ABSTRACT

The Bakken Formation is a thin but very widespread Mississippian-Devonian siliciclastic unit. In much of the Williston Basin three members can be recognized: a relatively coarse sandstone/siltstone unit (Middle Member) separates two black to brown, organic-rich shales (Upper and Lower Members). Because of their characteristic high radioactivity, the Upper and Lower Members are excellent subsurface markers. The Middle Bakken is an oil reservoir in North Dakota, Saskatchewan, and Manitoba.

In the Daly field in southwestern Manitoba, the Lower Bakken is absent and the Middle Member shows a sharp erosional contact with the underlying Devonian Lyleton Formation. The Middle Bakken Member consists of four lithofacies: i) wavy laminated-lenticular bedded sandstone; ii) horizontally laminated siltstone; iii) massive, bioturbated siltstone; and iv) breccia. These rocks are composed mainly of quartz, clay minerals, and dolomite, with minor amounts of anhydrite, calcite, halite, feldspars, hematite, and pyrite. The amount of matrix material (mainly clay minerals) generally increases upwards in the section, with a concomitant decrease in quartz content. Similarly, the proportion of authigenic dolomite increases from less than 20 percent at the base to over 40 percent at the top of the member.

Thin section and SEM examination show that the Middle Bakken Member underwent at least five diagenetic stages. The first stage involved dolomite formation primarily as small, euhedral rhombohedra coating detrital grains and filling pores. This occurred before substantial porosity reduction by compaction and most likely took place either penecontemporaneously with sedimentation or at shallow depths below the sediment-water interface during very early methanogenesis. At the same time, pyrite was precipitated locally and in some cases forms marker horizons that can be traced from well to well. During the second stage, illite was precipitated as rims and coats around the detrital grains. Minor secondary intragranular porosity formed as a result of dissolution of the authigenic dolomite crystals and detrital feldspar grains. The third stage is marked by the formation of quartz overgrowths on detrital grains. Following this, neoformed K-feldspar was precipitated. The final stage was the formation of anhydrite cement. These last two episodes were likely associated with the oxidation of pyrite to form elevated sulphate/low pH conditions, and the subsequent reaction of illite to form K-feldspar and the precipitation of calcium sulphate.