Geologic significance of microbialites and microbially influenced sedimentary structures of the Bakken Formation, Williston Basin, North Dakota

Abstract

The Late Devonian – Early Mississippian Bakken Formation, is a prolific petroleum system that is currently producing more than 1.4 million barrels of oil per day (NDDMR, 2019). Advances in horizontal drilling and hydraulic fracturing technology has caused a resurgence in production from the Bakken Formation, that was once thought to be uneconomic. Located in the Williston Basin of North Dakota, Montana and Canada, the Bakken Formation is composed of four members: Upper and Lower Bakken shales, Middle Bakken and Pronghorn. The Middle Bakken Formation is a significant unconventional hydrocarbon accumulation that is sourced by the organic-rich Upper and Lower Bakken shales. Six facies, A-F, have been identified within the Middle Bakken Formation by the Colorado School of Mines MUDTOC Consortia. Within the C, and E facies the presence of microbialites is observed. Classifying the type of microbialite that is present- Microbially Influenced Sedimentary Structure (MISS) or Stromatolites- allows for a more comprehensive understanding of the extrinsic controls on the deposition of the Middle Bakken Formation. MISS trap and bind detrital sediment. Stromatolites trap sediment and organically precipitate carbonate. Calcite content exhibits the strongest correlation to reservoir quality (R2=0.65). Trapping and binding of detrital sediment creates a framework that enhances reservoir quality, while organic carbonate precipitation occludes primary porosity. MISS, which engage in trapping but not precipitation, host better reservoir quality than stromatolites. Differentiation between MISS and stromatolites provides insight into the effects that different classes of microbialite have on reservoir quality.