Abstract: Relation of Early Carbonate Porosity to Depositional and Diagenetic Facies, Subsurface Cow Creek Limestone (Lower Cretaceous), South Texas

Robert G. Loucks

The Cow Creek Member of the 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 shallow open 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 material. Dominant facies on the shelf are oncolite packstone, terrigenous mudstone and shale, and mottled to interbedded carbonate wackestone and terrigenous mudstone.

Porosity distribution in the Cow Creek is controlled mainly by depositional facies; approximately 95 percent of the porosity is in the grainstones and boundstones. In the first stage of marine diagenesis primary interparticle and intraparticle porosity was as high as 60 percent. Early meteoric-phreatic diagenesis reduced primary porosity by cementation with calcite. However, abundant secondary moldic porosity was produced by the dissolution of unstable allochems. In the deeper subsurface diagenetic environment (more than 1,500 ft or 457 m) both the primary and secondary porosities were cement-reduced slightly by quartz, dolomite, and anhydrite. The remaining porosity is as high as 30 percent with related permeabilities up to 175 md.

AAPG Search and Discovery Article #90972©1976 AAPG-SEPM Annual Convention and Exhibition, New Orleans, LA