Abstract: Dolomitization in the Bakken Formation in South Eastern Saskatchewan, Canada
FERDOUS, HASAN S. and ROBIN W. RENAUT
The Devonian-Mississippian Bakken Formation is a thin, but widespread subsurface unit and stratigraphic marker within the Williston Basin. The formation comprises two radioactive, organic-rich black shales separated by a silty sandstone unit. Although the middle Bakken unit lacks well-developed reservoir characteristics in most of the Williston Basin, it is a productive hydrocarbon reservoir in Saskatchewan.
Facies-selective alteration processes have resulted in different diagenetic facies in the middle Bakken unit with different reservoir characteristics. The main reservoir facies is dominated by secondary porosity caused mainly by carbonate dissolution and dolomitization. Two different generations of dolomitization, one early, the other late, control the pore network in the middle Bakken sediments.
The early formed macro-dolomites (approx. 40 micrometers) show distinct zonation, and are associated with enhanced reservoir characteristics. These dolomites contain calcite inclusions suggesting that they probably replaced the precursor calcites. The presence of high organic matter and pyrite suggest that organogenic dolomitization may have been responsible for these early formed macro-dolomites. The late stage micro-dolomites (<10 micrometers) formed by precipitation of dolomite crystals in the pore spaces. Their intergrowth locally destroys pore connections, decreasing the overall reservoir quality. These micro-dolomites may have formed by the mixed water dolomitization, during subaerial exposure. The greatest development of micro-dolomites, which decreases reservoir quality, is observed close to the Bakken erosional edge, and at stratigraphically higher levels of the middle Bakken unit in SE Saskatchewan.
Search and Discovery Article #90945©1997 AAPG Pacific Section Meeting, Bakersfield, California