ABSTRACT

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 9300 to 15,000 ft (2835 to 4572 m). Depositional patterns were controlled by a diapiric shale uplift and related faults. Petrographic analyses show that primary porosity was reduced during early diagenesis by calcite cementation. However, dissolution of calcite cement, feldspar, and volcanic rock fragments, which took place 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 individual 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 and pore lining probably aided in preservation of porosity by inhibiting cementation.

Highest porosites and permeabilities are found where the sandstones have the highest secondary porosities, as determined by petrographic study. Porosity increases from about 15 percent to as much as 20 percent in the section from 9642 to 12,586 ft (2939 to 3836 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.