Abstract
Striking mineralogic and chemical changes occur in outcrops of a Permian redbed sequence overlying oil-productive parts of the prolific multireservoir oil accumulation at the Cement anticline, Oklahoma. Gypsum beds along the flanks are altered abruptly to erosion-resistant carbonate rocks at the crest of the fold in the Keechi Hills. Associated sandstones, typically red and friable in the surrounding region, are altered to pink, yellow, and white on the flanks of the anticline and to hard carbonate-cemented gray sandstone at the crest. The zone of cementation, confined to sandstone intervals, extends to a depth of at least 2,500 ft.
Calcitized gypsum exceptionally deficient in C13 and light-carbon/heavy-oxygen cements directly overlie petroleum-productive zones near regions where fluids have superior vertical avenues of communication (faults and an unconformity at shallow depths and of limited extent along the crest). Away from these avenues of leakage, the influence of hydrocarbons on the isotopic composition of the carbonate cements decreases systematically. Color changes in the sandstones are related to reduction and dissolution of iron in the presence of hydrocarbons.
Much of the hydrocarbons leaked from Missourian reservoirs beneath the crestal unconformity. Dense crude oil from stratigraphically discontinuous reservoirs along the basinward flank of the structure are associated with low-salinity pore water. Paraffinicity and salinity of waters decrease systematically with increasing depth of burial; these salinity variations, initially effected by ingress of water squeezed from expandable clays in the bordering basin, may have played a role in the selective solution of low-molecular-weight fractions. Water, vertically expelled along the crest, was desalted in passing from sandstone to shales. Large volumes of sandstone thereby were cemented off in shallow Permian rocks in places over the crest; the uncemented sandstones are petroleum-productive down the flanks.
