--> --> Building a Predictive Model for Stratigraphic Transitions and Lateral Facies Changes in the Cretaceous Almond Formation, Wyoming

AAPG Rocky Mountain Section Meeting

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Building a Predictive Model for Stratigraphic Transitions and Lateral Facies Changes in the Cretaceous Almond Formation, Wyoming

Abstract

The Cretaceous Almond Formation, located in the Greater Green River Basin, records deposition of coastal plain fluvial sandstones and shallow marginal-marine sandstones in a net-transgressive sequence along the western margin of the Cretaceous Interior Seaway (CIS) in the early Maastrichtian. The Almond Formation is an important hydrocarbon reservoir, with development along the Wamsutter Arch and the margins of the Washakie basin. Further development of the Almond petroleum system away from current production requires extending our understanding of lateral facies changes and sequence stratigraphic architecture beyond areas that have been previously studied. The aim of this research is to build a predictive model of the facies transitions and reservoir character along the Cherokee Arch. This structural feature marks the southern margin of the Washakie Basin and is roughly perpendicular to the shoreline of the CIS. Almond strata are exposed at either end of the arch, which is oriented E-W and spans the distance between the Rock Springs and Rawlins uplifts. Initial outcrop examination shows that lower Almond strata transition from fluvio-deltaic deposits in the west to shoreface strata in the east, while the upper Almond strata show marine influence consistently across the ~125 km separating the two outcrop localities. Preservation of shoreface strata and related near-shore fluvio-deltaics across large distances in the dip direction shows the large magnitude of the transgression with which it is associated, suggesting that the system gradient was likely very gentle leading to wide facies belts, and that reservoir continuity could be favorable over significant distances. Shoreface sands transition from relatively thick exposures near the Rock Springs uplift to much more attenuated outcrop to the east of the basin. Preservation of these sands to the east demonstrates a reworking of the paleoshoreline leaving primarily lower shoreface sediments. Further work to characterize the outcrops and integrate well data along the Cherokee Arch will add to initial outcrop observations and provide further detail to our understanding of this shoreface system, leading to better exploration decisions within the basin as well as a better understanding of transgressive systems along passive margins elsewhere in the geologic record.