Maceral Variations and Maceral Assemblages within the Upper and Lower Members of the Bakken Formation, Williston Basin, Saskatchewan, Canada.

Abstract

Argillaceous rocks are most often considered to be homogeneous entities, because, at the macroscopic scale, they appear to be so. The Bakken Formation within southern Saskatchewan contains two black shale members, the Upper and Lower Bakken Formation. This study has examined the organic matter within the Upper and Lower members of the Bakken Formation using fluorescence light microscopy on samples taken from core, at closely-spaced intervals across southern Saskatchewan. The organic constituents, or macerals, vary significantly both along with depth within a given borehole (i.e. temporally) and from one well to another (i.e. spatially). The macerals present in the Upper and Lower members of the Bakken Formation include: amorphous organic matter identified as the maceral bituminite with minor amounts of alginite, and ‘degraded alginite’. Bituminite is the dominant maceral and forms the groundmass of the Bakken Formation black shales. Bituminite was unequivocally distinguished from clay or other inorganic material by prolonged exposure to ultraviolet/blue light for a designated time period, using the ‘alteration effect’ of liptinites. Two sub-macerals of bituminite were distinguished, identified as bituminite A and bituminite C. These two sub-macerals ‘substitute’ for one another across a given east-west transect, such that: the sub-maceral bituminite A is the dominant maceral within the centre of the study area, whereas bituminite C occurs as the groundmass along the peripheral area of the sub-crop preceded by a transitional area where there is a mixture of both bituminite A and bituminite C. The considerable variation in maceral assemblages, both with depth and along a given section, have implication toward the hydrocarbon generative potential of these source rocks and, in order to comprehend the variation in organic matter, the maceral assemblages were grouped into organic facies. Four groups of organic facies were identified and defined, each with their own respective sub-division based upon the type of groundmass (i.e. either bituminite C or bituminite A) and both the amount or type of microfossils present. A number of differing depositional environments for each organic facies are proposed using the presence or absence and type of microfossils (i.e. algae, acritarchs and spores) within each organic facies (and organic facies subdivision). The Bakken Formation black shales are interpreted to have been deposited under anoxic conditions, where changes in water chemistry and the availability of light were the primary mechanisms responsible for the variations in the type and amount of organic matter deposited.