Paleostructural Control on Facies Distribution and Reservoir Quality: Seminole San Andres Unit (Permian, Guadalupian), West Texas

Mark D. Sonnenfeld,1 Laura C. Zahm,2 Grace L. Ford, K. Lyn Canter,3 Steven J. Buckner,2 Laura S. Foulk,2 Charles Kerans,4 Scott L. Pluim,5 Michele Simon,5 and Scott W. Tinker6

ABSTRACT

Ongoing reservoir characterization at Seminole San Andres Unit (SSAU), a mature CO2 flood just NE of the Central Basin Platform, has integrated 12,000 feet of core description (42 wells), 600+ well logs, and inverted 3-D seismic. Fifteen key facies range from subtidal-argillaceous dolomudstone and fusulinid wackestone to peritidal laminites and microkarst residuum. Principal reservoir facies are fusulinid dolowackstone, fusulinid-peloid grain-dominated dolopackstone, and coated-grain dolograinstone. A cycle-hierarchy consistent with the regional outcrop framework was defined first in 1-D and 2-D from cored wells, then tested through 3-D log and seismic correlation. Depositional facies’ control on reservoir properties persisted throughout the paragenetic sequence from primary porosity through later dolomitization. Consequently, facies distributions were mapped across the SSAU for most 5th-order cycles to detail platform evolution. Profound paleostructural control is demonstrated by alternating crestal and peripheral distributions of grain-dominated reservoir facies. 3-D seismic shows pronounced thickening of the lower San Andres on structure, reflecting keep-up sedimentation during late Leonardian sea-level rise. Upper San Andres progradation off paleostructure largely equalized differential topography and "attached" the Seminole and Central Basin Platforms. Within SSAU there is a northern, more coalesced paleohigh, whereas several NNE-SSW paleohighs segment the more complex southern area. Recognizing paleostructure at the onset of reservoir characterization yields the following: 1) more deterministic understanding of facies distributions; 2) an orientation guide for true depositional dip- and strike-oriented stratigraphic cross-sections requisite for high-resolution correlation; 3) preferred directions for facies and porosity distribution; and 4) cycle-based facies maps useful as interim field management tools during model-building.

1Marathon Oil Co., Littleton, CO 80160, phone: 303-697-3972, e-mail: mdsonnenfeld@aol.com,

2Marathon Oil Co,

3Applied Geoscience Inc, Boulder, CO,

4Bureau of Economic Geology, Austin, TX,

5Amerada Hess Corp, Houston, TX,

6Bureau of Economic Geology