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Quantifying Reservoir Complexity in a Tight Oil Play by Integrating Detailed Facies Analysis With Stratigraphic Architecture Analysis From a Digital Outcrop Model

Abstract

Abundant facies heterogeneities found in tight reservoirs such as the Upper Cretaceous Wall Creek Member (WCM) of the Frontier Formation in the Powder River Basin (PRB), Wyoming, make the prediction of reservoir behavior challenging. Well and seismic data cannot adequately resolve the uncertainties in lithofacies distribution and internal sedimentary architecture that maintain an important control on fluid flow in the reservoir. Here we present results from a 1km2 digital outcrop model (DOM) of the WCM, exposed along the eastern limb of the Tisdale Anticline on the western margin of the PRB. In the study area a series of intersecting canyons provide a near three dimensional exposure of the WCM. Nine stratigraphic sections are integrated with the DOM to constrain lithofacies distribution in the outcrop. Two main stratigraphic packages previously interpreted as a wave-influenced delta, deposited during a relative highstand, and an overlying tidally modulated bar complex deposited during a relative lowstand compose the strata in these outcrops. In the wave-influenced delta, laterally continuous beds interpreted as delta front deposits coarsen upward into thick, planar and low angle trough cross bedded sandstone beds. Each of these beds are largely correlative across the study area and compose laterally continuous flow units in depositional dip and strike direction. Conversely, the tidally influenced delta package is highly heterolithic with a multitude of depositional elements that are stacked hierarchically. The most common depositional element are tidal dunes that stack into composite dunes and bars. Tidal dunes display relatively high width to length ratios. Results from DOM analysis demonstrate a strong lateral bed discontinuity in depositional dip direction, and a magnitude greater lateral continuity in the depositional strike direction. This geometry coupled with the presence of bed bounding discontinuous mud beds results in strong flow anisotropy, with preferred fluid migration along strike, perpendicular to what is expected from tidal bars elongated in the depositional dip direction. Results from this study provide essential input parameters for a high resolution outcrop analogue geomodel, to address the impact of multiscale heterogeneities on fluid flow, and provide important input for future well placement, completion strategies, and more accurate resource volume calculations in the WCM and in similar heterolithic reservoirs around the world.