Controls on Composition and Distribution of Lacustrine Organic-Rich Rocks of the Green River Formation, Wyoming

George J. Grabowski Jr. and Kevin M. Bohacs

Lake type controls the accumulation of organic-rich rocks; climate and hydrology determine lake type. The amount and composition of organic matter, and the thickness, variability and lateral extent of organic-rich rocks, differ in freshwater, alkaline and saline lake deposits, as illustrated by the Eocene Green River Fm.

The Luman Tongue was deposited in freshwater, thermally stratified lakes. It consists of sequences of lacustrine shales, 100's of feet thick, that are variable in composition. Littoral coals and shales with less than 2% TOC are composed of Type-III kerogen (HI 58-300 mg/gC). Profundal shales have up to 7% TOC that is a mixture of Types I and III kerogen (HI <580). Low TOC values are due to poor preservation in the lake sediments and dilution by detrital deposition. Input of land-derived organic matter causes much variability in organic-matter type.

The Laney Member was deposited in alkaline, chemically stratified lakes with littoral marshes. Well-defined sequences 10-20 ft. thick characterize this member. Littoral shales have less than 7% TOC that is a mixture of Types I and III kerogen (HI 235-650). Profundal dolomitic lime mudstones have up to 30% TOC enriched in Type I kerogen (HI 500-650). TOC and HI values are typically highest near the base of each sequence, where the lake was at its maximum extent, deposition from land was restricted to littoral areas, and the bottom sediments were depleted in oxygen.

The Wilkins Peak Member was deposited in saline, chemically stratified playa lakes. Organic-rich rocks 1-5 ft. thick occur at the bases of several sequences that are 3-10 ft. thick. These organic-rich dolomitic lime mudstones have 5-20% TOC composed of Type-I kerogen (HI 650-1100). TOC and HI values vary little laterally, reflecting an absence of diluting sedimentation or input of organic matter from land.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California