Print this pageGeochemical Aspects of Organic-Rich Mudstones (Oil Shales) From the Parachute Creek Member, Green River Formation, Eastern Uinta Basin, Utah, U.S.A.
Abstract
The Green River Formation (GRF) of the Uinta Basin contains some of the world’s richest oil shale, concentrated in 8 organic-rich mudstone (ORM) intervals (up-section from R1 to R8) from the Parachute Creek Member. The richest interval is R7 — the Mahogany Oil Shale Zone (MOSZ) and comprises variably rich ORM deposited in a stratified, alkaline, lacustrine environment. Ongoing studies relate to several beds of oil shale sampled from the lower R8 zone, ~60 to 130 m above the base MOSZ, from both core and outcrop. SEM analyses indicate the presence of disseminated heavy-metal accumulation within both matrix and non-particulate organic material in samples from phosphatic (Pr) intervals. Analyses have indicated potential relationships between organic material (OM) and trace-element variability, with systematic enrichment patterns of lanthanides, actinides, and toxic heavy metals occurring in Pr intervals of ORM.
The relationship between trace-element enrichment and organic material is examined using inorganic (LA-ICP-MS), and organic (GC-MS) geochemical data to highlight compositional changes related to the nature and type of OM, source rock maturity trends, and organic richness in the GRF. Isoprenoid/n-alkane ratios vary significantly between Pr and non-Pr samples in the Upper GRF, and maturity indices such as CPI indicate differences in respective OM populations — most notably in the long chain n-alkane distribution — within these thin, toxic heavy-metal enriched intervals. Local [paleo]pore-water chemistry is not precisely understood, but some contribution to the differences in trace-element geochemistry might perhaps be ascribed to variations in the nature and type of preserved OM, which has been found to differ between Pr and non-Pr intervals.
AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018