RESERVOIR CHARACTERIZATION OF THE LOWER TRIASSIC SINBAD LIMESTONE MEMBER OF THE MOENKOPI FORMATION BASED UPON SURFACE EXPOSURES IN THE SAN RAFAEL SWELL, UTAH

The Lower Triassic Sinbad Limestone Member of the Moenkopi Formation 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 (roadcut panels and closely spaced stratigraphic sections) of this thin (15 m), mixed carbonate-silicilastic unit in the San Rafael Swell permit detailed analysis of its vertical and lateral reservoir heterogeneity. Vertically, the Sinbad comprises three subunits (basal skeletal-oolitic limestone, middle fine-grained hummocky cross-stratified siliciclastic interval, capped by cross-bedded oolitic dolograinstone) reflecting a single transgressive-regressive cycle. Hydrocarbon-lined interparticle and separate vug (largely molds) pores (1 to 5%) characterize the skeletal-oolitic limestones with permeability ranging from 0 to 100 md. Marine and meteoric cements have occluded much of the depositional porosity. Low permeability and low porosity characterizes the middle silicilastic unit. The best reservoir qualities (permeability up to 400 md) occur in portions of the dolomitized oolitic grainstones that form the upper 2 to 3 m of the Sinbad Limestone. Lateral variation is most pronounced in the middle siliciclastic unit where offshore shale grades laterally into hummocky cross-stratified sandstone. Individual beds and microfacies within the basal limestone and sequence-capping dolostone display a large degree of lateral homogeneity and regional persistence. This study will not only aid in future Sinbad exploration, but will serve as a model for parasequence-scale intervals that might form components of larger reservoir volumes.