Mineralogic controls on fault displacement-height relationships

Abstract. Understanding the distribution and geometry of subsurface faults is critical for predicting fault penetration and associated leakage of fluids such as groundwater, hydrocarbons, and injected anthropogenic waste through sealing intervals. Fault dimensions are often underestimated due to the resolution limits of seismic reflection data, which only image portions of faults with sufficient displacement to offset seismic reflectors. To address this fault underestimation problem, we quantify relationships between host rock composition and fault displacement gradients using a well-exposed outcrop of normal faults in mechanically layered sedimentary rocks in the footwall to the west branch of the Moab Fault, Utah. We integrate high-resolution digital photogrammetry, structural mapping, X-ray diffraction (XRD) mineralogy, and Schmidt rebound measurements to analyze how mineralogy and mechanical properties influence fault displacement vs. height relationships. Our results indicate that normal fault displacement gradients tend to be higher in less competent beds and lower in more competent strata, and that fault displacement gradient is positively correlated with clay content and negatively correlated with strong minerals (e.g., quartz, feldspar, dolomite). Outcrop-derived relationships are used to build a predictive framework that uses fault displacement and mineralogy to predict fault height. We apply this framework to a worked seismic interpretation example and demonstrate that fault dimensions are likely substantially underestimated in conservative seismic interpretations. Our results highlight the importance of mechanical stratigraphy in controlling fault geometry and provide a data-driven approach for estimating sub-seismic fault dimensions, with implications for reservoir characterization, fluid containment, and geohazard assessment.