ABSTRACT

Cored intervals of the late Paleocene Lobo sandstones of Webb County, Texas were studied between depths of 9066.5 and 9900.5 ft. (2763.5-3018 m). Diagenesis has extensively modified these sandstones, both compositionally and texturally. Porosities and permeabilities within the sandstones range from 6-28 percent and 0.01-10.0 md, respectively. Porosity and permeability are dependent on depositional facies and diagenetic history.

Density and sonic logs, coupled with petrographic analysis, were used to delineate the burial history of the sandstones. Primary porosities as high as 40 percent were reduced to 25 to 30 percent through a combination of compaction, bioturbation, and quartz cementation during the first 3000 ft. (914 m) of burial. Calcite cement, up to 26 percent, infilled essentially all remaining pore spaces, decreased porosity to irreducible levels between 3000-6500 ft. (914-1981 m). During deep burial secondary porosity was created by the dissolution of calcite cement and framework grains. This last stage of sandstone diagenesis appears to be in response to the generation of abnormally high fluid pressures in surrounding shale sections.