Source rock potential of upper cretaceous marine shales in the Wind River Basin, Wyoming: Chapter 8 in Petroleum systems and geologic assessment of oil and gas resources in the Wind River Basin Province, Wyoming

Seventy-eight samples collected from marine shales from
the Cretaceous Mowry Shale, the lower part of the Frontier
Formation, and the lower shaly and upper sandy members
of the Cody Shale in the Wind River Basin, Wyoming, were
analyzed using Rock-Eval and total organic carbon analysis
to determine the source rock potential. Results indicate
that the Mowry Shale has a generative potential based on
organic richness that is considered to be fair to very good,
and hydrogen indices and S₂
/S₃
ratios indicate a capability to
generate both oil and gas. Maps of the Mowry indicate that
it is most organic rich and oil-prone in the eastern part of the
basin. Results of total organic carbon and Rock-Eval analyses
for the Frontier Formation indicate that it is composed of
mainly type III gas-prone kerogen, with organic richness
levels that are generally poor to fair. Results of similar
analyses of samples from the lower shaly member of the
Cody Shale show a generative potential ranging from fair to
excellent, and hydrogen indices and S₂
/S₃
ratios indicate that
it is capable of generating both oil and gas. Maps showing
the distribution of kerogen types and organic richness for
the lower shaly member of the Cody Shale are similar to the
Mowry and show that lower shaly member of the Cody is
more organic rich and more oil-prone in the eastern part of the
basin. Analyses of samples of the upper sandy member of the
Cody Shale indicate that it has little or no potential as a source
rock. Thermal maturity mapping based on vitrinite reflectance
measurements in the coal-bearing post-Cody Upper
Cretaceous and Paleocene rocks shows that Upper Cretaceous
marine shales in the deeper parts of the Wind River Basin are
thermally mature to overmature with respect to hydrocarbon
generation.