Stratigraphic and Diagenetic Controls on Reservoir Architecture of a Non-Reefal Icehouse Isolated Platform—Sacroc Unit, Horseshoe Atoll, Texas

C. Kerans
Bureau of Economic Geology, The University of Texas at Austin, Austin, TX

The detailed cycle- and sequence-scale architecture of icehouse carbonate platforms and associated flow units is one of the least well understood and documented carbonate reservoir styles. The Sacroc Unit of the greater Horseshoe Atoll, Permian Basin, provides an excellent example of such an icehouse system. Extensive data for this unit, including more than 1600 wells, several thousand feet of core, and an extensive production history from primary through waterflood and CO₂, makes this field ideal for documenting icehouse architecture. This giant field produces oil from both Canyon and Cisco (latest Pennsylvanian through earliest Permian) reservoirs that include a spectrum from layered, conformable flow units, to karstcontrolled non-stratiform flow paths.

Major stratigraphic drivers for the Permo-Pennsylvanian section at Horseshoe Atoll are (1) rapid supersequence-scale (2nd order) tectonic subsidence associated with development of the Ouachita- Marathon foreland basin, (2) composite and high-frequencysequence scale eustacy that caused bundling of strata into sequences separated by major episodes of exposure, karsting, and erosion, (3) moderate-amplitude eustatic shifts at the cycle-scale (5th order?) producing cycles that fill accommodation and result in organized layered reservoirs, and (4) cycle-scale (5th order?) eustatic events with high-amplitude (>10 m) high-frequency events where accommodation space is rarely filled and strong exposure and leaching of cycle tops is characteristic.

The effect of the second-order driver is a repeated backstepping of the platform from Strawn through Canyon and finally Cisco pinnacle development. Prolonged exposure in the middle Cisco termi nated platform growth at least locally in the Horseshoe Atoll. Exposure/erosion at sequence boundaries produced a series of surfaces of truncation, with local development of lowstand/transgressive wedges on the flanks of the platform. The effects of these major erosion surfaces on fluid flow and compartmentalization is still uncertain. Early-middle Canyon high-frequency-cycle-scale eustatic shifts produced systematic upward-coarsening tight-to-porous cycles that define strongly layered reservoir heterogeneity by stratigraphically controlled permeability pathways that appear continuous throughout the reservoir. Late Canyon and early Cisco highfrequency cycles show increased eustatic amplitude with greater unfilled accommodation, local exposure, cycle-scale karstification, and development of abundant moldic and touching-vug porosity. Future development of this reservoir should be carried out using a static model that incorporates these fundamental stratigraphic and diagenetic heterogeneity styles in order to best model sweep efficiency and remaining hydrocarbon saturation.

AAPG Search and Discovery Article #90905©2001 AAPG Southwest Section Meeting, Dallas, Texas