Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
West Texas Geological Society Fall Symposium, Midland, Texas, October 25-26, 2001
Reservoir Architecture at SACROC and the CanyonCisco Transition
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. Here I contrast the fundamentally different reservoir styles of layered Canyon shelf strata with the fractured and possibly karst-modified Cisco section.
This study is based on data from well log and core-based examination of the northern third of the unit including 550 wells and 3500 ft of core. 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.
The transition from shoal/exposure-capped Late Canyon depositional cycles to mud-dominated crinoidal facies with associated pinnacle development is consistent with the gradual backstepping and eventual drowning of the Horseshoe Atoll in the SACROC area. This is consistent with several studies that have suggested that the irregular upper surface topography of the platform is not erosional. However, the extensive fracturing and newly defined abrupt facies changes are best viewed in terms of subaerial erosion. These different interpretations yield important differences in reservoir property distribution and must be solved before confident 3D models are constructed.
Kerans, Senior Research Scientist