Abstract

The Upper Devonian-Lower Mississippian Bakken Formation in the Williston Basin is one of the most prolific onshore petroleum systems in the continental U.S. It consists of a middle mixed carbonate-siliciclastic member, sandwiched between the two organic-rich black shales. The focus of this study is to highlight the facies architecture and sequence stratigraphy of the middle Bakken member that forms the reservoir.

The middle Bakken contains eleven depositional facies, both carbonates and siliciclastics. Most of the middle Bakken sediments are siltstones. The eleven facies are arranged in three or four parasequences at the margin of the basin and up to six parasequences in the central basin. The lower two to four parasequences show an increase in depositional energy up-section whereas the top one to two reflect laterally varying stacking patterns and a successive decrease in depositional energy. Many of these parasequences are correlatable over more than a hundred miles through the study area, but not every parasequence is traceable from the deepest to the shallowest parts of the basin.

The basal two to four parasequences show an overall progradation of the sedimentary system. During this initial decrease in accommodation space, shoreface and foreshore sandstone, siltstone and oolite sediment bodies successively stepped down the ramp profile. This progradation culminated in the deposition of the foreshore and shoreface sediments close to the basin center, thereby exposing earlier formed basin margin sediments. The upper, retrograding portion of the middle Bakken member shows a successive deepening of the basin setting the stage for the deposition of the upper member black shale. Whereas the parasequences represent fourth-order cycles, the shallowing from the lower member into the middle and the successive deepening into the upper member are interpreted as a fluctuation of third order.

Candidates for conventional reservoir facies within the middle Bakken depositional system are sandstones and siltstones representing shoreface and foreshore sediments. These rock types are concentrated at the tops of parasequences although depending on the position within the basin, not every parasequence top may show suitable reservoir characteristics. The margins developed shoreface and foreshore facies earlier than the center, resulting in potential reservoir siltstones and sandstones being present in different stratigraphic levels along a transect through the basin. An uncritical lateral correlation of siltstone and sandstone bodies from the basin margin to the depocenter will cross time lines and does not reflect original depositional geometries.