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The Spraberry Sandstone is a Lower Permian (Leonard) unit restricted entirely to the subsurface of the Midland basin of West Texas. Production of oil from the Spraberry reservoir began in 1949 in east-central Dawson County. Permeability of the reservoir rocks ranges from 0 to 2.5 md, and porosity averages 10%. Major production is made possible by fracture porosity.
The Spraberry Sandstone is approximately 305 m thick and consists of upper and lower sandy and silty members separated by a middle member consisting of predominantly dark argillaceous carbonate rock and calcareous shale. Detailed study of sedimentary structures, vertical sequences, and petrography suggests that the Spraberry represents sedimentation from a series of coalescing submarine-fan complexes. The dark, finer grained intervals within the Spraberry may be the source beds for the petroleum being produced from the formation, although no published data exist to support this.
Samples from the finer grained intervals in two cores from the Tex-Harvey and Pegasus fields (Midland County) were analyzed by means of various geochemical techniques to determine their source-bed potential. Total organic carbon content of the samples ranged from 0.68 to 2.42 wt. %. Soluble organic matter extracted via Soxlet ranged from 1,892 to 6,598 ppm by weight. The carbon preference index as determined from gas chromatograms of the paraffinic-napthenic fraction ranged from 1.14 to 1.22. Kerogen coloration values were between 2.0 and 3.2, with the mode being 2.3. Hydrocarbons comprised from 0.65 to 8.6% of the total organic carbon and 17 to 71% of the total extractable organic matter. Compared with criteria commonly used to judge source-bed potential, the Spraberry has most requi ements and probably contained its own source beds.
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