Microbially-Mediated Dolomite Associated with the Oil Shales of the Lacustrine Green River Formation, Green River, Uinta, and Piceance Creek Basins

Long Ma and Henry S. Chafetz
University of Houston, Houston, TX

The Green River Formation is well-known for its significant oil shale deposits. The dolomites in the organic-rich “oil shales” are iron-bearing, relatively calcian, generally anhedral to euhedral dolomicrite and dolomicrosparite. These dolomicrite generally occur as either bright laminae alternating with hydrocarbon stringers/laminations or clots, and not uncommonly associated with pyrite spheres, cyanobacterial shealths, and authigenic quartz (Magadi-type cherts?). Putative bacteriomorphs (filamentous, knobby, and spherical features) are associated with these in situ precipitated micritic dolomite crystals.

The oil shales have been deposited under anoxic conditions, which were favorable for the thriving of both sulfate reducing bacteria and methanogenic bacteria. Positive δ¹³ C values (up to + 12.5 ‰) for the dolomite in the oil shales is significantly higher than for non-oil shale dolomites and indicates that bacterial methanogenesis has played an active role. Positive δ³⁴ S values of authigenic pyrite (up to + 43‰, with a mode of + 25 ‰), however, are contrary to the low isotopic values normally produced by bacterial sulfate reduction process. However, positive sulfide δ³⁴ S values caused by BSR process have been reported in other freshwater lake sediments. BSR process, therefore, may have influenced dolomite precipitation. Thus, given the probable low sulfate concentration in water and abundance of organic matter in ancient Lake Gosiute and Lake Uinta, it is our interpretation that bacterial methanogenesis was the dominant bacterial process in the formation of dolomites in the oil shales of Green River Formation.