Alluvial architecture and predictive modeling of the Late Cretaceous John Henry member, Straight Cliffs formation, southern Utah

Five detailed measured sections (each >175 m), two outcrop gamma ray logs, 2100 paleocurrent measurements, and detailed facies interpretation of high-resolution composite photographs were used to document the vertical and lateral variations of the alluvial architecture of the John Henry Member of the Upper Cretaceous Straight Cliffs Formation (southwest Kaiparowits Plateau, southern Utah). These data interpretations correlate to 24 measured sections and logged cores from previous work in fluvial and paralic facies in the John Henry Member and complement previous correlation to marginal marine facies ∼60 km basinward of the study area. The proximal fluvial strata of the ∼230 m-thick member is divided into seven depositional units (DU) from stratigraphic base to top: DU-0, tidally-influenced channel belts; DU-1, highly amalgamated, laterally accreting channel belts with tidally-influenced channel belts in the upper section; DU-2, laterally extensive (>1.2 km wide), laterally accreting channel belts; DU-3, isolated channel belts (<150 m) embedded within thick floodplain muds; DU-4, clusters of laterally restricted (<500 m), laterally accreting channel belts and isolated channel belts; DU-5, laterally extensive, locally amalgamated, downstream accreting channel belts; and DU-6, highly amalgamated, downstream accreting channel belts. An unconformable surface at the base of DU-1 demonstrates a basinward shift in facies and is interpreted to represent erosion during base level fall. DU-0 correlates basinward to tidally-influenced channel belts and coastal coal mires in the south-central Kaiparowits Plateau that are coeval with the landward extensive, net progradational shoreface sandstones. An unconformity bound, fining upward trend limited to DU-1 through DU-3 in the western plateau is broadly consistent with previous interpretations of a northeast trending incised valley system and fill, which is correlative with a basinward shift and subsequent net transgression of shoreface sandstones. A coarsening upward trend from DU-4 through the overlying Drip Tank Member is consistent throughout alluvial facies of the entire John Henry Member and corresponds to net progradation of landward extensive shoreface sandstones. Reservoir characterization based on grain size, average porosity, net to gross estimates, channel belt size, internal connectivity (heterogeneity) and channel belt connectivity show that these defining trends in fluvial architecture (fining upward and coarsening upward) correspond to trends in reservoir analog quality (upward decreasing and upward increasing) and demonstrate that revised depositional models of alluvial architecture may be used to predict reservoir quality.