Abstract

Many existing and potential hydrocarbon reservoirs are present in the Permian Basin in Upper Pennsylvanian carbonates deposited in former slope-to-basin settings during sealevel lowstands, and in Lower Permian (Wolfcampian and Lower Leonardian) resedimented periplatform carbonates (debris-flow breccias and grainflow sands) deposited during sealevel highstands. Despite differences in their timing and depositional origin, however, styles of diagenesis and porosity evolution are remarkably similar in these strata. Canyon and Cisco lowstand reservoirs south of the Horseshoe Atoll, for example, typically compose reworked limestone-clast breccias and in-situ banks (reefs, shoals). Porosity is represented by vugs in limestone clasts, and by allomolds, interparticle pores, and vugs in the autochthonous limestones; dissolution-enlarged fracture porosity also is present. These pores may have formed during early post-depositional subaerial exposure of the rocks. Many of these pores, however, contain leached saddle dolomite, which indicates that they were once at least partially occluded and that renewed dissolution post-dated dolomite precipitation. Petrographic observations and oxygen isotope values of the dolomite indicate a mesogenetic timing of this last dissolution event. In contrast, at least initial dolomitization of most Lower Permian resedimented carbonate reservoirs occurred syndepositionally and during early burial, wherein porosity was not created, and originally porous limestone breccias and sands underwent porosity obliteration during progressive burial as a result of compaction and cementation. Porosity formation in these limestones and replacive dolomites also resulted from late dissolution in the mesogenetic environment. Timing of porosity formation can be determined through oxygen isotope thermometry.