Examination of porosity and permeability (reservoir quality) data, as determined by whole core, acoustic log, and petrographic analyses of lower Tertiary sandstones along the Texas Gulf Coast, made it possible to delineate areas most favorable for development of hydrocarbon reservoirs. Deep (about 3,350 m [11,000 ft] or greater) Wilcox sandstones exhibit no systematic regional reservoir-quality trends. Along the lower and parts of the middle and upper Texas Gulf Coast, deep Wilcox sandstones are tight, but in other parts of the middle and upper Texas Gulf Coast, porosity exists at depth. Yegua sandstone porosity is intermediate between that of the Vicksburg and Wilcox sandstones. Vicksburg sandstones have the poorest reservoir quality of all the deep sandstones. Frio sandstones improve systematically in reservoir quality from the lower to the upper Texas Gulf Coast owing to grain composition and geothermal gradient.Wilcox sandstones are poorly to moderately sorted, fine-grained, quartzose lithic arkoses that become richer in quartz from the upper to the lower Texas Gulf Coast. Most rock fragments are metamorphic or volcanic. Yegua sandstones are moderately sorted, fine-grained, lithic arkoses to quartzose lithic arkoses. Volcanic and carbonate rock fragments are common along the lower Texas Gulf Coast, whereas volcanic and metamorphic rock fragments are common along the middle and the upper Texas Gulf Coast. Vicksburg sandstones are poorly sorted, fine-grained lithic arkoses. Rock fragments are mainly volcanic clasts containing lesser amounts of carbonate and minor amounts of metamorphic clasts. Frio sandstones range from poorly sorted, fine-grained, feldspathic litharenites to lithic arkoses along the lower Texas Gulf Coast to poorly sorted, fine-grained, quartzose lithic arkoses to subarkoses along the upper Texas Gulf Coast. Volcanic and carbonate rock fragments are common along the lower Texas Gulf Coast.Although they vary in composition, lower Tertiary sandstones exhibit similar diagenetic sequences that may be idealized as follows:Surface to shallow subsurface diagenesis (0 to 1,200 m+/- [0 to 4,000 ft+/-]) began with the formation of clay coats on framework grains, dissolution of feldspar, and replacement of feldspar by calcite. Minor amounts of kaolinite, feldspar overgrowths, and Fe-poor calcite were locally precipitated. Porosity was commonly reduced by compaction and cementation from an estimated original 40 percent to less than 30 percent. Intermediate subsurface diagenesis (1,200 to 3,400 m+/- [4,000 to 11,000 ft+/-], involved dissolution of early-formed carbonate cements and subsequent cementation first by quartz overgrowths and later by carbonate cement. Cementation may have reduced porosity to 10 percent or less, but this trend could have been reversed by later dissolution of feldspar grains, rock fragments, and possibly carbonate cements. Porosity was restored in some sandstones to more than 30 percent, but some porosity was later reduced by cementation by kaolinite, Fe-rich dolomite, and ankerite.Deep subsurface diagenesis (>3,350 m +/- [>11,000 ft+/-]) was a continuation of late-stage Fe-rich and Fe-poor carbonate cement precipitation. Plagioclase was albitized during this stage. Differences in intensity of diagenetic features that were related to changes in rock composition and geothermal gradient distinguish areas of high reservoir quality in the deep subsurface along the Texas Gulf Coast. Vicksburg and Frio reservoirs along the lower Texas Gulf Coast have extensive late-formed carbonate cements, whereas along the upper Texas Gulf Coast late-formed carbonate cements are minor. Wilcox reservoirs show no simple regional trend; quartz and carbonate are the dominant porosity-reducing cements, and their precipitation was governed by local chemical and physical conditions.
Loucks, R. G., Dodge, M. M., and Galloway, W. E., 1986, Controls on Porosity and Permeability of Hydrocarbon Reservoirs in Lower Tertiary Sandstones along the Texas Gulf Coast: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 149, 78 p.