Comparing Biases of Fault Zone Permeability Magnitudes and Inferred Conceptual Models - Global Multidisciplinary Compilation and Mapping

Abstract:

Although fault zones have been studied worldwide, there have been no global mapping, compilation and meta-analysis of interpretations of the fault zone permeability structures and/or methodological biases. To investigate biases in data collection sources we review ~2000 published studies and reports and summarize categorical data from over 600 cases, including ~200 studies with reported fault zone permeability, transmissivity, or diffusivity estimates from the fault damage zone, fault core, whole fault zone, and protolith. The data are categorized into fault zone permeability structures (e.g. barrier, conduit, barrier-conduit, etc.) and are evaluated with respect to the type of fluid flow or permeability observation, the data collection source (e.g. studies in structural geology, hydrogeology, tunneling, mining, engineering, etc.), and on the scale of measurement. Our results show that the combined conduit-barrier fault zone structure is observed in only 15-20% of the cases (but up to 60% of structural geology cases if paleo-conduit studies are included). The barrier structure is observed in ~30% of the faults in structural geology, hydrogeology, and mining studies, and in over 40% petroleum engineering studies, but in less than 10% in tunnel engineering and rarely in geothermal engineering. The barrier nature of faults is detected primarily with qualitative observations (water levels and pressures, water geochemistry), and is difficult to measure in the subsurface. Some hydrogeological observations favour the detection of hydraulic barriers or conduits, but not both equally. Therefore, the frequency of fault zone conceptual models (barriers/conduits) globally or within a region may be a result of measurement bias and not of actual conditions. We also compare reported permeability values at three scales of measurement: matrix permeability, small scale fractured bulk permeability, and whole fault zone permeability. The quantitative permeability anisotropy or scaling ratio between the fault core or fault damage zone and protolith is only reported in fewer than 30 studies (excluding numerical/parametric modelling with assumed permeability). We use the global data summaries and statistical comparisons to comment on the multidisciplinary understanding of fluid flow in fault zones.