Regional sedimentology and diagenesis of the middle Bakken member : implications for reservoir rock distribution in Southern Saskatchewan

Abstract

The Devonian-Mississippian Bakken Formation is a prominent marker-horizon in the subsurface stratigraphy of Saskatchewan. The formation consists of three distinct members: black shale of the Lower Bakken Member, siltstone-sandstone of the Middle Bakken Member, and black shale of the Upper Bakken Member. This study was initiated to evaluate the Middle Bakken Member from several perspectives: lithofacies, and depositional environment; petrography, diagenesis, and cement distribution; and, petrofades and reservoir distribution. The Middle Bakken Member is dominantly a sandy siltstone unit, with an average thickness of 12 m in Saskatchewan. Six different lithofades have been defined in this middle member. (i) MB1, massive silty sandstone; (ii) MB2, ripple laminated sandy siltstone; (iii) MB3, horizontally laminated siltstone; (iv) MB4, thinly bedded sandstone; (v) MBS, interbedded sandy siltstone; and (vi) MB6, bioturbated siltstone. These six lithofacies types represent two different facies associations each consisting of three lithofacies types. Each facies association represents a fining-upward trend of shallow marine origin, with local variations in lateral and vertical facies distribution, change, and thickness. Fades Association 1 (FA1) was deposited in a shallow marine environment, in which MB1, MB2, and MB3 were deposited during a relative rise in sea level, producing a fining-upward sequence. Later, sea level dropped abruptly for a short duration, followed by a rapid sea-level rise that led to deposition of Fades Association 2 (FA2) comprising lithofacies MB4, MBS, and MB6, in another fining-upward sequence. Petrographic, diagenetic, and geochemical studies of the Middle Bakken Member help to establish a regional diagenetic-facies model in southern Saskatchewan. Four diagenetic-facies types recognized in the model are: (i) diagenetic-facies A: dolomitic-calcareous quartz wacke facies; (ii) diagenetic-facies B: calcareous-dolomitic quartz wacke facies; (iii) diagenetic-facies C: feldspathic-dolomitic quartz sub-arenite fades; and (iv) diagenetic Facies D: pyritic-dolomitic quartz wacke facies. These diagenetic facies are characterized by variable major and minor cements, and porosity distribution across southern Saskatchewan. Dolomite is the most widespread cement, and occurs in three different forms: (i) massive, non-planar, dolomite cement (20-70 [mu]m); (ii) planar, macro-dolomite rhombs (30-50 [mu]m); and (iii) euhedral, micro dolomite rhombs (~10 [mu]m). Middle Bakken rocks exhibit a variable amount of secondary porosity, which is the main contributor to the reservoir quality. Most secondary pores are of enlarged intergranular type, and have their widest distribution in lithofaaes MB4 and MB5 in eastern and western Saskatchewan. Creation of secondary porosity is attributed to the organic acids released during diagenesis of the organic-rich black shales overlying and underlying the Middle Bakken Member, and to the infiltration of weakly acidic meteoric waters. Reservoir potential of the Middle Bakken Member was analyzed in terms of petrofacies, by combining lithology and diagenetic features with reservoir characteristics. Six petrofacies types have been established, based on a synthesis of the complex relationships between the parameters studied during porosity, permeability, and mercury injected capillary pressure analysis. The petrofacies types are: (i) petrofacies 1a: non-reservoir unit; (ii) petrofacies 1b: marginal reservoir unit; (iii) petrofacies 1c: non-reservoir unit; (iv) petrofacies 1d: main reservoir unit; (v) petrofacies 1e: potential reservoir unit; and (vi) petrofacies 1f: non-reservoir unit. The Middle Bakken Member rocks, in general, have a similar framework composition across the study area. However, regional differences exist in matrix and cement content and distribution, and reservoir characteristics. Variable post-depositional modifications are considered as dominant control on the regional differences in eastern, central, and western Saskatchewan.