Bedded Gypsum in the Edwards Group, Texas: Its Role in Reducing Porosity and Enhancing Permeability

Susan D. Hovorka

The Lower Cretaceous Edwards Group was deposited in shallow subtidal, grainstone-bar, and supratidal environments on the San Marcos Platform and slightly deeper, entirely subtidal environments in the Maverick Basin. On the platform, facies relationships suggest that bottom-grown gypsum precipitated in shallow, extensive brine pools that had developed behind grainstone bars during gradual sea-level rise. In the basin, sedimentary structures in nodular anhydrite that is interbedded with dark, laminated carbonate mudstone suggest that gypsum formed as cumulates and bottom-grown crusts in an intermittently density stratified water column. Euxinic carbonates accumulated beneath stratified marine-over-hypersaline waters. Evaporation, density equalization, and overturn allowed g psum to precipitate on the bottom.

On the platform, gypsum has been entirely removed by calcitization and dissolution. Poikilotopic calcite spar fills porosity in grainstones associated with former gypsum. Calcite partly replacing gypsum crystals creates vuggy porosity and low permeability. Collapse breccia that formed by gypsum dissolution has calcite-occluded porosity, but conduit permeability is moderate to high. Fabrics in brecciated and calcitized gypsum are similar across all platformal facies, irrespective of burial history or structural setting. Gypsum removal probably occurred in one or more Cretaceous fresh-water flushing events, either syndepositionally or at the top-Edwards sequence boundary.

In the basin, anhydrite is preserved in the deeper subsurface. Several episodes of gypsum dissolution and brecciation before and after asphalt migration can be identified. Local silicification and calcitization of sulfates preserve textures; otherwise, gypsum has been removed, and the permeability of the overlying strata has been increased by fracturing.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995