Proximal Bakken Facies – A New Play Type in the Late Devonian-Early Mississippian Williston Basin Succession, North Dakota

Abstract

The Bakken Formation in the Williston Basin of North Dakota typically shows a four-fold subdivision: a mixed siliciclastic-carbonate middle member is sandwiched between two black shale units, the upper and lower Bakken members, and underlain by a discontinuous sandy to muddy basal unit, the Pronghorn member. Mostly siliciclastic, the Bakken in places shows carbonate oolites, and algal/microbial mounds. However, cores from the southern margin of Bakken subcrop are distinctly different from classical Bakken cores: (1) they lack a lower Bakken shale or it is strongly condensed; however, up to cm-size shale clasts are common at the Three Forks-Bakken transition; (2) the Pronghorn member, if present, thickens and becomes more sandy towards the margin; (3) the lower half of the middle Bakken contains mud-rich carbonates; and (4) the mud-rich carbonates exhibit pink and green colors, typical for the Three Forks Formation, however, its textures and diverse ichnofacies clearly separate it from this underlying unit.

The shale clasts are interpreted as reworked lower Bakken shale which was likely widely distributed, but largely removed by the transgression that deposited the middle Bakken member. The scattered presence of the Pronghorn probably reflects a local control, likely pre-existing relief during the initial Bakken transgression. The carbonates in the basal middle Bakken probably show transgressive conditions and may therefore be time-equivalent to a thin carbonate lag at the base of the middle member which is present further distally. Only the overlying siliciclastic units correspond to the regression mirrored by the bulk of classical middle Bakken. The pink and green colors of much of the marginal middle Bakken could result from partly reworking Three Forks sediments, and/or sedimentary conditions similar to Three Forks times. The trace fossils, however, reflect hospitable living conditions unlike during Three Forks deposition.

The distribution, organic-richness, and thermal maturity of the upper Bakken shale likely plays a key role for the proximal Bakken play. In conjunction, the lateral facies changes within the middle Bakken will also be crucial for the distribution of locally generated and accumulated hydrocarbons, as well as the potential for laterally migrated and accumulated oil and gas. The proximal Bakken therefore has to be evaluated as a potentially new play type that may in places show significant undiscovered reserves.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018