Stratigraphy And Depositional Environments of the Bakken Formation (Upper DevonianILower Mississippian) In West-central Saskatchewan

Abstract

Abstract The Upper Devonian/Lower Mississippian Bakken Formation of west-central Saskatchewan comprises three members: the lower black shale member (LBSH), the middle sandstone member and the upper black shale member (UBSH). Seven different facies have been defined in the middle sandstone member:oolitic bioclastic grainstone facies (OBG);lower bioturbated siltstone facies (LBS);thinly-bedded bioturbated sandstone facies (TBBS);thinly-bedded sandstone facies (TBS);interbedded sandstone-siltstone-shale facies (ISSS);upper bioturbated siltstone facies (UBS);transitional facies (CSST). The upper black shale member (UBSH) is a facies equivalent of facies six and seven in the middle member. During the early stages of deposition of the Bakken Formation, the basin waters were relatively deep and the lower Bakken shale member (LBSH) was deposited below storm wave-base under anoxic conditions. Shallowing before, or at the beginning of middle Bakken deposition resulted in the development of oolitic shoals (OBG) in a shallow, open marine setting and fine-gniined siliciclastic sediments (LBS) accumulated basinward of the shoals. Subsequent deepening of the basin waters was accompanied by progradation of terrigenous sediments. These were distributed and reworked by storms and tidal currents (TBS, TBBS and ISSS), ultimately burying the oolitic shoals. Late Middle Bakken deposition was marked by continued basin deepening with possible localized brackish or lagoonal conditions (CSST) and open marine offshore shelf conditions (UBS). With continued transgression, sediment supply was reduced and sediments remained in flooded river-valley systems. The upper Bakken shale member represents basin deepening coincidental with transgression, sediment starvation and a return to anoxic marine conditions (UBSH) similar to, but less widespread than those of the lower Bakken shale. Introduction The Upper Devonian/Lower Mississippian Bakken Formation is a predominantly siliciclastic, marine unit in the subsurface of the Williston Basin and parts of southeastern Alberta. In west-central Saskatchewan the Bakken Formation varies from 0 to more than 31 m thick. The original depositional thickness commonly is not represented as a result of sub-Cretaceous erosion. In the study area. Bakken strata unconformably overlie the Big Valley Formation. They are unconformably (locally conformably?) overlain by the Lodgepole Formation, or are unconformably overlain by Cretaceous sands or the Success Formation (JurassicCretaceous?). The Bakken Formation has long been a useful marker unit because of its distinctive log signature. Recently, oil has been successfully recovered from the formation by horizontal drilling(1). Oil is produced in southeast and west-central Saskatchewan, the area of this study (Figure 1), and in southwest Manitoba. The Bakken Formation has been subdivided into three informal Member (2), all of which are recognized where the Bakken has been identified. They are the Lower black shake member, the middle sandstone member, and the upper black shale member (Figure 2). The purpose of this paper is to describe the Bakken facies and associated rock-types in west-central Saskatchewan (Figure 1) using evidence from the study of about 50 cores and associated well-log data, and to make a preliminary interpretation of the depositional history of the formation in the study area.