Lower Vicksburg sandstones (Oligocene) at McAllen Ranch field in Hidalgo County, Texas, consist of thin sandstones interbedded with shales. The sands were deposited by turbidity currents as channel and overbank deposits. The sandstones produce gas from depths of 9,300 to 15,000 ft (2,800 to 4,500 m). Depositional patterns were controlled by a diapiric shale uplift and related faults. Petrographic analyses show that primary porosity was reduced during early diagenesis owing to calcite cementation. However, dissolution of calcite cement, feldspar, and volcanic rock fragments, which occurred after deep burial, led to secondary porosity development. Dissolution is evidenced by the formation of intergranular porosity, oversized pores, grain molds, and by microporosity within i dividual grains. Dissolution was followed by precipitation of quartz overgrowths, formation of authigenic clay minerals (kaolinite, chlorite, illite, smectite, and vermiculite), and by precipitation of iron-rich calcite cement. Scanning electron microscopy confirms that clay minerals are primarily authigenic and uniformly distributed. Chlorite grain coatings

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and pore lining probably aided in preservation of porosity by inhibiting cementation.

Highest porosities and permeabilities are found where the sandstones have the highest secondary porosities as determined by petrographic study. Porosity increases from about 15% to as much as 20% in the section from 9,642 to 12,586 ft (2,939 to 3,836 m). This increase is related to the abnormally high pressure gradient of about 0.92 psi/ft (20.8 kPa/m) and to an elevated geothermal gradient of about 2°F/100 ft. Optimum reservoir properties are present where late stage cementation by clays and iron-rich calcite has not been extensive.

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