BASIN SCALE PATTERNS OF ORGANIC MATTER ENRICHMENT IN LACUSTRINE MUDROCKS, EOCENE GREEN RIVER FORMATION, WYOMING (Invited Presentation)

The Green River Formation of the western U.S. is widely considered to represent the largest oil shale resource in the world. According to recent estimates by the U.S.G.S, deposits in Wyoming, Colorado, and Utah together have a potential yield of ~4.3 trillion barrels of oil in place, equal to more than twice the world’s presently known recoverable oil reserves. Carbonate-bearing oil shale was deposited by alkaline lakes that ranged from freshwater to hypersaline, with approximately half of the assessed resource attributable to the latter. Maximum organic enrichment of oil shale in the Bridger basin in Wyoming displays two distinctly different patterns, that can be related to lacustrine facies association (lake type). In hypersaline lake deposits (underfilled lake type), the histogram of Fischer Assay gallons per ton (GPT) displays a modal value of less than 2 GPT, and then decreases approximately exponentially to a maximum value of ~60 GPT. This distribution is interpreted to reflect deposition by a relatively shallow lake that fluctuated across a low-gradient basin floor, with rich oil shale deposited only during episodes during maximum transgression. Fresh to brackish lake deposits (overfilled to balanced-fill) have similar maximum values, but modal values are much higher at 10-12 GPT. These distributions are interpreted to reflect more continuous organic matter accumulation in lakes that routinely intersected rocky, high-gradient basin bounding uplifts. In all cases, the highest average organic enrichment and organic matter burial rates coincide with the areas of greatest inorganic sediment accumulation rate. Rather than representing stratigraphically condensed intervals, the richest oil shales therefore correspond to areas with the greatest rates of dilution by inorganic sediment. We infer that organic matter preservation was strongly influenced by the geographic distribution of bottom water anoxia, which coincided with the most rapidly subsiding areas of the basin. Rapid burial of organic matter beyond the influence of lowstand oxidation may have further aided preservation.