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).

This report summarizes geologic, engineering, and production information on 24 low-permeability, gas-bearing sandstone reservoirs in 13 basins in the United States. The information compiled in this report comes from publicly available sources: Gas Research Institute and Department of Energy project reports, applications by gas producers to state regulatory agencies for tight formation designation, and published literature.

In a five-county area of South Texas, geopressured-geothermal reservoirs in the Paleocene-Eocene Wilcox Group lie below medium- to heavy-oil reservoirs in the Eocene Jackson Group. This fortuitous association suggests the use of geothermal fluids for thermally enhanced oil recovery (TEOR). Geothermal fairways are formed, where thick deltaic sandstones are compartmentalized by growth faults. Wilcox geothermal reservoirs in South Texas are present at depths of 11,000 to 15,000 ft (3,350 to 4,570 m) in laterally continuous sandstones 100 to 200 ft (30 to 60 m) thick.