4) and high lake (S5) occur during climatic cooling, accompanied by increased precipitation.

The organic deposition of the Green River oil shale is related to three factors: production, destruction, and dilution. The pattern of organic richness variation within 400k year sequences suggests a net-productivity-driven organic depositional model modified by variations in dilution related to climate. Inorganic geochemistry proxies (P, Al, V/Cr, C13, O18) suggest net productivity plus dilution by siliciclastics and/or evaporites also controls the average organic richness variation over the long term lake history. This is evident in the variation between Green River rich-zones (R), expressed here as changes in average oil yield in gal/ton. During S1, stratified conditions first developed, and moderately high net productivity and diminishing detrital dilution occurred as the climate dried, resulting in increasing richness (R0-21gal/ton, R1-27gal/ton). High productivity and low dilution peaked in early S2, resulting in very high richness (R2-39gal/ton). Richness then declined as evaporate precipitation increased (R3-25gal/ton). S3 shows decreasing average organic richness from an early high to a minimum at the end of S3, when increased saline mineral dilution occurred at the peak of the climate optimum (R4-36gal/ton, R5-21gal/ton). Net productivity increased during subsequent climate cooling, with a return to wet conditions (S4, S5). Diminished saline dilution resulted in increasing organic richness (R6-24gal/ton, R7-30gal/ton).

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California