The Sequence Stratigraphy and Production Potential of the Pronghorn Member of the Bakken Formation

Sonnenberg, Stephen; Johnson, Rebecca

The Bakken Petroleum System is a highly productive continuous tight oil play. According to the USGS, 3.0 to 4.3 billion barrels of undiscovered, recoverable oil remain in the Bakken Petroleum System (BPS). The Upper Devonian to Lower Mississippian BPS lies in the 30,000 square mile intracratonic Williston Basin of North Dakota, South Dakota, Montana, and Saskatchewan.

The Bakken Petroleum System consists of the Three Forks Formation and the Bakken Formation. The organic-rich Upper and Lower Shale Members of the Bakken represent anoxic open marine depositional environments and act as source and seal within the Petroleum System. The Middle and Pronghorn Members of the Bakken represent shallow subtidal to open marine depositional environments and act as reservoirs. The dolomitic Upper Three Forks "D bench" acts as an additional proven reservoir within the Bakken Petroleum System. This study focuses on the lowermost Pronghorn Member of the Bakken Formation and its relationship to the underlying Upper Three Forks. The facies of the Pronghorn are laterally discontinuous and vary greatly in thickness throughout the basin. While the limestone cap and upper siltstone lithofacies have distinct well log characteristics, other lithofacies are indistinguishable in logs. For well log correlations the Pronghorn was broken into two facies: the upper siltstone (PRNR silt) and the facies below and including the limestone cap (PRNR1).

The stratigraphy of the Pronghorn Member is interpreted as an overall deepening. The PRNRN1 facies represent a Transgressive Systems Tract (TST) overlying the Upper three Forks "D bench." The limestone cap is laterally more continuous than the lower lithofacies and represents a transgressive surface of erosion (TSE). The upper siltstone lithofacies transitions into the Lower Bakken Shale member, with increasing shale content upwards. The facies change is marked by the loss of fine-grained sandstone and silt sized quartz moving upwards into the shale.

The greatest potential for Pronghorn target wells lies in southwestern North Dakota. In much of the basin, the lower Pronghorn sandstone and sandy dolomite facies (PRNR1) appears to be in communication with the Upper Three Forks "D bench", and can augment Upper Three Forks laterals. However, the tight silt to shale upper Pronghorn facies (PRNR silt) may act as a vertical migration barrier in some areas, reducing oil movement from the Lower Shale into the Upper Three Forks.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013