PALACAS, J. G., R. M. FLORES, C. W. KEIGHIN, and D. E. ANDERS, U. S. Geological Survey, Denver, CO

ABSTRACT: Organic Geochemical Typing of Oils in the Wind River Basin, Wyoming

Distribution of biomarkers, including tricyclic and pentacyclic terpanes (m/z 191), steranes (m/z 217), monoaromatic (m/z 253) and triaromatic steroids (m/z 231), coupled with gas chromatographic analyses of saturated hydrocarbons, were used successfully to classify Wind River Basin oils into three principal genetic types: (1) Tertiary Fort Union, (2) Cretaceous, and (3) upper Paleozoic Phosphoria.

Tertiary Fort Union oils are characterized by (1) pristane/phytane Pr/Ph ratios generally much greater than 2.0, and as high as 8.0 or more (for condensates), (2) very low concentrations of tricyclic terpanes relative to pentacydic terpanes, (3) the abundance of regular steranes in the order C<29>> C<28>> C<27>, and (4) relatively low amounts of diasteranes in the region between the regular C<27> steranes and the two prominent diasterane peaks known as the C<27> 13 (beta) (H), 17 (alpha) (H) 20S and 20R diasteranes.

In contrast, Cretaceous oils have (1) Pr/Ph ratios commonly 1.6 to 2.0, (2) slightly but significantly higher amounts of tricyclic terpanes relative to pentacyclic terpanes, (3) a regular sterane distribution of C<27>> C<28>> C<29>, and (4) very high relative concentrations of the 'intermediate' diasteranes described above. Both Cretaceous and Tertiary oils commonly contain less than 0.2% sulfur.

Phosphoria-type oils, prominent in upper Paleozoic reservoirs but known also to charge Triassic and Jurassic reservoirs (such as the Nugget Sandstone), are distinctly different. Phosphoria oils are distinguished by (1) large amounts of tricyclic terpanes, commonly dominating over the pentacydic terpanes, (2) very low quantities of diasteranes in contrast to Cretaceous and Tertiary oils, (3) a regular sterane distribution order of C<27>> C<29>> C<28>, and (4) C<27> terpane Tm/Ts ratios greater than 1.0 (in contrast toratios <1.0 for Cretaceous and Tertiary oils). Furthermore, the Phosphoria group of oils are known for their high sulfur content, generally 2-4%.

Successful typing of oils in the Wind River Basin permits identification ofmigration pathways (e.g., stratigraphic vs. structural), identification of oil source rocks, and time of oil migration. This geochemical technique may be used as a tool to target potential areas of exploration in a basin.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.