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Mississippian rocks in the southern Texas Panhandle constitute a complex sequence of carbonate deposits formed in a platform-to-basin setting. Following relatively rapid transgression and inundation of the area from the north and east, the Hardeman basin area was characterized by outer platform conditions in which isolated carbonate buildups developed surrounded by relatively deep water. The Palo Duro basin to the west was the site of shallow-water, inner platform deposition. In intermediate areas, limestone turbidites, perhaps derived from carbonate buildups to the east, accumulated in quiet water. After this initial transgression, an upward-shallowing trend resulted in the formation of ooid to skeletal shoals throughout the area.
Although current production in the area is coincident with the distribution of organic-rich Upper Mississippian shales in the eastern part of the Hardeman basin, TOC studies indicate that potential carbonate source rocks are present in the western Hardeman and eastern Palo Duro basins. Mississippian rocks in the Palo Duro basin proper have little source rock potential. Vitrinite reflectance studies indicate that Hardeman basin rocks are well within the oil window. However, correlative deposits at
equivalent depths in most of the Palo Duro basin are only marginally mature.
Although thermal maturity seems to be mirrored by the present geothermal gradient, and source rock quality appears related to depositional setting (depth of water), successful exploration outside currently productive areas will require a detailed analysis of organic geochemistry and depositional facies.
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