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Average porosity from porosimeter analyses of Vicksburg sandstone core plugs from McAllen Ranch gas field is approximately 15%, but average porosity from point counts is only 6.5% due to exclusion of microporosity. Average permeability is less than 1.5 md. These low porosities and permeabilities are due to extensive diagenetic modification of a chemically unstable sand.
The Vicksburg Formation in Hidalgo County was deposited in a deltaic environment. Most of the reservoir sandstones are submature fine-grained lithic arkoses and feldspathic volarenites containing less than 30% quartz. Primary porosity was most commonly occluded by precipitation of authigenic minerals, predominantly calcite cement. The onset of extensive calcite precipitation occurred at relatively shallow burial depth, approximately 3,000 ft (915 m), in the Hidalgo County area. Carbonate cements are abundant in most samples to depths of more than 13,600 ft (4,145 m). Most porosity present is secondary porosity which formed by dissolution of cements and grains.
Diagenetic modification is much greater in Vicksburg sandstones of Hidalgo County than in other Tertiary sandstones of the Texas Gulf Coast. This is due to the chemically unstable mineral assemblage and the high thermal gradient in this area.
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