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.
Description:
A Thesis Submitted to the Faculty of Graduate Studies and Research In Partial Fulfillment of the Requirements for the Degree of Master of Science in Geology, University of Regina. xvi, 221 p.