Document Type

Article

Publication Date

4-19-2013

Abstract

In the past twenty years, the fracture research field has experienced exponential growth, but there is still debate about how to best sample and characterize natural fracture networks. A vast majority of studies lack a comprehensive evaluation of variables that control fracture behavior, and few studies take into account either fracture aperture or observational bias in the characterization of fracture systems. In addition, most fracture research has been limited to either the microscopic or macroscopic scale. I investigated fracture networks at the transition between the micro- and macroscopic scale at the wellexposed Stillwell anticline in west Texas. The excellent cross-sectional exposure of the asymmetric anticline provided the opportunity to analyze fracture systems within a single limestone bed at different structural positions, including the forelimb, the forelimb hinge, the middle limb, the backlimb hinge, and the backlimb. At each structural position, I measured fractures’ orientation, fill, morphology, length, and aperture within a rectangular observation area. Because observational bias can strongly affect outcrop data, I used a new multi-step method to account for the unequal probability of encountering fractures based on each fracture’s orientation relative to the observation plane and the orientation of each fracture within the rectangular shape of observation area. Based on these relative orientations, I weighted each fracture, assigning an integer-based correction factor. Optical imagery showed that these fracture systems are mostly composed of calcite veins with multiple generations of fracture fill. Statistical data suggest that fracture intensity, aperture, and fracture length data are significantly different at each structural position, and fracture intensity appears to be directly related to strain. In fold hinges, where bed curvature is v greatest, fracture intensities are highest and fracture lengths are lowest. In contrast, in the forelimb, where shear strain is at a maximum, fracture intensity is lowest and fracture lengths are highest. This suggests that fracture initiation and propagation are strongly affected by structural position, which is likely controlled by the how stresses are applied to limestone beds throughout the formation of the fold system. These results demonstrate that analysis of fracture networks at a transitional scale can provide significant insight about fracture systems and their evolution at different positions in a fold system. In many low porosity oil and gas reservoirs, natural fractures control the permeability of the system, so these results might also help predict permeability changes in similar subsurface fold systems.

Comments

Non-Honors Thesis

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