Chemostratigraphy, Mineral Association and Water Chemistry Modeling of the Green River Formation, Piceance Basin, Colorado: An Integrated Method to Unveil the Evolution of the Uinta Lake

Abstract

The Eocene Green River Formation (GRF) of Colorado in the Piceance Basin (PB) is estimated to contain the largest oil shale deposits in the world, and is a well-documented classic example of a lacustrine depositional system. The GRF provides an opportunity to define the chemical evolution of its rich source rock. Understanding the chemistry of these sedimentary rocks across the basin can give us an insight into the interaction of clastic input, carbonate mineral sedimentation, saline mineral precipitation, and organic matter deposition. A systematic profile of major & trace elements was generated by handheld X-ray Fluorescence (XRF) measurements made on the Douglas Pass area and cores “Shell 23X-2” & “John Savage 24-1”, which represent the two end members of the PB, the basin margin and the basin center, respectively. Quantitative mineral identification was completed by X-ray Diffraction (XRD). Integrating these data, mineral stability diagrams have been constructed and the chemistry under which these minerals formed is simulated via Geochemistry Workbench. Boak et al.(2013) defined three mineralogic units in the Piceance Basin: 1) the Lower Mineralogic Unit, with illite in the basin center and multiple clay types in the basin margin, reflecting less saline, alkaline conditions; 2) the Middle Mineralogic Unit, defined by the appearance of dawsonite in the basin center, indicating increased salinity, alkalinity and CO2 concentration; 3) the Upper Mineralogic Unit, characterized by the disappearance of dawsonite, and the appearance of analcime in the basin margin, corresponding to increased silica activity, and possibly lower CO2. Trace metal redox indicators (U, Mo, V, Cr, Ni, Cu, Zn, etc) provide support to the interpretation of Lake Uinta as stratified, with a deep suboxic, and periodically anoxic zone in the basin center, underlying a shallow, less persistently suboxic zone in the basin margin. The distinct mineral association between the basin margin and the basin center shows the variation in water chemistry of Lake Uinta in terms of silica activity, alkalinity, salinity and CO2 concentration. The integration of XRF, XRD and water chemistry modelling is critical to reveal the evolutionary history of Lake Uinta in the Eocene era.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017