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Differentiation of Mississippian-age Shale Units of Central and Eastern Nevada Using Petrophysical, Mineralogical, and Geochemical Characteristics

Don E. French, Jerome P. Walker, James H. Trexler, and Patricia H. Cashman
3704 Hayden Drive, Billings, MT

Organic-rich rocks of Mississippian age, commonly identified as the Chainman Shale, can be subdivided over a large area into upper and lower units based on characteristics of wireline logs, mineralogy, and organic-carbon content. The boundary between the units is sharp; the lower unit has high resistivity with greater variability compared to that of the upper unit. Gamma ray curves also have greater intensity and variability in the lower unit compared to the upper. Changes in the log curves coincide with systematic variation in the source-rock characteristics. The lower unit has about twice the total organic carbon content and 5 times the pyrolysis yield as the upper unit. X-ray diffraction of surface samples and well cuttings shows that the lower unit has substantially more quartz than clay compared to the upper unit. Dolomite and calcite are commonly present in the lower unit and absent from the upper, whereas kaolinite is common in the upper and rare in the lower unit. These differences are manifested in outcrops. The upper unit is softer and weathers as an expansive soil that is difficult to sample. The lower unit is brittle and weathers as light gray to light tan chips. The data indicate the units are separated by an unconformity that can be associated with the early Chesterian C2 unconformity of Trexler and others (this volume). The unit below the C2 unconformity is the likely source rock for 95% of the oil produced in Nevada and is a target for unconventional-resource development.

AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013