Lower Silurian Medina Group reservoir rocks at Athens and Geneva fields in Crawford County, Pennsylvania, consist of very fine-, fine-, and medium-grained red and gray sandstones. The sandstones were deposited as bars and tidal deltas in a transitional marine setting. The sandstones produce gas from depths of 4,650 to 5,000 ft (1,395 to 1,500 m). The productive intervals are characterized by low average porosity and permeability (4.0% and <0.1 md) and low reserves.

Petrographic analyses show that primary porosity was extensively reduced during burial diagenesis by the precipitation of quartz and feldspar overgrowths. This stage of chemical compaction resulted in the reduction of intergranular porosity to irreducible lamellar porosity in the very fine to fine-grained intervals. These intervals also functioned as an extra stratal source of dissolved silica which precipitated as pore-filling cement in adjacent medium-grained intervals. Silica cementation was followed by the formation of authigenic clays, which diminished any remaining effective intergranular porosity.

Secondary porosity developed after deep burial. A variety of secondary sandstone pore textures are present in the Medina reservoirs at Athens and Geneva fields. Fracturing and grain shrinkage, coupled with remaining lamellar porosity, provided adequate passage for leaching brines formed during the generation of hydrocarbons in the adjacent shales. Additional secondary porosity formed through dissolution of sedimentary material and authigenic cement. Dissolution is evidenced by oversized and elongate pores, corroded grain margins, inhomogeneous packing, and microporosity within individual grains and cement. Some secondary porosity was subsequently reduced by the precipitation of carbonate and anhydrite cements concomitant with the entrapment of hydrocarbons. Secondary porosity was furt er reduced by grain alteration in the feldspathic intervals, as evidenced by a well-developed replacement fabric in these zones.

Adequate porosity for commercial production is found where the sandstones have the highest secondary porosities as determined by petrographic examination. Optimum reservoir development occurs where late-stage cementation by carbonate, anhydrite, and alteration product clays has not been extensive.

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