Jurassic-aged Fractures and Grikes in Mississippian Carbonate: a Re-evaluation of Fracture, Fold, and Thrust Relationships and the Implications for Fractured Hydrocarbon Reservoirs, Rocky Mountain Thrust Front, Montana

Geraghty, Emily M., James W. Sears
University of Montana, Missoula, MT

The discovery of Jurassic-aged fractures enlarged by karst (grikes) and filled with sandstone in the upper part of the Mississippian carbonate section along the Montana Rocky Mountain thrust front calls for a complete re-evaluation of the relationship between fracturing, folding, and thrusting in this regionally significant hydrocarbon reservoir. The key research site is at Swift reservoir, west of Dupuyer, Montana, because of excellent exposures and extensive previous work. The recognition of the fractures’ Jurassic age indicates that fractures previously interpreted as "type I & II" and kinematically linked to extension within fault-propagation folds, in fact, predate these structures by at least 80 million years. Furthermore, because the fractures are sand-filled and deformed, their reservoir potential is likely to be significantly different than that modeled for open fractures associated with bending of the limestone within fault-propagation folds. Therefore, determining the distribution and geometry of the Jurassic grikes, differentiating them from younger fractures that are kinematically related to the local structure, will improve oil drilling efficiency and recovery.

Results of summer fieldwork, accounting for grike distribution, fill characteristics, and evidence of deformation, indicate that a significant set of these fractures trend NNW (350). Their geometric relationship to the folding and faulting at the field site is depicted using Geographic Information System (GIS) software and shows that the fractures are distributed over both the fold limbs and fold crest suggesting a pre-fold age. Future work at other sites throughout the Montana thrust belt will determine the regional extent of the fracture network.