Abstract

Permian shallow-water carbonate reservoirs in the Permian Basin are highly heterogeneous because of complex variations in depositional facies produced by high-frequency sea-level rise and fall. Accordingly, establishment of a cycle stratigraphic framework is fundamental to defining reservoir heterogeneity in these rocks. Because nearly all of these reservoirs have experienced multiple episodes of dolomitization and sulfate emplacement, however, permeability is also a function of diagenetic overprint. The extent to which di-agenesis can affect permeability development is dramatically displayed in the Grayburg Formation (middle Permian) at South Cowden field, West Texas.

Three scales of cyclicity contribute to original depositional facies heterogeneity in the Grayburg; high-frequency cycles, averaging 3 meters in thickness, constitute the fundamental architectural element in the main reservoir interval. Despite original depositional heterogeneity due to this cyclicity, however, permeability development has substantially resulted from two diagenetic events: (1) dolomite alteration in vertically burrowed wackestones and packstones and (2) late alteration and removal of anhydrite.

Dolomite alteration in vertically burrowed wackestones and packstones has produced irregular vertical zones of higher permeability in mud-dominated bases of high-frequency cycles in leeward ramp-crest highstand successions. Because dolomite alteration is concentrated in burrowed highstand successions, the distribution of resultant permeability trends is partly constrained by patterns of long-term accommodation and high-frequency cyclicity. Anhydrite diagenesis, which is characterized by conversion to gypsum or by complete removal of sulfate, is developed along basinward margins of the field and crosscuts original depositional framework.

Altered dolomite in burrowed zones exhibits an average of twice the porosity and an order of magnitude greater permeability than adjacent unaltered rocks because of rock-fabric differences. Especially where sulfate removal has occurred, altered zones contain intercrystalline and touching-vug pores in contrast to intercrystalline and separate-vug pores characteristic of unaltered zones. These rock-fabric types also display distinctly different porosity-permeability relationships.

Pore types can be traced across the field by comparing acoustic and neutron-density porosity logs. Once rock-fabrics are delineated, appropriate porosity-permeability transforms can be applied to predict permeability across the reservoir. Mapping of zones of anhydrite diagenesis and associated permeability development by means of cores and acoustic log porosity signatures demonstrates that alteration and removal of sulfate and attendant permeability enhancement is highly irregular at the interwell scale.