Gas Reservoir Quality Variations and Implications for Resource Development, Frio Formation, South Texas: Examples from Seeligso

Discovery of volcanic glass detritus in fluvial deposits of the middle Frio Formation (Seeligson and Stratton fields) of South Texas has resulted in a new classification of reservoir types (type I and type II) within this formation. Type I reservoirs consist of sandstones that do not contain volcanic glass and have an average permeability of 36 md (geometric mean). Although calcite cement can strongly inhibit permeability in type I reservoirs, the development of secondary porosity by the dissolution of feldspar, volcanic rock fragments, and calcite cement has led to a well-developed intergranular porosity. Type II reservoirs, which are present in the medial third of the middle Frio Formation, consist of sandstones that contain volcanic glass detritus and volcanic ash matrix. These sandstones have a poorly developed intergranular pore system and an average permeability of 2.6 md (geometric mean). Secondary porosity (moldic and intragranular) may be well developed in these sandstones, but the bridging of intergranular pore throats by calcite and analcime cements and by matrix effectively isolates secondary pores and reduces permeability. Permeability is further reduced by the filling of secondary pores by mixed-layer chlorite-smectite (corrensite). Volcanic glass in the medial third of the middle Frio Formation is detectable on gamma-ray logs by elevated API counts in both sandstones and mudstones. Because alteration of volcanic glass strongly affects permeability, gamma-ray log response can be used to assess resource potential in the middle Frio Formation of South Texas. In addition, the Frio interval containing volcanic glass can be used as a chronostratigraphic unit (tentatively thought to represent the 30- to 34-mya eruptive episode in the Trans-Pecos region) for interfield correlation in fluvial deposits.