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Geologic Controls on Injection Related Reactivation of Basement Faults

Abstract

Generation of earthquakes by injection of fluids at depth in sedimentary basins is a growing issue worldwide. The largest felt earthquakes generated in this manner originate in basement rock beneath sedimentary injection horizons. We examine geologic controls on transmission of fluid pressure across basal nonconformities into basement faults in an attempt to improve risk assessment for proposed injection sites. Although a considerable amount of previous work has investigated the controls on fluid flow in fault zones in sedimentary and crystalline basement rocks, few, if any, have investigated the hydrologic coupling of the two (faulted sedimentary/basement) systems. We have developed conceptual models of fault-zone permeability architecture of faulted nonconformities and refined the models using data from outcrop analogues in New Mexico and Colorado. Outcrop characterization included description of fault-zone architecture and basement and sedimentary section lithologic variation, and permeability distribution using a portable permeameter. The diagenetic history of the sites has also been investigated with an emphasis on determining the history of past fluid flow across the basement/sedimentary interface. Key variables that influence the permeability architecture of such coupled systems include: the degree of weathering of the nonconformity, timing of deformation relative to lithification, likelihood of fault mineralization, basal lithology, and the nature of mechanical infiltration of sediment into basement fracture networks. Our conceptual models will constrain numerical models to assess the risk of induced seismicity for a given site using commonly available geologic data.