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Grayburg Formation Reservoir-Scale Architecture and Sequence Stratigraphy, Permian Basin


A regional study was conducted on the Lower Middle Permian (Guadalupian) Grayburg Formation, a mixed carbonate (dominant)-siliciclastic (subordinate) composite sequence. The Grayburg was studied in the: 1) type section, Northwest Shelf; 2) Guadalupe Mountains; 3) Apache Mountains; 4) Eunice Monument complex of unitized oil fields, northwest Central Basin Platform (CBP); and 5) McElroy field, southeast CBP. All of these areas have been correlated in strike and dip view. Reference sections have been proposed for each area. Grayburg Formation is composed of six proposed members, which in stratigraphic order from base to top are: 1) Premier Sandstone Member; 2) Lower Dolostone Member; 3) Metex Dolostone and Sandstone Member; 4) Loco Hills Sandstone Member; 5) Upper Dolostone Member; and 6) Stone Canyon Dolostone and Sandstone Member. Hierarchy of Grayburg Formation sequence stratigraphy is: 1) one composite sequence; 2) two simple sequences; 3) 10 high frequency sequences; 4) 21+ cycle sets; 5) 82+ cycles; 6) numerous beds/lamina. Bedding, cyclicity, and rock types change systematically down-dip to up-dip via changes in accommodation space, wave/storm energy, and tectonics. High resolution sequence stratigraphy of one apparent cycle, utilizing 20 closely spaced measured sections in the Guadalupe Mountains, identified five shingled cycles from up-dip to down-dip in a distance of 3,333 m (10,935 ft), which identified more cyclicity than individual vertical measured sections suggest. Deposition was upon a tectonically modified, distally steepened ramp in an: 1) inner ramp (non-reservoir–lateral seal); 2) ramp crest shoal (reservoir); 3) middle ramp (reservoir), and outer ramp (non-reservoir) depositional setting. The inner ramp was evaporitic. The ramp crest contains peloid-ooid dolopackstone-dolograinstone. The middle ramp contains fusulinid-poor and fusulinid-rich strata. The outer ramp contains solitary sponges, small sponge clusters (1 m, 3 ft), and massive sponge bafflestone. Most of the outer ramp and part of the middle ramp was eroded during post-Grayburg Formation subaerial exposure in the Delaware Basin. Sequence stratigraphy has been found to be highly useful in helping solve day-to-day production-related problems in Grayburg Formation oil fields.