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Because much of the petroleum contained in giant structure traps in the United States has been discovered, the petroleum resources of the future will be from complex petroleum traps involving structure, lithofacies distribution, and diagenetic alteration. The Black Warrior basin of northwestern Alabama is an excellent basin to prospect for combination petroleum traps; to date 35 petroleum fields have been discovered. The key to successful prospecting in this basin involves the delineation of local structure and determination of reservoir size, morphology, and quality. Mississippian sandstone reservoirs presently have the greatest petroleum potential, the Carter and Lewis sandstones being the most economic of these reservoirs. The Carter was deposited as part of a high-con tructive, elongate to lobate delta which prograded from northwest to southeast into the basin. Specific deltaic environments identified include distributary channel and mouth bar, distal-bar, prodelta, and interdistributary bay. The Lewis accumulated as a series of elongate, northwest to southeast trending sand bodies on a shallow marine shelf. Specific environments identified include central-bar, interbar, and shelf.
Carter distributary-mouth bar and distal-bar lithofacies and Lewis central-bar lithofacies constitute the primary Mississippian reservoirs in the basin. These sandstones are usually fine grained, well sorted, subangular to subrounded quartzarenites or sublitharenites. Primary interparticulate porosity has been reduced through the development of quartz overgrowths and/or calcite cementation. Porosity is principally secondary and involves leaching of carbonate allochems, calcite cement, and/or matrix. The Carter prodelta and interdistributary-bay shales and the Lewis marine shales make excellent petroleum source rocks. These shales contain amorphous, algal, and herbaceous kerogen. The state of alteration of the kerogen indicates that the thermal history of the basin has been favorable f r the generation and preservation of hydrocarbons. The hydrocarbon-trapping capabilities of these strata have been enhanced because of their association with normal faults.
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