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Sequence Stratigraphy of the Lower Pierre Shale in Southern Powder River Basin, Wyoming, USA

Abstract

The Upper Cretaceous section of the southern Powder River Basin in the Rocky Mountain region includes the lower Pierre Shale which consists mostly of progradational shale sequences and two productive sandstone members encased in these shales. Eight members of the lower Pierre Shale is defined in the southern Powder River Basin. These members are progradational highstand deposits of the Gammon Ferruginous Member; lowstand prograding wedge deposits of the Shannon Sandstone, the Unnamed Member, and the Sussex Sandstone; transgressional Ardmore Pedro Bentonite Beds and Sharon Springs members; and highstand deposits of the Mitten Black Shale and Red Bird Silty members. The Shannon and Sussex members are known targets for oil production. The Sharon Springs Member has relatively high organic carbon content. Therefore, determining its continuity throughout the study area and its source rock potential was one of the main focuses of this study in addition to building the sequence stratigraphic framework for the lower Pierre Shale interval. Based on an integrated research of 1490 raster well-log data with three cores and including an in depth sequence stratigraphic analysis, this study demonstrates that the lower Pierre Shale interval is a Type 1 Ramp Margin Sequence. The sequence stratigraphic framework was built based on well-log correlations, core descriptions, isopach maps and three-dimensional surface maps of each member. A depositional model connecting the Bighorn Basin to the Southern Powder River Basin was created and it demonstrates that the Shannon Sandstone, Unnamed, and Sussex Sandstone Members were deposited as an encased lowstand prograding wedge in a thick shale sequence hundreds of miles basinward from the stratigraphically equal Mesaverde lowstand sandstones of the Bighorn Basin. The shift of facies across such long distances is explained with the forced regression that has taken place during the deposition of the uppermost Gammon Ferruginous Member and the following deposition of Shannon and Sussex sandstone members. Source rock analyses within the Sharon Springs interval demonstrate that member is moderately organic rich with Type III – gas prone kerogen and thermally mature. Based on these data, there is no evidence for the contribution of the Sharon Springs Member to the oil found in the Shannon and Sussex members.