ABSTRACT

Lower Vicksburg sandstones form multiple gas reservoirs at depths that range from 9300 to 15,400 ft (2836 to 4697 m). Core examination shows that the sandstones display ordered sequences of sedimentary structures within beds that average about 4 ft (1.2 m) in thickness. Thicker sandstones are massive below and horizontally laminated above and represent turbidites of the AB type which are probably of channel origin. Thinner sandstones are dominated by laminated and rippled beds that represent turbidites of the ABCD and BCD types and are probably of overbank origin. Reservoir sandstones appear to represent channel deposits that were closely bounded by levee sediments. Isopach maps show that the sandstones are narrow, linear bodies which have dip trends in the upper part of the section. However, deeper sandstones in the east part of the field show anomalous strike trends.

Average grain size of the sandstones is 0.13 mm (fine grained), and bed sets typically show textural gradation. Average detrital composition is 16% monocrystalline quartz, 35% feldspar, 39% rock fragments, 9% matrix, and 1% other grains. Total cement, mostly calcite, averages 26% of bulk volume. Porosities range from 7 to 24% and permeabilities from less than 0.1 to 118 md. Higher permeabilities are found in thicker channel turbidites.

Structure within the field appears to be dominated by a deep-seated shale uplift which caused the formation of a major growth fault. The fault has about 600 ft (183 m) of throw on the downdip side of the shale uplift, but fault plane dip decreases with depth and becomes essentially parallel to bedding. Below the fault, the Jackson Shale is abnormally pressured and probably folded. Early shale uplift by buoyancy controlled sand distribution by diverting turbidity flows from their normal dip trends. Continued uplift caused faulting on the basinward flank, and the fault shows continuous growth with increased thicknesses of lower Vicksburg intervals on the downthrown side. Shale uplift soon ended because trends of later sandstones in the lower Vicksburg Formation are not greatly affected. However, the growth fault was active through the end of lower Vicksburg deposition. This pattern of early shale uplift and subsequent growth faulting contrasts with previous ideas that attribute shale uplift and faulting to loading by rapid deposition of overlying sediment.