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Petrology and Hydrocarbon Reservoir Potential of Subsurface Pottsville (Pennsylvanian) Sandstones, Black Warrior Basin, Mississippi
Russell H. Beard (1,2), Maurice A. Meylan (2)
The Black Warrior Basin of Mississippi and Alabama is a Paleozoic foreland basin developed between the North American craton and the Appalachian and Ouachita orogenic belts. The basin fill consists of a Middle Mississippian to Lower Pennsylvanian clastic wedge transitional in character between Appalachian molasse and Ouachita flysch. Pottsville (Pennsylvanian) sandstones, shales, coals, and thin conglomerates make up the greater portion of the wedge, thickening to 11,000 feet in NE Mississippi. Although the outcropping and near-surface Pottsville is economically important as a source of coal in Alabama, only minor amounts of gas have been derived from the subsurface Pottsville of Mississippi (Clay and Monroe Counties). Production from the Black Warrior Basin, mostly gas, is contributed chiefly by Chesterian (Mississippian) sands and limestones from the shallower part of the basin, especially Monroe County.
Cores of Pottsville sandstones from four wells in the deeper part of the Black Warrior Basin (Calhoun and Choctaw Counties) have been examined to determine their petrography, diagenetic history, and reservoir quality. This part of the basin is relatively unexplored, and the primary objective of the study was to determine if suitable hydrocarbon reservoirs are present.
Based on Folk's classification, the sandstones studied are mostly very fine- to fine-grained sublitharenites and litharenites, consisting of about 65%-75% monocrystalline quartz with lesser amounts (6%-8%) of polycrystalline quartz, metamorphic rock fragments (10%-18%), feldspar (2-6%), miscellaneous grains (2-4%), and matrix (2%). The framework grains are interpreted as having an Ouachita provenance, and probably represent recycled orogenic-belt detritus.
The Pottsville sandstones examined apparently were deposited in a nearshore marine delta fringe environment. Evidence for this includes glauconite, carbonates, and marine invertebrate fossils, along with plant debris. The original depositional texture has been markedly altered, particularly by "over-compaction" and suturing of quartz grains. Porosity has also been reduced by ductile grain deformation which has contributed to a "pseudomatrix". Partial to complete pore filling in places has been accomplished by silica, calcite, siderite, and chlorite cementation. Porosity reduction has been partially offset by dissolution of feldspar and rock fragments, yielding secondary porosity. Measured porosity values range from 1.2 to 14.7%, averaging 7.6%, indicating that reservoir quality is generally poor to fair.
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