Competing Effects of Depositional Architecture and Diagenesis on...Reservoir Development: Grayburg...South Cowden Field, West T

In this study, 28 cores were used to define the depositional facies, cyclicity, and sequence stratigraphy of a typical upper Guadalupian carbonate platform reservoir in the Permian Basin. Integration of facies and petrophysical data, however, demonstrates that reservoir quality is largely a function of late diagenetic overprinting of depositional facies attributes by dolomite diagenesis and anhydrite alteration and removal. As in most carbonate successions, reservoir development in the Grayburg Formation (Guadalupian, Permian) in South Cowden field is the result of a complex interplay between depositional and diagenetic processes. Although some diagenesis follows depositional architecture, much of the postdepositional alteration of these shallow- water carbonates crosscuts facies, resulting in a hybrid of depositionally and diagenetically controlled rock fabrics and flow units. Core and wireline log study shows that the Grayburg at South Cowden field is the result of deposition at three scales of cyclicity. The Grayburg constitutes part of a single long-duration accommodation cycle that began with a major sea-level rise and flooding of the preexisting San Andres platform and ended with a major basinward shift in facies and deposition of overlying Queen Formation tidal-flat successions. Within the Grayburg part of this long-duration cycle, four higher frequency sea-level- rise events, termed high-frequency sequences (HFS), can be recognized. Each of these is characterized by extensive flooding of the platform by outer ramp fusulinid-rich facies and subsequent highstand aggradation of grain-dominated peloidal facies. Platform flooding at the base of HFS 3 documents a regional sea-level maximum flooding event that is correlatable throughout the Permian Basin in both outcrop and the subsurface. High-frequency cyclicity is variably expressed in the Grayburg, reflecting variations in platform accommodation and delivery of siliciclastics. In transgressive lower Grayburg sequences (HFS 1, HFS 2), cyclicity is poorly displayed because of lack of internal organization in outer ramp fusulinid-dominated succession and facies amalgamation in high-energy windward ramp-crest successions. Highstand sequences (HFS 3, HFS 4), on the other hand, display well-developed, highly correlative, high-frequency cycles because of low platform relief and sufficient facies contrast between base-of-cycle mud-dominated facies and cycle-capping peloid/ooid grain-dominated deposits of the leeward ramp crest. Although the entire formation has been altered by multiple episodes of diagenesis, two major diagenetic events have exhibited strong local controls on reservoir character: dolomitization and sulfate alteration and removal. Tan, isotopically depleted dolomite is locally common along vertical burrows in highly cyclic mud-dominated packstones and wackestones of the HFS 4 Grayburg highstand succession. Later alteration and removal of anhydrite is focused in structurally low sections along the eastern and southern (downdip) margins of the field. Where these diagenetic zones overlap, original depositional heterogeneities are muted.Tan dolomite exhibits an average of twice the porosity and an order of magnitude greater permeability than adjacent gray-brown rocks because of rock-fabric differences. Especially where sulfate removal has occurred, tan zones contain intercrystalline and touching-vug pores in contrast to separate-vug pores characteristic of unaltered zones. These rock-fabric types display distinctly different porosity-permeability relationships. Highest permeabilities and continuity are developed along the margins of the field where sulfate diagenesis has created an extensive system of touching vugs and interconnected pores. In this part of the field, depositional framework has a limited impact on reservoir architecture and flow. By contrast, the interior of the field is characterized by intercrystalline and separate-vug pores whose distribution is strongly controlled by depositional facies framework.