Quantitative Analysis of the Green River Formation, Piceance Basin, Colorado: A Useful Tool to Elucidate the Depositional Process of the Paleolake

Abstract

The Eocene Green River Formation (GRF) in the Piceance Basin (PB) of Colorado is estimated to contain the largest oil shale deposits in the world, and is a well-documented example of a lacustrine depositional system. Understanding the depositional environment and sedimentary processes in the basin depends upon evaluating the distribution of the mineral assemblages and organic matter across the basin. Quantitative analysis of the GRF can be achieved by detailed geochemical data derived from Handheld X-ray Fluorescence (HHXRF) validated with limited Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). Sampling in the Douglass Pass area represents the basin margin and the basin center area is characterized by two complete cores, Shell 23X-2 and John Savage 24-1, sampled at the U.S. Geological Survey Core Research Center. Outcrop and core samples were analyzed through the full stratigraphic section by both methods. Major elements, Si, Ca+Mg, Al+K, Na are good proxies for quartz, calcite-dolomite, clay-feldspar, and salinity, respectively. Major elements define lithologic trends and indicate mixed deposition of siliciclastic and dolomitic siltstone/mudstone, shale, sandstone, and limestone in the basin margin; whereas in the basin center, major elements characterize the variations of siliciclastic, dolomitic siltstone/mudstone, limestone and saline zones through the section. The mineral profiles calculated from major elements match well with the mineralogy data generated from X-ray Diffraction (XRD), implying the reliability of XRF data for the major elements, which can also be confirmed by comparing with ICP-OES data. Trace elements, especially trace metal elements (Mo, U, V, Cr, Ni, Cu, As, Mn, Co, Zn) are used to characterize redox conditions of lake Uinta. Besides trace metal proxies (Mo, U) in marine shales, Arsenic (As), Copper (Cu), and Zinc (Zn) are working well in evaluating the reducing conditions of the lacustrine system in PB. Hierarchical clustering analysis was conducted on both the basin margin and the basin center and 6 chemofacies were generated to characterize the depositional process of the basin. From the basin margin, facies 6 (high Al, Si, K) dominates in the lake stage 2 (transitional lake), corresponding to the high quartz, K-Feldspar, Albite, Clay Minerals and low Calcite and Dolomite from the Mineralogic profiles; from the basin center, another 6 chemofacies were extracted but with different geochemical signals. A sharp decrease of redox indicator signals occurred around R5, corresponding to the sharp decrease in dawsonite, quartz, implying increased silica activity and decreased salinity. By integrating geochemical data (XRF +ICP-OES) and mineralogical data (XRD), a quantitative depositional process of the basin can be defined across the Piceance Basin through the Eocene time.

AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019