Temporal variability in fluvial deposition and the implications for static reservoir connectivity: John Henry Member, straight cliffs formation (Cretaceous, Utah)

Accurately predicting hydrocarbon recovery from fluvial reservoirs is challenging due to uncertainties associated with alluvial architecture. Fluvial deposits exhibit rapid lateral and vertical facies changes that result in discontinuous and disconnected sand bodies. Subsurface interpretations are further limited due to the scarcity and indirect nature of subsurface data. Accurate predictions of alluvial architecture, and thus sand body connectivity, are essential for good reservoir management practices. When used as analogs for subsurface reservoirs, outcrop-based studies can provide useful data for more informed predictions of subsurface fluvial organization. Exposures of the Upper Cretaceous John Henry Member along the flanks of the Kaiparowits Plateau of southern Utah offer an opportunity to study variability in alluvial architecture and address its impact on reservoir connectivity. A detailed study of the fluvial strata of the John Henry Member was conducted at Bull Canyon (southwestern Kaiparowits Plateau). This study leveraged a wealth of digital outcrop data as well as an integrated paleomorphodynamic workflow that estimates paleochannel morphology and formative paleohydraulic conditions. Outcrop observations and results of the paleomorphodynamic workflow were combined to build 3-D conceptual models of river system morphology in the John Henry Member. The conceptual models were then used to constrain model input parameters for object-based facies simulation. Static reservoir connectivity at four well configurations was calculated on 30 facies model realizations. The results of this study show that outcrops of the John Henry Member at Bull Canyon exhibit significant variability in fluvial style over a 180-meter (591- feet) thick interval. The lower John Henry Member (DUs 1, 2, and 3) was influenced by backwater hydraulics at the time of deposition, resulting in a stratigraphic architecture characterized by straight, narrow, and dispersive channel belts. Results of the static reservoir connectivity analysis show that, in 40% of model realizations, reservoir connectivity in at least one unit of the lower John Henry Member is 0%. This implies that backwater hydraulic effects may induce a reservoir dimensionality that negatively impacts static reservoir connectivity. This study highlights the importance of integrating paleomorphodynamic workflows along with more traditional outcrop observations to conceptually characterize fluvial systems and constrain model input parameters.