RUI: Coupled Fold-fracture Evolution in the Stillwell Anticline, West Texas

Trinity University, San Antonio TX

Investigators

Abstract

The linked formation and evolution of faults, folds, and fracture systems continue to be an important area of research because of the inherent importance of these features to the movement of fluids in the subsurface. A reconnaissance investigation of the well-exposed Stillwell anticline in west Texas has revealed along strike variation in fold geometry tied to different stages of fold evolution; the fold?s geometry supports formation controlled by propagation of an underlying basement fault system. Past research has revealed a range of factors that play important roles in the development of fracture systems in this context, including the influence of initial and transitional fold geometries, the influence of pre-existing fractures, the presence or absence of interlayer slip, and the effects of the underlying fault systems that are responsible for fold development. However, few studies have integrated a quantitative description of fracture strain and width or explored how fracture cement textures can be used to unravel the timing of fracture opening or relict porosities associated with fracture networks. The goal of this project is to integrate an array of evidence in an attempt to answer four primary research questions including: 1) Can structural position within an anticline system be used as a predictor of fracture intensity?; 2) Can mechanical stratigraphy be correlated with fracture stratigraphy?; 3) How to do the distributions, intensities, orientations, and cements of fractures sets vary before, during, and after fold formation?; and 4) What other (non-fracture-related) modes of deformation accommodate strain at different positions within a fold system? The excellent exposure of the anticline provides the opportunity to directly observe fractures, faults, and deformation bands within the folded layers, which can be used to help constrain the kinematic evolution of the fold. In addition, detailed mapping of cross-sectional fracture networks in concert with Schmidt hammer testing of these lithologies, to provide meter-scale semi-quantitative information about the rigidity of deformed beds, will generate a detailed mechanical stratigraphy that can be evaluated in the context of fold and fracture formation. Closely linked with analysis of mechanical behavior, the research team will apply fracture intensity analysis based on both layer thicknesses and structural position within the fold system, taking the wide spectrum of fracture sizes into account. This approach permits evaluation of the scale-dependence of fracture intensity, which can be a serious problem in the evaluation of permeability in subsurface core-based studies. In addition, standard microscopic petrographic analysis and scanning-electron-microscope-based cathodoluminescence imaging will help document growth textures in opening-mode fracture cements to determine synkinematic changes in permeability. Finally, computer kinematic modeling of fault-related fold formation will be used to constrain the predicted distribution of strain across the fold system in order to relate the timing of fracture formation to stages of fold evolution. The results of the proposed research will provide a new view of how permeability evolves during fold formation. Important decisions regarding subsurface water and petroleum exploration are commonly made with incomplete data. If the proposed research can answer the questions posed above, then this coupled fold-fracture investigation of the Stillwell anticline system will provide a model that should reduce uncertainty in permeability evaluation in less well-constrained systems worldwide. In addition to the scientific objectives of the research, the project is integrating undergraduate field instruction with field research, ensuring that undergraduate researchers gain a thorough overall understanding of fundamental geological processes in the context of their own work. In addition, Trinity University undergraduate students will have the opportunity to travel to research labs to perform analyses with faculty at larger research institutions; will disseminate results of their research through independent research papers, senior theses, and presentations at both regional and national meetings; and all project research results and subsequent products will be made accessible to all Trinity students and to the general public through a collaborative project with the Trinity University library.