ORGANIC MATTER DISTRIBUTIONS IN OIL SHALE ZONES OF THE GREEN RIVER FORMATION, PICEANCE BASIN

The oil shale resources of the Eocene Green River Formation in the Piceance Basin of northwestern Colorado have been studied extensively in order to quantify their potential for development. The quantity and variety of data collected to assess the oil shale and other mineral resources provide unique opportunities to examine ancient lake systems. In this study, the database of Fischer assay results from the Piceance Basin oil shales accumulated by the U.S. Geological Survey and former U.S. Bureau of Mines (~300,000 measurements; ~1000 wells) in the 1960’s and 1970’s was used to study organic matter deposition and preservation in this lacustrine petroleum system. The organic richness of Green River oil shale was examined in each of the stratigraphically defined rich and lean zones within the lacustrine sequence, specifically the total resource (barrels per acre, BPA) normalized to zone thickness. For offshore areas away from coarse clastic inputs, there are strong linear correlations between thickness and BPA in predominantly laminated zones and moderate correlations in zones with large amounts of laterally transported oil shale. BPA and thickness are also moderately correlated for saline-rich zones but only in areas where saline minerals have been leached by groundwater. In general, rich zones have a higher yield per unit thickness average (~1500 BPA/ft) than the lean zones (~1000 BPA/ft). Because the oil shale organic matter varies little from zone to zone, this difference does not appear to be related to preservation, but rather primarily to dilution and possible variations in organic productivity. Many lean zones appear to represent lake regressions when coarse clastics reached further into the lake. Maps of thickness-normalized resource will be presented for selected rich and lean zones to illustrate the geographic and stratigraphic variations in organic matter distribution through the lacustrine sequence. The distributions will be discussed in the context of the established lake history model for Eocene Lake Uinta and influences on organic richness, particularly dilution by detrital inputs and changes in lake chemistry.