Integrated Geologic and Geophysical Modeling of a Major Isolated Icehouse Carbonate Platform Reservoir SACROC Unit, Scurry County, Texas
Dr. Charles Kerans and Dr. Fred P. Wang
Bureau of Economic Geology
John A.& Katherine G. Jackson School of Geosciences
The University of Texas at Austin
SACROC unit forms the largest producing unit of the Pennsylvanian-age Horseshoe Atoll play in the foreland of the Fort Worth Basin. Since discovery in the 50's, primary, secondary, and tertiary recovery activities have been extensive, with over 1500 wells in this, the first CO2 flood in West Texas. In spite of this history, only the basics of the stratigraphic and petrophysical architecture are understood. This study is based on data from well log and core-based examination of the northern third of the unit including: 550 wells, 3500 ft of core, and 26 sq mi of 3-D seismic data.

The 700 ft thick reservoir column consists of Canyon and Cisco carbonates that change from layered cyclic open-shelf subtidal cycles with minimal diagenetic overprint (early and mid Canyon) to high-energy shoal-related cycles with frequent exposure surfaces (late Canyon-early Cisco) and increased evidence for cycle and sequence-scale erosion. Early Cisco deposition was characterized by dramatic changes in depositional style including growth of pinnacle reefs and the formation of complex fractured muddy crinoid-dominated facies that resemble Waulsortian deeper-water buildups. Seismic data was used extensively in constructing the stratigraphic framework and allowed significant advances in understanding of the stratigraphic architecture that were not possible with logs alone.

Seismic data were also fully integrated into 3-D geologic model through the following processes

  • Seismic data were first inverted to impedance and then porosity using a range of approaches including standard Hampson Russell software and neural-network technique.

  • Two geologic models with same layering were built in time and depth spaces. The seismic and inverted data cubes were first loaded in a 3-D time model, and then copied into the 3-D depth model.

  • 3-D porosity distributions were modeled using the seismically inverted porosity data constrained by 450 wireline logs.

The end result of this modeling effort, utilizing modern geologic, geophysical, and modeling practices, is a 3-D volume that is drastically different from that previously generated. Huge volumes of the platform previously modeled as layercake can be shown to consist of erosionally generated slope wedges associated with major icehouse eustatic falls. Complex promontories and reentrants similar to the present-day Bahama platform mark the edges of the field, and large windward-leeward asymmetries control reservoir quality distribution. This modern model of Sacroc should greatly aid ongoing efforts for enhanced recovery using WAG (water alternating CO2) processes and related practices.