Abstract

Nine facies were identified in the Bakken Formation of west-central Saskatchewan:

  1. lower black shale facies (LBSH);

  2. oolitic biochastic grainstone facies (OBC);

  3. lower bioturbated siltstone facies (LBS);

  4. thinly-bedded bioturbated sandstone facies (TBBS);

  5. thinly-bedded standstone facies (TBS);

  6. interbedded standstone-siltsone-shale facies (ISSS);

  7. uppser bioturbated siltstone facies (UBS);

  8. transitional facies (TRANS); and

  9. upper black shale facies (UBSH).

During the early stage of deposition of the Bakken Formation, the basin was relatively deep and the lower Bakken shale member (LBSH) was deposited below storm wave-base under anoxic conditions. Changes at the beginning of middle Bakken lime resulted in the development of the oolitic shoals (OBG) in a shallow. Open marine selling; fine-grained siliciclastic sediments (LBC) accumulated basinward of the shoals. Subsequent deepening of the basin was accompanied by 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, and open marine offshore shelf conditions prevailed (UBS). The upper Bakken shale member represents a return to anoxic marine conditions (UBSH) similar to but less widespread than those in the lower Bakken shale.

Introduction

The Bakken Formation is a dominantly siliciclaslic unit in the subsurface of the Williston Basin and parts of southeastern Alberta. The formation has long been a useful marker horizon because of easily recognized log characteristics Recently, the Bakken has been important to the oil industry because of successful exploitation by horizontal drilling. It is an oil producer in southwest Manitoba. and in southeast and west-central Saskatchewan. the area of this study (Figure 1).

The Bakken Formation was informally subdivided by Nordquist (1953). into three members. The lower black shale member. middle sandstone member and upper black shale member (Figure 2) are recognizable everywhere that the Bakken has been identified.

The aim of this paper is to describe the Bakken facies in west-central Saskatchewan (Figure 1) using evidence from

the study of approximately 50 cores and well-log data, and to make a preliminary interpretation of the depositional history.

FACIES OF THE BAKKEN FORMATION

The Bakken, a tripartite formation is composed of nine facies that can be recognized and correlated throughout the study area.

Facies A) Lower Black Shale (LBSH)

The shales of the LBSH are generally dark grey 10 black and rarely green Karma (1991) reported that the upper and lower Bakken shales contain silica, clav minerals {illite after glauconile), organic matter, sulfides, and carbonales. They are homogeneous, fissile to well indurated, waxy, non calcareous to slightly calcareous, pyriliferous, and raniferous Amorphous organic material is abundant. Most commonly as wisps and flecks parallel lo clay laminae Parallel clay lamination is the only sedimentary structure observed. Subangular to subrounded, equant to elongate. sill- Sized monocryslalline quartz grains are scattered through the lower shale, or are concentrated in laminae one gram thick.

Facies B) Oolitic Bioclastic Grainstone (OBG)

The OBG is light grey and composed of bioclasts, including articulated and disarticulated, broken brachiopod shells and spines, crinoid ossicles, large branching bryozoans, foraminifera and ostracodes.

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