Sequence stratigraphy of the lower Pierre Shale in southern Powder River Basin, Wyoming, USA

Abstract

Powder River Basin is one of the biggest interior sedimentary basins in the Rocky Mountain region. The Upper Cretaceous section of the southern Powder River Basin includes the Niobrara Formation, which is one of the most significant source rocks of the Western Interior Cretaceous Seaway, and it is overlain by lower Pierre Shale which consists mostly of progradational shale sequences and two productive sandstone members encased in these shales. In the southern Powder River Basin, the lower Pierre Shale is made up of eight members. These members are progradational highstand deposits of the Gammon Ferruginous Member; lowstand prograding wedge deposits consisting 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 sandstone members are known targets for oil production. The Sharon Springs Member of the lower Pierre shale 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, this study demonstrates that the lower Pierre Shale interval is a Type 1 Ramp Margin Sequence. While previous studies were primarily focused on individual sandstone members or parts of the lower Pierre Shale section in relatively limited areas, this study provides an in depth sequence stratigraphic analysis of the lower Pierre Shale interval. The sequence stratigraphic framework was built based on well-log correlations, core descriptions, isopach maps and three-dimensional surface maps of each member, and one master west east oriented cross section throughout the study area. According to this sequence stratigraphic analysis, a depositional model connecting the Bighorn Basin to the Southern Powder River Basin was created. The depositional model demonstrates that the Shannon Sandstone, Unnamed, and Sussex Sandstone Members were deposited as an encased lowstand prograding wedge between the progradational Gammon Ferruginous Member and the transgressional Ardmore Pedro Bentonite Beds and Sharon Springs members 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 farther in the basin compared to the synchronous shoreline sands. Source rock analyses within the Sharon Springs interval demonstrate that member is moderately organic rich (TOC wt. % 1.27-2.72) and it is thermally mature (Tmax 437-440 °C) within the study area. However, the kerogen type within the member is Type III and the organic matter is gas prone. Based on these data, there is no evidence for the contribution of the Sharon Springs Member to the oil accumulations found in the Shannon and Sussex Sandstone Members.