Abstract: Porosity Development and Distribution in Shoal-Water Carbonate Complexes--Subsurface Pearsall Formation (Lower Cretaceous), South Texas

R. G. Loucks

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: (1) shoal-water carbonate complex and (2) 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, aragonite cement (now calcite), 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, finely crystalline equant to bladed rim, medium-crystalline equant, and syntaxial calcite, reflect an oxidizing water chemistry varying in magnesium and low in iron. Leaching of aragonite shells created moldic porosity. Magnesian calcite and aragonite grains were stabilized to calcite in this stage. Later with initial burial, medium crystalline to coarsely crystalline equant calcite cement, low in iro and magnesium, precipitated from a regional meteoric groundwater system. Finally, at depths below 2,000 ft (600 m), quartz overgrowths, anhydrite, iron-rich baroque dolomite, and iron-zoned coarsely crystalline equant calcite were precipitated. Hydrocarbons created a reducing environment. Iron and silicon were released during diagenetic dewatering of adjacent shale. The shale dewatering, along with stylolitization, produced an hydraulic pump that forced water through the rocks.

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

AAPG Search and Discovery Article #90967©1977 GCAGS and GC Section SEPM 27th Annual Meeting, Austin, Texas