ABSTRACT

The Lower Cretaceous Pearsall Formation in the subsurface of South Texas consists of contemporaneous carbonate and terrigenous clastic facies deposited in two major depositional systems: shoal-water carbonate complex and open, shallow-water shelf. Carbonate facies that were deposited in the high-energy environments are porous grainstones and boundstones. These are surrounded by a halo of nonporous lower energy packstones and wackestones. The open shelf contains more terrigenous material than the carbonate shoal. Dominant facies on the shelf are oncolite packstone, terrigenous mudstone and shale and mottled to interbedded carbonate wackestone and terrigenous mudstone.

Four stages of diagenesis are recognizable in Pearsall carbonate grainstones. Micrite envelopes, former aragonite cement, and broken grains are the dominant features in the first diagenetic stage which occurred in the marine environment. The next stage of diagenesis took place in a series of local meteoric-phreatic environments produced by partial subaerial exposure. The cements, fine-crystalline equant to bladed rim, medium-crystalline equant, and syntaxial calcite, indicate an oxidizing water chemistry varying in Mg and low in Fe. Leaching of aragonite shells created moldic porosity. Mg-calcite and aragonite grains stabilized to calcite in this stage.

Later with initial burial, medium- to coarse-crystalline equant calcite cement, low in Fe and Mg, precipitated from a regional meteoric ground-water system. Finally, at depths over 2,000 feet, quartz overgrowths, anhydrite, Fe-zoned baroque dolomite and Fe-zoned coarse-crystalline equant calcite were precipitated. Hydrocarbons created a reducing environment. Dewatering of juxtaposed shale released Mg and Si and along with stylolitization, produced a hydraulic pump pushing water through the rocks.

Approximately 95 percent of the high porosity and permeability is contained in the grainstone and boundstone facies; therefore, distribution of the porosity in the Pearsall Formation can be predicted from mapping depositional facies. Two major forms of porosity are primary interparticle and secondary moldic porosity.

1. Published with permission of the Director, Bureau of Economic Geology, The University of Texas at Austin

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