Potential Source Rocks in Eocene Green River Formation, Wyoming, Colorado, and Utah
Ronald C. Surdam, Kevin R. Adler
The Eocene Green River Formation of Wyoming, Colorado, and Utah has long served as a standard for lacustrine deposition. Because lacustrine depositional systems recently have become exploration targets, the depositional fabric of potential source rocks in the Green River Formation is of increased interest. The potential source rocks (oil shale) are laminated kerogeneous carbonates (calcite, dolomite, or both). These oil shales typically range in total organic carbon (TOC) from 4 to 30 + wt. %, in hydrogen index (HI) from 500 to nearly 1,000, and in oil yield from 5 to 60+ gal/ton. In brief, these potential source rocks display significant variation in organic character.
The potential source rocks occur in various depositional settings. They are found in marginal lacustrine settings associated with lithologies representing strandlines and carbonate mud flats, and in lake-center deposits. In Wyoming, regardless of their position with respect to the geometry of ancient Lake Gosiute, the most organic-rich rocks typically occur either just below or just above evaporitic sequences, or in both places. This stratigraphic position optimizes the following factors: (1) dominance of organic productivity, (2) sparsity of epifaunal grazers, (3) low siliciclastic input, (4) inflow of nutrient-rich waters, and (5) generation of chemical stratification. Ultimately, the optimum condition for deposition of oil shale (e.g., Mahogany oil shale bed in the Piceance Creek b sin) is the expansion of the hydrographic basin subsequent to a prolonged lowstand (evaporitic in part).
AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.