ABSTRACT

Two distinct reservoir types (type I and type II), differentiated on the basis of framework mineralogy, diagenetic history, and reservoir quality, are present in the Oligocene middle Frio Formation in Seeligson and Stratton fields, South Texas. Type I reservoir sandstones are feldspathic litharenites to litharenites having an average composition of Q₃₀F₂₅L₄₅. Lithic fragments are predominantly volcanic and carbonate rock fragments. These sandstones underwent early calcite precipitation, after which secondary porosity developed (dissolution of feldspar, rock fragments, and calcite cement). Kaolinite or chlorite and pyrite postdate secondary porosity and locally influence porosity and permeability trends. Porosity averages 20% and ranges between 7% and 28%. Permeability averages 400 md and ranges between 0.007 md and 4,000 md. The large variation in porosity and permeability in these gas reservoirs is directly related to the irregular distribution of calcite cement and the development of secondary porosity.

Type II reservoir sandstones, which occur in the medial third of the 2,000- to 2,500-ft thick (600- to 760-m) middle Frio Formation, are lithic arkoses to feldspathic litharenites having an average composition of Q₂₈F₃₁R₄₁. Lithic fragments are predominantly volcanic rock fragments; carbonate rock fragments are rare. These sandstones contain abundant unabraded to slightly abraded glass shards and volcanic ash matrix. Devitrification of volcanic glass detritus has resulted in analcime precipitation, which precedes and overlaps in time with an early calcite cement phase similar to that found in type I reservoirs. Secondary porosity, which developed through the preferential dissolution of volcanic glass and feldspar, postdates these cementation events and has resulted in the development of moldic and intragranular porosity that can be as high as 18%. Mixed-layer illite-smectite lines much of the secondary pores. Porosimeter porosity averages 18% and ranges between 5% and 27%. Permeability averages 5.5 md and ranges between 0.005 md and 78 md. Although average porosity in type II reservoirs is similar to that in type I reservoirs, much of it is ineffective. The precipitation of calcite and analcime in intergranular pores and illite-smectite in secondary pores, combined with abundant volcanic ash matrix, results in poor communication between pores and a corresponding decrease in reservoir quality.