Hydrogeological implications of fault behaviour from in situ pressure measurements of the Horrane Fault in the Surat Basin (Great Artesian Basin, Australia)

In order to assess the impact of faulting on the production and compartmentalisation of a coal seam gas (CSG) reservoir, seismic reflection surveys and subsequent drilling were undertaken to locate and then intersect the Horrane Fault in the eastern Surat Basin, in the Great Artesian Basin of Australia. The investigation of the Horrane Fault in the area of Cecil Plains on the Darling Downs identified faulting at depths of 50-600 m within the Walloon Coal Measures. Pressure data collected during the drilling show differential pressures on each side of the fault. The differential is interpreted to show depletion on the western side of the fault due to the CSG production field located west of the fault. On the eastern side of the fault, the pressures were near hydrostatic, and thus the fault is acting as a lateral seal. Regional modelling currently simulates faults as having permeability based upon the fraction of the units off-set across the fault. This would lead to exaggerated predictions of impact not supported by the data from this study. Conceptual models of the Surat Basin are currently based upon the premise that connectivity by direct linkages due to the faults enhances groundwater-level changes due to water extraction and that the behaviour of the faults is poorly understood. This study represents the first detailed evaluation of fault behaviour in the eastern Surat Basin and can be used to update current conceptual models and inform regional groundwater impact assessments.