--> --> Evaluation of Resource-Play Potential of the Mississippian Chainman Shale of the Great Basin, Nevada

AAPG Rocky Mountain Section Meeting

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Evaluation of Resource-Play Potential of the Mississippian Chainman Shale of the Great Basin, Nevada


The lower part of the Mississippian Chainman Shale has been identified as the principal source rock for conventional accumulations that have produced 45 MMBO in Railroad Valley, Nevada. Resource potential is indicated by a deep well in Railroad Valley, where the lower Chainman is overpressured and produced a few hundred barrels of oil during tests. Regional investigations show that the lower Chainman is present in an area covering more than 30,000 sq mi, is up to 3,500 ft thick, has good to excellent source quality, and has mineralogy conducive to hydraulic fracture treatment. But the distribution of the shale is uneven due to episodes of erosion. Maturity indices from outcrop samples are generally low, indicating that generation sites and over-pressured conditions are probably associated with burial in Neogene basins. These conditions indicate that the resource-play potential must be evaluated for each Neogene basin, case-by-case. The Neogene basins are evaluated using spreadsheets that combine various local and regional data. A paleogeologic map of the region shows the distribution of the shale and overburden of later Paleozoic through early Cenozoic deposits. Geophysical and outcrop data show the distribution of Neogene strata relative to the paleogeology. Local geothermal gradients based on subsurface data establish the depth of burial required for generation conditions. Thickness of the Chainman is estimated from an isopach map and local data. Pyrolysis of local samples provides an estimate of generation potential of the shale. The spreadsheets are used to prioritize target areas and identify remedial tasks, such as additional source-rock analyses or outcrop work to verify paleogeologic interpretations. Oil-in-place estimates for resource targets are 2 to 240 BO/ac-ft, and 2 to 146 MMBO per section. Resource potential for 13 assessed sites ranges from 7 to 700 MMBO based on 5% and 10% recovery factors. The spreadsheets show that, in addition to the source-rock quality, the thickness of the lower Chainman Shale is an important factor in the estimates. The great thickness of the resource target implies that it might be successfully developed with vertical instead of horizontal wells. Also, a successful resource play will necessarily drill many conventional faultblock traps that would not otherwise be evaluated.