INFLUENCE OF DIAGENESIS ON REGIONAL AND FOLD-RELATED FRACTURE SIZE DISTRIBUTION

Fracture size distributions influence permeability, but sampling subsurface fractures is difficult. Consequently accurate predictions that can be verified using readily sampled parts of fracture size distributions are useful. Fracture aperture frequency analysis has been proposed as a method for using microfracture observations to estimate macrofracture abundance (average spacing). The effectiveness and accuracy of this approach, however, is difficult to assess with core observations alone owing to sparse sampling that typically affects macrofracture observations. We test the reliability of the approach in a regional outcrop study of Cambrian Flathead sandstone in and near Grand Teton National Park. Samples span an area ~30×60 kms and include (1) gently tilted low-strain areas >10 km distant from major faults, (2) transects along and across major Laramide basement-involved reverse faults and associated folds, and (3) samples from fault zones accommodating tri-shear deformation. Indurated Flathead quartzarenites have regionally variable quartz cement volumes reflecting differences in burial history/thermal exposure, but retain primary rock and macrofracture porosity which SEM-CL analysis demonstrates resembles that in many productive tight-gas sandstones. Sparse populations of micro- and macrofractures are present in the least deformed rocks. These aperture size distributions can be described by a power-law distribution and fit a slope of -0.8 to measured data, which is effective for predicting macrofracture abundance. Samples adjacent to reverse faults have the highest microfracture abundance. These samples also include porous macrofractures that have localized crack-seal texture in quartz deposits and porous quartz-cemented breccia. Measurements along transects allow comparison of fracture size distributions between rock with the same composition and thermal history but differing fold-related strain. Results are in broad agreement with structural diagenetic models that account for fracture size distributions by mechanical interaction of quartz precipitation and fracture growth.