Abstract 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 northwestern 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.

A sub–Middle Jurassic unconformity is exhumed at Swift Reservoir, in the Rocky Mountain fold-and-thrust belt of Montana. The unconformity separates late Mississippian Sun River Dolomite of the Madison Group (ca. 340 Ma) from the transgressive basal sandstone of the Middle Jurassic (Bajocian-Bathonian) Sawtooth Formation (ca. 170 Ma). North-northwest–trending, karst-widened fractures (grikes) filled with cherty and phosphatic sandstone and conglomerate of the basal Sawtooth Formation penetrate the Madison Group for 4 m below the unconformity. The fractures link into sandstone-filled cavities along bedding planes. Clam borings, filled with fine-grained Sawtooth sandstone, pepper the unconformity surface and some of the fracture walls. Sandstone-filled clam borings also perforate rounded clasts of Mississippian limestone that lie on the surface of the unconformity within basal Sawtooth conglomerate. After deposition of the overlying foreland basin clastic wedge, the grikes were stylolitized by layer-parallel shortening and then buckled over fault-propagation anticlinal crests in the Late Cretaceous–Paleocene fold-and-thrust belt. We propose that the grikes record uplift and erosion followed by subsidence as the Rocky Mountain foreland experienced elastic flexure in response to tectonic loading at the plate boundary farther to the west during the Middle Jurassic. The forebulge opened strike-parallel fractures in the Madison Group that were then karstified. The sandstone-filled karst system contributes secondary porosity and permeability to the upper Madison Group, which is a major petroleum reservoir in the region. The recognition of the fractures as pre–Middle Jurassic revises previous models that have related them to Cretaceous or Paleocene fracturing over the crests of fault-propagation folds in the fold-and-thrust belt, substantially changing our understanding of the hydrocarbon system.

Abstract This one-day field trip examines two Neoproterozoic sections in the Portneuf Narrows area, SE of Pocatello, Idaho. Rocks to be examined on the first traverse belong to the Scout Mountain Member, Pocatello Formation, and include <710 Ma glacial diamictites, dolomite cap, 667 Ma tuff, and upper caplike carbonate. The second traverse, in Blackrock Canyon, exposes the cyclic Blackrock Canyon Limestone, with upward-shallowing siliciclastic to carbonate cycles and microbial mounds.