Burial Dissolution of Anhydrite in Permian Reservoirs, Origin and Petrophysics

Floyd J. Lucia and Stephen C. Ruppel
University of Texas at Austin, Austin, TX

Burial dissolution and porosity enhancement is currently a popular concept for porous, deeply buried, limestone reservoirs. Whereas much of the evidence for burial dissolution of carbonates is questionable, a number of dolostone reservoirs in the Permian Basin display good evidence of significant anhydrite removal in the burial environment. Textural evidence indicates that significant volumes of nodular, poikilotopic, and pore filling anhydrite have been dissolved from at least four Permian dolostone reservoirs. Anhydrite removal creates an assortment of fabrics ranging from simple molds of anhydrite crystals and nodules to a complex fabric of small collapse features and open fractures. In general, these fabrics exhibit enhanced permeability values compared to a simple porous matrix model. In some cases, permeability is enhanced 100 fold. In two reservoirs nearly all anhydrite has been dissolved and in other reservoirs anhydrite has been selectively dissolved within what are interpreted to be paths of ground water movement. Invasion by fresher waters is suggested because the connate water in these fields has lower salinity than typically found in Permian dolostone reservoirs. The dissolving fluids are thought to be sourced from the Guadalupe Mountains to the west and from the Midland Basin to the East.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas