Fracture distribution on the Swift Reservoir Anticline, Montana: implications for structural and lithological controls on fracture intensity

Hannah Watkins (Corresponding Author), Clare E. Bond, Adam J. Cawood, Mark A. Cooper, Marian J. Warren

Research output: Contribution to journalArticle


Where primary porosity and permeability of a rock are unfavourable for hydrocarbon production, fractures can improve reservoir potential by enhancing permeability. Higher fracture intensity may create a better connected fracture network, improving fractured reservoir quality. Investigations into the controls on fracture intensity commonly conclude that either structural or lithological factors have the greatest influence on fracture abundance. We use the Swift Reservoir Anticline in north western Montana to investigate how fracture intensity varies throughout the structure, and determine that although structural factors do influence fracture intensity, lithology is the main control at outcrop.

The Swift Reservoir Anticline exposes bedding surfaces of the Mississippian Castle Reef Formation dolomite. Field data indicates that fracture intensity is highest in the fold forelimb, decreasing into the backlimb except in outcrops of coarse dolomite where fracture intensity is low, regardless of structural position. Field fracture intensity correlates with whole rock quartz, kaolinite and porosity percentages. We suggest porosity and composition influence bulk rock mechanical properties, which, in turn, control the fracture intensity at outcrop. Fracture intensity has a stronger relationship with lithological than structural factors, therefore we suggest that the key to predicting fracture intensity in the subsurface here is understanding how lithology varies spatially.
Original languageEnglish
Pages (from-to)209-228
Number of pages20
JournalGeological Society Special Publications
Early online date16 Jan 2019
Publication statusPublished - 2020


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