--> Impacts of Depositional Processes on Reservoir Quality in Tidally-Influenced Lithofacies of the Upper Cretaceous John Henry Member, Straight Cliffs Formation, Southern Utah

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Impacts of Depositional Processes on Reservoir Quality in Tidally-Influenced Lithofacies of the Upper Cretaceous John Henry Member, Straight Cliffs Formation, Southern Utah

Abstract

Sand bodies deposited along tidally-influenced shorelines can ultimately be reservoirs for significant hydrocarbon resources. When compared to more proximal fluvial or more distal shoreface deposits, tidal deposits are found to be more heterolithic. This is a result of transport and deposition of sand grains during flood and ebb conditions and silt or finer sediment during slack tides. Due to the variable energy of tidal deposits, porosity and permeability are reduced, particularly in the vertical direction as a result of interlaminated sands and silts. Few integrated thin section to outcrop studies of tidal deposits have addressed reservoir quality of tidal facies as a result of depositional processes. This study presents a unique core to outcrop scale study from a paralic section in the Upper Cretaceous John Henry Member of the Kaiparowits Plateau. An in-depth core study, including benchtop measurements on core plugs coupled with a cm-scale core description and thin-section analysis was performed to study and quantify the heterolithic nature of paralic deposits from the grain scale up to outcrop scale (e.g., reservoir quality and distribution). Borehole cores were described at cm-scale resolution and interpreted within a hierarchical framework to capture and describe the sampled rock properties within a predictive, multi-scale depositional framework. Lithofacies categories are the basis for the core description and represent observations at the smallest scale (based on grain size, sedimentary structures, ichnofacies, etc.). Lithofacies are grouped into lithofacies assemblages (stacked beds of lithofacies) which are in turn evaluated for size, shape, and stacking patterns and interpreted as depositional environments. Sampling within this framework, core plug samples were taken and used to measure P-wave and S-wave velocities, density, porosity, and permeability. Thin sections were made from the core plug samples and analyzed for mineralogy, cementation, clay content and type, grain size and sorting. Core plug and thin section data was used to evaluate reservoir quality and compare tidal and non-tidal deposits. Tidal deposits were found to have higher density and lower porosity, permeability, and Vp/Vs ratio than other marginal marine samples. This indicates that tidally-influenced deposits may contain reservoir beds but have overall lower quality than other marginal marine deposits.