This study of the Frontier Formation of the Southwestern Powder River Basin examines the stratigraphy, sedimentology, petrography, and oil and gas potential of the Frontier sandstones in the southwestern part of the Powder River Basin. The study area covers approximately 180 townships and more than 6400 square miles. This report, representing four man-years of geologic research, provides the explorationist with a chronostratigraphic framework for the Cretaceous Frontier Formation and its members. The Frontier study in the southwestern part of the Powder River Basin, using 962 wells, 11 measured sections, and 51 cores, establishes a stratigraphic framework that is essential for explorationists in this play.

     In this study, the Frontier Formation is divided into three members, the Belle Fourche, Emigrant Gap, and Wall Creek, in ascending order. Eighteen picks were carried within the Frontier. Most were correlated throughout the study area; a few were recognized only locally. The correlated picks use bentonite beds, the tops of coarsening-upward sequences, and regionally significant unconformities. Regional cross sections and maps document the distribution of stratigraphic intervals defined by these picks throughout the southwestern part of the Powder River Basin.

     In this report, the Frontier Formation is subdivided into mappable, chronostratigraphic members, which are bounded by bentonite beds, disconformities, or unconformities. This chronostratigraphic methodology facilitated mapping of time-bounded lithofacies sequences

and reconstruction of paleogeography through time. Physical correlations indicate there are three regionally significant unconformities within the Frontier Formation in this area of the Powder River Basin. Two bound the Emigrant Gap Member, and the third occurs within the upper part of the Belle Fourche Member. In addition to the unconformities, the Frontier includes numerous disconformities. These most commonly occur at the tops of upward-coarsening sequences and record post-depositional erosion associated with transgression. In many instances, the amount of erosion associated with these disconformities was slight. In other cases, particularly in areas underlain by active basement structures, the amount of erosion was substantial. Reworking of sediment on these transgressive disconformities is responsible for the formation of some of the principle reservoir sandstones.

     The Frontier play consists of three principal reservoir sandstone types consisting of shoreface, transgressive lags, and reworked shelf sand bodies, including prodelta "plumes." The origins and distributions of these sandstone types are portrayed within the context of the complicated transgressive-regressive history of the Frontier. The analysis of the stratigraphy, sedimentology, and petrographic characteristics in this report is designed to assist exploration and production geologists in understanding the complex stratigraphic relationships, reservoir sandstone geometries, diagenetic history, and hydrocarbon production within the Frontier play trend. The study discusses the potential for Frontier reservoirs throughout the southwestern part of the Powder River Basin.