Volume of methane dissolved in formation waters in Tertiary sandstone reservoirs is directly related to sandstone volume and porosity, formation temperature and pressure, and salinity of formation fluids. Volume of potential reservoirs, located on regional cross sections below 8,000 ft (2,438 m; approximate base of oil production), was determined from isopach and net-sandstone maps of each formation. Plots show variation of formation pressure, temperature, and salinity with depth in all wells on cross sections. Porosity was determined by whole-core analyses, supplemented by sonic and resistivity logs.

The Texas Gulf Coast was divided into 24 subdivisions for detailed reservoir mapping and calculation of methane resource. Subdivision boundaries were defined by structural provinces and by major fault zones. Subdivision 2, an area of 5,300 sq mi (13,727 sq km) along the lower Texas coast, illustrates methods for evaluating the amount of methane dissolved in formation waters. In the lower Frio Formation, an in-place resource estimate of 61 quads (1 quad = 1 Tcf) was calculated by averaging parameters in 1,000-ft (305 m) intervals, a method similar to that used by the U.S. Geological Survey. A comparable estimate, 56 quads, was calculated by using parameters averaged for the entire lower Frio Formation. Total in-place methane of 144 quads for all formations in Subdivision 2 was determin d using the latter method. Similar evaluation of all subdivisions will define the entrained methane resource of the onshore Texas Gulf Coast.

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