RUPPEL, STEPHEN C., Texas Bureau of Economic Geology, The University of Texas at Austin, Austin, TX; and C. L. HEDRICK and S. L. DOROBEK, Department of Geology, Texas A&M University, College Station, TX

ABSTRACT: Dolomitization and Porosity Evolution in Shallow-Water Carbonates: The Role of High-Frequency Sea-Level Fall

It has long been suggested that the commonly observed relationship between the occurrence of dolomite and porosity in carbonate hydrocarbon reservoirs is due to porosity gain associated with mole-for-mole replacement by dolomite. Although the efficacy of this mechanism has been recently challenged, no alternative processes have been advanced. In highly cyclic, Leonardian (Lower Permian) reservoir successions in the Permian Basin of West Texas, there is evidence that porosity evolution is intimately related to dolomitization, but that both are a function of diagenesis associated with repeated fall in relative sea level.

The Leonardian of the Permian basin comprises approximately 700 m of highly cyclic, shallow-water carbonate sediments. At least three orders of cyclicity can be recognized in these rocks, ranging from thin, high-frequency, fifth-order (meter scale) cycles to intermediate, fourth-order (10-20 m) cycles, to thick (100-200 m), third-order cycles. Porosity is greatest fourth-order cycle tops and decreases downsection in both tidal-flat capped and subtidal-capped, upward-shallowing cycles. In incompletely dolomitized sections, dolomite is similarly concentrated immediately below tops of fourth-order cycles and decreases down section. Combined facies and geochemical evidence suggest that fourth-order cycle tops underwent early diagenesis during sea-level lowstand. Cycle tops on paleotopogra hic highs are capped with tidal-flat sediments indicative of subaerial exposure, and karsting is locally common. Stable isotopes of both carbon and oxygen suggest isotope depletion immediately below cycle tops and increasing enrichment downsection.

The preferential development of both porosity and dolomite at the tops of fourth-order, upward-shallowing cycles argues for episodic early dolomitization associated with early meteoric diagenesis at sea level lowstand. These data neither support nor exclude mole-for-mole replacement as a contributing mechanism in porosity evolution. They do illustrate, however, that early dolomitization associated with episodic sea-level fall may play an important role in the development of porosity in shallow-water carbonate platform successions.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.