The Niobrara formation in the Southern Powder river basin, Wyoming: An emerging giant continuous petroleum accumulation
The Niobrara Total Petroleum System (TPS) covers an extensive area across the Rocky Mountain Region, US. In the Powder River Basin (PRB), the petroleum system consists of source beds in the Upper Cretaceous Niobrara Formation as well as reservoirs in the Upper Cretaceous Frontier, Turner, Niobrara, Sussex, Shannon, Parkman, Teapot, and Teckla. The Niobrara is a deep-water hemipelagic carbonate mudrock and is Coniacian to early Campanian in age and approximately 150-650 ft thick. The formation, where productive, has low porosity (< 10%), low permeability (<0.01 md), and pore throat sizes less than 0.1 micron. The immature-mature present-day depth boundary is approximately 8000 ft. The formation is subdivided informally into three units in the PRB (A, B, and C). The units consist of cycles of marls and chalks. Geologic factors related to successful exploration and development include excellent source-rock quality, source-rock maturity, reservoir thickness, matrix and fracture porosity and permeability development, high geothermal gradients, overpressure, oil gravity, gas-oil ratios, and regional fracture development. The accumulation covers a large area and is a potential giant accumulation. Recent drilling success by several operators indicates that productive reservoir conditions exist in the southern PRB (Figure 1). In this area, the Niobrara Formation consists of three marly chalk facies, informally referred to in descending order as A, B, and C benches that are separated by marls, similar to the Denver Basin (Figure 2). The basal Fort Hays Member of the Niobrara Formation is absent in the southern PRB (Weimer and Flexer, 1985; Taylor, 2012). The main reservoir target currently in the southern PRB is the Niobrara B chalk/marl interval. Porosities are 6-12% and permeabilities are less than 0.01 mD (Taylor, 2012). Hydrocarbons are generated from mainly a marine type II kerogen with total organic carbon content (TOC) averaging 2-3 wt.% for the interstratified organic-rich source beds (Anna, 2009).