SEDIMENTARY PETROLOGY AND RESERVOIR CHARACTER OF A POST-EXTINCTION CARBONATE PLATFORM: THE EARLY TRIASSIC (SMITHIAN) SINBAD LIMESTONE MEMBER OF THE MOENKOPI FORMATION, UTAH

The Early Triassic (Smithian) Sinbad Limestone Member of the Moenkopi Formaton has produced minor amounts of oil in the Grassy Trail Creek Field near Green River, Utah and is present below much of central Utah, including the recently discovered Covenant Field. Superb outcrops of this thin (15 m), mixed carbonate-siliciclastic unit in the San Rafael Swell permit detailed analysis of its vertical and lateral reservoir heterogeneity. Vertically, the Sinbad comprises three grain-dominated facies associations: (A) a basal, storm-dominated, well-curculated skeletal-oolitic-automicritic limestone association, (B) a middle storm-dominated, restricted-marine hummocky cross-bedded peloid-siliciclastic association, and (C) a capping peritidal, cross-bedded oolitic-peloidal dolograinstone association. Lateral variation in lithology is most pronounced in the upper part of the lower limestone where storm-dominated beds pinch our over a distance of one kilometer. Otherwise, individiuual beds, microfacies, and hardground surfaces display a high degree of lateral homogeneity and regional persistence. The lack of mud and presence of automicrite in low-diversity grain-dominated packstones are likely the result of deposition under anachronistic conditions reflecting the aftermath of the end-Permian mass extinction. Diagenesis is strongly tied to microfacies in this mud-lean unit and is represented by fibrous marine calcite cement, micritization, compaction, dissolution and cementation of aragonitic grains, meteoric cement, and dolomitization. Hydrocarbon-lined vugs and interparticle pores characterize the skeletal limestones.