Source-rock Analysis of the Lower Member of the Lewis Shale, Washakie Basin, Wyoming

A.V. Rigoris, I. Pasternack, J. B. Curtis, and Neil F. Hurley. Department of Geology and Geological Engineering, Colorado School of Mines, 1516 Illinois Street, Golden, CO 80401, phone: 303 273 3887, fax: 303 273 3574, [email protected]

Turbidite sandstones within the Lewis Shale (upper Cretaceous) comprise an emerging tight-gas resource in the eastern Green River Basin, Wyoming. The purpose of this research was to provide a Lewis Shale source-rock characterization. This study helped to clarify the relationship between the potential source-rocks and the gas-oil production and rock properties from the lower 600 ft (183 m) of the Lewis Shale. A total of 145 representative shale samples from nine cores were collected from 411 ft (125 m) of core; 3-ft (1-m) composite samples were analyzed to determine source-rock richness, kerogen type and thermal maturity. Rock-Eval pyrolysis and total organic carbon (TOC) data interpretations show that many of the samples have good to very good (1.0 – 5.5 wt.%) organic carbon contents. The samples are thermally mature and the depth of burial is between the beginning and peak of the “oil window”. Hydrogen index (HI) and oxygen index (OI) data were plotted on a modified van Krevelen plot to determine kerogen type. The plot indicates that the organic matter is Type II and III kerogen and is likely to generate gas and minor oil. A relationship between TOC and wireline gamma ray log response was observed, which suggests that high gamma ray response in the formations corresponds to intervals with high TOC values. TOC data and normalized gamma ray values were plotted to determine the nature of the dependency between these parameters. A good correlation factor (76%) was found, indicating that TOC and gamma ray responses are correlated.