The Niobrara Total Petroleum System (TPS) covers an extensive area across the Rocky Mountain Region, USA. 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, and Paleocene and Eocene Fort Union and Wasatch formations, respectively. The Niobrara is a deep-water hemipelagic carbonate mudrock and will be the subject of this paper.
The Niobrara in the southern PRB is Coniacian to early Campanian in age and approximately 150 to 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 8,000 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.
The main reservoir target is the B chalk/marl zone.
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.
Recent drilling success utilizing horizontal drilling and multi-stage hydraulic fracture stimulation suggest the opportunity for a large (giant) unconventional petroleum accumulation.
The Niobrara Formation in the southern PRB is an emerging continuous hydrocarbon play. 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).
