FRACTURE AND JOINT ANALYSIS OF UPPER DEVONIAN DOLOMITE ACROSS SOUTHWEST AND NORTH-CENTRAL MONTANA: 3D MODELING OF STRUCTURALLY CONTROLLED DIAGENESIS

Intersecting brittle fractures and joints in rock units may be modeled as networks through which subsurface fluids preferentially migrate. Fracture and joint networks can volumetrically comprise the majority of the pore space in carbonate rocks, and these networks can strongly influence diagenetic alteration across many scales of observation. Permeability within carbonate rocks is often dependent on fracture/joint connectedness and the relative percentage of extension versus shear fractures. This field-based investigation documented the orientation, spacing, aperture and frequency of fractures and joints in Upper Devonian (Frasnian Stage) dolomite in southwest and north-central Montana. Dolomite units were studied in a variety of structural environments, including the fold-and-thrust belt along Montana’s Rocky Mountain Front, the Little Belt and Big Snowy Mountains of central Montana, and a Laramide perched basement wedge in southwest Montana (Bridger Range). Despite obvious differences in structural environment, dolomite beds were always systematically fractured with major sets parallel and perpendicular to bedding, and left- or right-oblique to bedding. Fractures and joints were measured by the selection method on outcrops and analyzed with 3D modeling software to explore larger-scale characteristics. The network of fractures and joints at the mesoscopic scale was well connected with apertures of 1-3 mm. Microscale fractures weren’t considered in the model, as many of them lacked measurable aperture and were not consistently connected to one another. There was also evidence of hydrothermal fluid alteration along mesoscopic fractures in the form of de-dolomitization, calcite precipitation and hydrothermal breccia conduits bounded by fractures parallel to bedding. Changes in lithology between beds, including de-dolomitized vuggy zones, tight dolomite and dolomitized stromatoporoids, appear to have influenced the placement of fractures and joints, and ultimately fluid migration pathways. The results of this study suggested that the role of different fracture and joint sets, such as those parallel and perpendicular to bedding, were the most significant in structurally controlled diagenesis, whereby warm fluids migrated through the network and altered existing lithology.