Pronghorn member of the Bakken Formation, Williston Basin, USA: lithology, stratigraphy, reservoir properties, The

Abstract

In the North Dakota portion of the Williston Basin, the Pronghorn Member of the Bakken Formation produces hydrocarbons in Sanish, Parshall, and Billings Nose Fields. In other areas of the basin, the Pronghorn is not present, unproductive, or untested. This study describes the lithology, sequence stratigraphy, and reservoir properties of the Pronghorn Member. The Upper Devonian and Lower Mississippian Bakken Petroleum System (BPS) is present within potions of the 30,000 square mile intracratonic Williston Basin. The Bakken Petroleum System consists of the Upper Devonian Three Forks Formation and the Upper Devonian and Lower Mississippian Bakken Formation. The Bakken Formation contains four members. In ascending order these are: (1) the Pronghorn Member (2) the Lower Bakken Shale (3) the Middle Bakken Member, and (4) the Upper Bakken Shale. The organic-rich Upper and Lower Bakken Shale members were deposited in anoxic marine environment during relative sea level highstand. These shales are both source and seal within the BPS. The Middle Bakken and Pronghorn members were deposited in shallow subtidal to open marine environments and are reservoirs within the Petroleum System. The Three Forks Formation is divided into three units: Lower, Middle, and Upper. The formation consists of interbedded dolomite, siltstone, sandstone, and anhydrite beds that were deposited in sabkha and intertidal environments. The Upper Three Forks silty dolomite beds are reservoirs. This study focuses on the Pronghorn Member and its relationship to the underlying Upper Three Forks Member and the overlying Lower Bakken Shale. The facies of the Pronghorn Member include in ascending order: (1) heavily burrowed, well-sorted, angular to subrounded, very fine to fine-grained sandstone (2) interlaminated shale and burrowed dolomitic siltstone (3) fossiliferous lime mudstone to lime packstone, and (4) shale with thin interlaminated siltstone and very fine to fine-grained sandstone. Pronghorn Lithofacies 1 is equivalent to the "Sanish Sand" of North Dakota. Pronghorn facies are laterally discontinuous and vary in thickness. Facies discontinuity may, in part, be related to the energy of the depositional setting. Underlying Prairie Salt dissolution may have produced local thickness variations. The Pronghorn Member was deposited as part of an overall deepening cycle. A regionally correlative, unconformable surface of erosion lies at the contact between the Upper Three Forks and Pronghorn Lithofacies 1. A second significant surface lies at the contact between Pronghorn Lithofacies 3 and Lithofacies 4. This surface is marked by a skeletal lag associated with a Transgressive Surface of Erosion (TSE). The contact between Lithofacies 4 and the Lower Bakken Shale is variable, and is transitional and locally sharp and erosional. The limestone of Pronghorn Lithofacies 3 and the upper shale of Pronghorn Lithofacies 4 have distinct well log characteristics. The other lithofacies are commonly indistinguishable on logs. For well log correlations, the Pronghorn was divided into two facies: the upper silty shale Lithofacies 4 (PRNR ) and Lithofacies 1, 2, and 3 (PRNR1). The PRNR1 well log facies has good reservoir potential, with the highest production potential present in the southwestern portion of the North Dakota. The gross reservoir map of the PRNR1 well log facies correlates with current and proposed production from the Pronghorn in southern North Dakota. Currently, the Pronghorn is not reported as a separate producing zone, so petrophysical maps of the Pronghorn were compared to production values reported for the Three Forks Formation. Where fracture-stimulated, the lower Pronghorn interval (PRNR1) is probably in communication with the Upper Three Forks. The upper silty shale of the Pronghorn Lithofacies 4 (PRNR) is possibly a vertical migration barrier between the oil-rich Lower Bakken Shale and the Upper Three Forks. In targeting the Upper Three Forks, drilling through thick and tight Pronghorn Lithofacies 4 may decrease production potential.