Heavy Metal Oil Shale From the Upper Green River Formation, Uinta Basin, Utah

Abstract

The Green River Formation of the Uinta Basin in Utah hosts part of the world's largest oil shale deposit. This deposit is concentrated within eight organic-rich zones, with the richest zone, R7 or the Mahogany Oil Shale Zone, acting as a regionally correlated datum that marks the base of the informal "upper GRF". A series of inorganic geochemical studies (XRD, SEM, EDS, microprobe, LA-ICP-MS) have been performed on several of the R7 and overlying R8 organic-rich mudstone beds to investigate the nature and diagenetic origin of many of the metal enrichments associated with these beds.

To date, sulfides of Fe, Cu, As, Pb, Zn have been encountered as anherdal (grain) to euhedral (diagenetic crystal) particles of widely ranging (μm to mm) size and form (specular, blocky, framboidal) that are widely distributed across the samples. However, HgS, containing minor Mo, Tl, and Po, has only been encountered as microscopic crystals enclosed in a blocky calcium fluorapatite cement beneath a mat-like organic structure. This blocky cement, along with microcrystalline CFA arranged as globular aggregates (likely fossilized microbes) also contain W, either adsorbed on, or substituting into, the CFA lattice. In vertical profile, thick phosphatic intervals exhibit zoning of Th, U and REE enrichments resulting from preferential substitution into the CFA crystal lattice. It therefore appears that early, shallow substrate, biogenically induced precipitation of phosphate has played a crucial role in sequestering numerous metals from deep Lake Uinta.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018