Factors controlling reservoir quality in tertiary sandstones and their significance to geopressured geothermal production. Annual report, May 1, 1979-May 31, 1980.

Differing extents of diagenetic modification is the factor primarily responsible for contrasting regional reservoir quality of Tertiary sandstones from the Upper and Lower Texas Gulf Coast. Detailed comparison of Frio sandstones from the Chocolate Bayou/Danbury Dome area, Brazoria County, and Vicksburg sandstones from the McAllen Ranch Field area, Hidalgo County, reveals that extent of diagenetic modification is most strongly influenced by (1) detrital minerology and (2) regional geothermal gradients. Vicksburg sandstones from the McAllen Ranch Field area less stable, chemically and mechanically, than the Frio sandstones from the Chocolate Bayou/ Danbury Dome area. Vicksburg sandstones are mineralologically immature and contain greater proportions of feldspars and rock fragments than do Frio sandstones. The reactive detrital assemblage of Vicksburg sandstones is highly susceptible to diagenetic modification. Susceptibility is enhanced by higher than normal geothermal gradients in the McAllen Ranch Field area. Thus, consolidation of Vicksburg sandstones began at shallower depth of burial and precipitaion of authigenic phases (especially calcite) was more pervasive than in Frio sandstones. Moreover, the late-stage episode of ferroan calcite precipitation that occluded most secondary porosity in Vicksburg sandstones did not occur significantly in Frio sandstones. Therfore, regional reservoir quality of Frio sandstones from Brazoria County is far better than that characterizing Vicksburg sandstones for Hidalgo County, especially at depths suitable for geopressured geothermal energy production.