The Green River Shale: an Eocene Carbonate Lacustrine Source Rock

Abstract

The Green River Formation was deposited in a series of saline, alkaline lake basins in the western United States which formed as a result of Laramide tectonics. Deposition occurred largely during the Eocene. Although there is considerable lithologic variability, the unit may be described, in general, as a dolomitic marl. The unit exhibits a wide range in organic enrichment (0.2 to 33.7 wt.% organic carbon) and hydrogen generation potential (up to 370.6 mg HC/g rock). Although the kerogen does display some variability, it generally displays a close affinity to the type I reference curve. The kerogen is isotopically depleted with most kerogen δ13C values being less than - 30‰ relative to PDB. Pyrolysis-gas chromatography indicates that the generated products would be high wax crudes. Variability in the geochemical attributes are noted both stratigraphically and areally. The most organically enriched horizons, with the highest hydrocarbon generation potentials, are restricted to the Mahogany Zone within the Parachute Creek Member. Deposition of this material is thought to be associated with the transgressive lake phases (i.e., fresher water episodes). Areally the most organic enriched units, with the highest generation potential and the most hydrogen-enriched organic material are found within the Piceance Creek basin. Leaner and poorer quality organic material is found toward the orogenic belt. Much of the unit is thermally immature and has not yet entered into the main phase of petroleum generation and release. Observational data and thermal maturity modeling suggest that the main phase of hydrocarbon generation and release is attained at a depth of approximately 2600 m in the vicinity of Bluebell-Altamont Field. Although much of the unit is thermally immature, available data indicate that oil-in-place in excess of 1.9 × 1010 barrels can be attributed to this unit. An additional 1.5 × 1013 barrels of oil are believed to be bound within the immature oil shales. The general exploration strategy for Green River-derived oils is directed toward the identification of stratigraphic traps in marginal lacustrine sandstones of the Green River Fm. or red bed facies of the Wasatch Fm. which are in close proximity to the thermally mature Green River.