Sequence Stratigraphy in Mixed Lake Systems, Organic Richness, and Climate — Green River Formation, Lake Uinta, Part II, Organic Richness
Sarg, Rick; Tanavsuu-Milkeviciene, Kati; Jufang, Feng
The Green River Fm. lake deposits (E-M Eocene) in the Piceance Creek basin are largely composed of kerogen-rich and kerogen-poor mudstones (clay and carbonate). The organic deposition of the Green River oil shale is related to three factors: production, destruction, and dilution. The pattern of variation 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 controls the average organic richness variation on the scale from depositional units to depositional sequences (100 and 400 Ky) over the long term lake history (6 My). On a sequence scale peaks in organic richness occur during lake level rises and at maximum floods, and vary, related to dilution, during high lake times.
Over long-term lake history, variation occurs between Green River rich-zones (R), expressed in gal/ton average oil yield. Long term lake evolution is defined by lake stages: S1-Fresh-Mesosaline Lake, S2-Transitional Lake, S3-Highly Fluctuating Lake, S4-Rising Lake, and S5-High Lake. Lake stages correlate to the Eocene climate optimum. S1 formed during the warming phase of the climate optimum and represents change from fresh to mesosaline conditions, suggesting a change from abundant rainfall and high runoff, to more seasonal and dryer climate. Moderately high net productivity and diminishing detrital dilution occurred as the climate dried, resulting in increasing richness (R0-21gal/ton, R1-27gal/ton). Increased seasonality, and flashy runoff began during S2, indicating restricted evaporative lake conditions when oil shale changed from clay mudrock to siliceous dolomitic mudstone; and peaked during S3, at the maximum of the climate optimum, when arid conditions prevailed, and nahcolite and halite are abundant. 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 organic content, when increased saline mineral dilution occurred at the maximum warm time (R4-36gal/ton, R5-21gal/ton). The ensuing lake level rise (S4) and high lake (S5) occur during climatic cooling, accompanied by increased precipitation. Net productivity increased with a return to wet conditions. Diminished saline dilution resulted in increasing organic richness (R6-24gal/ton, R7-30gal/ton).
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013