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Biogenic Permeability in the Bakken Formation, Canada

Abstract

The Late Devonian-Early Carboniferous Bakken Formation, a prolific producer of oil from low-permeability reservoir in the Williston Basin of USA and Canada, is assessed for the potential of secondary production from bioturbated levels into fractures. For this purpose, core from various fields in Southern Saskatchewan were assessed. Methods of analysis include detailed ichnological description, minipermeametry, thin-section petrography and microCT from samples deemed characteristic of reoccurring ichnofabrics. Three recurrent ichnofabrics were recognized: (1) highly burrowed (BI = 5) a regularly laminated sandstone-mudstone unit dominated by small Planolites and Phycosiphon; (2) a Nereites missouriensis-ichnofabric with subordinate Phycosiphon and Planolites; and (3) a cross-stratified, locally cryptically bioturbated sandstone. The Planolites/Phycosiphon ichnofabric shows permeability to air (Ka) that range between 0.01 and 0.50 mD. The highest K are associated with sandstone laminae and sand-filled burrows. Mudstone beds are typically admixed with some sand and many sand-filled burrows penetrate the mudstone beds. As such, the permeability is more isotropic and homogeneous than comparable laminated fabrics. The Nereites missouriensis ichnofabric has similar permeability ranges, wherein Ka ranges from 0.01 to 0.20 mD. The highest permeabilities are associated with unburrowed matrix and silty haloes associated with Nereites missouriensis represent the mode of permeabilities, which is near 0.1 mD. The silty haloes represent biogenically reworked zones that extend as far as 4 mm from the causative burrow. These coalesce and as with the Planolites/Phycosiphon examples, the bioturbated texture presents an isotropic and homogeneous medium compared to laminated examples. X-Bedded sandtones range in permeability from 0.05 to 0.70 mD. Levels characterized by cryptic bioturbation tend to occur in the upper part of this range, typically between 0.20 and 0.70 mD. Cryptically bioturbated levels present more homogenous permeability distributions compared to zones where the cross lamination is well preserved. From these observations, it is ascertained that bioturbated levels within the Bakken Formation do not reduce the reservoir quality, but in fact enhance certain parameters such as isotropy and homogeneity. These characteristics suggest that bioturbated levels stand to contribute to fracture permeability; due to the isotropic nature of the biogenic permeability, the angle of incidence between the fracture and burrowed-bed would not influence hydraulic communication into the fracture.