Mineralogy of the Green River Formation, Wyoming: Mining Old Data in the Light of New Models

Abstract

The Eocene Green River Formation (GRF), one of the richest source rock in the world, provides an important example for understanding formation of organic rich sedimentary rocks in lacustrine and possibly other environments. Tanavsuu-Milkeviciene and Sarg (2012) subdivided the GRF in Colorado into six stratigraphic stages in the evolution of Lake Uinta, and Boak et al. (2013) showed how mineralogic and geochemical changes related to the stratigraphic evolution. Major GRF member-level boundaries in Wyoming appear to correspond to stage boundaries in Colorado. X-ray Diffraction (XRD) data from the ERDA Blacks Fork No 1 well in the Green River Basin shows mineralogic transitions like those in the Piceance Basin. Peak height values for primary mineral peaks are not actual abundances, but can be used to identify trends in GRF mineralogy, by analogy with quantitative data from Colorado. The Tipton Member in the Blacks Fork well corresponds to the Transitional Lake Stage in Lake Uinta, with transitions from Dolomite+ Calcite+ Quartz+ Pyrite+ minor Feldspar, and Illite/ Feldspar >0.2 to Dolomite+ Siderite+ Analcime + K-feldspar+minor Pyrite with much lower Quartz, as well as Illite/ Feldspar <0.2. The Wilkins Peak Member corresponds to the Fluctuating Lake Stage in Lake Uinta, and is composed of Dolomite+Quartz+ Albite+K-feldspar±Pyrite, with Illite/ Feldspar <0.2 except in sandstone intervals. The unit is distinguished by the presence of shortite [Na2Ca2(CO3)3] and zones with Trona [(Na3(CO3)(HCO3)‡2H2O], Nahcolite [NaHCO3] and Halite. Calcite appears primarily in the upper part in association with sandstones. The Laney Member is characterized by Dolomite+ Calcite+Quartz+Analcime+ Feldspar+minor Pyrite and Illite/ Feldspar <0.2, as well as local aragonite. Sodium mineral changes in the Transitional Lake reflect increasing salinity; the sequence Analcime – Albite – Shortite different from but parallel to Albite – Dawsonite – Nahcolite found in Colorado. The disappearance of shortite just below the top of the Fluctuating Lake Stage may parallel the disappearance of dawsonite in Colorado. Calcite+Quartz/ Dolomite+ Feldspar may reflect the degree of authigenic mineral formation near the sediment/ water interface. It decreases sharply in the upper Tipton, rises through the lower Wilkins Peak, fluctuates through the interval of periodic sand sheet deposition in the upper Wilkins Peak, and rises again at the Wilkins Peak/Laney boundary, where saline minerals stop being deposited. This ratio is also distinctly lower in richer oil shale. Mineral relationships suggest that redox conditions, salinity, pH, and silica activity were all important in creating the organic and sodic enrichment of the GRF. The assemblages may have relevance to less extremely enriched sedimentary rocks. ‡ Now Statoil ASA, Arkitekt Ebbells veg 10, Rotvoll Norway

AAPG Datapages/Search and Discovery Article #90193 © 2014 Rocky Mountain Section AAPG Annual Meeting, Denver, Colorado, July 20-22, 2014