Multi-tracer and hydrogeophysical investigation of the hydraulic connectivity between coal seam gas formations, shallow groundwater and stream network in a faulted sedimentary basin

Abstract

Abstract Characterization of geological fault networks in sedimentary basins targeted for coal seam gas development typically use deep-penetrating seismic data. Such methods can reveal the presence of numerous fault zones at depths greater than 100\u202fm but not necessarily show if these extend to the surface or provide a significant degree of hydraulic connectivity between deep and shallower geological formations. Our combined multi-tracer and hydrogeophysical study used transient electromagnetic surveys to image the subsurface in a sedimentary coal seam gas basin (Gloucester Basin, NSW, Australia) and indicated that deep faults (greater than 200\u202fm) from seismic analysis may extend to within metres of the ground surface. Analysis of environmental tracers in surface water and groundwater samples near fault traces showed the presence of hotspots of deeper groundwater from the coal seams discharging into the stream network via the alluvial aquifer. The isotopic composition of methane was extremely variable in groundwater (δ13C-CH4 from –81.6 to –29.2‰ V-PDB) and surface water (δ 13C-CH4 from –56.0 to –19.9‰ V-PDB), suggesting several sources of methane being recycled in the connected alluvial aquifer and stream network. A 4He hotspot (1.67\u202f×\u202f10−6\u202fcm3\u202fSTP\u202fg−1) in the alluvial aquifer close to a fault line showed high methane (463\u202fµg\u202fL−1) with a thermogenic signature (δ 13C-CH4 of −62.4‰ and δ2H-CH4 of −174.7‰). The Gloucester Basin, which has experienced multiple episodes of complex restructuring, displays high dilation tendency of fractures close to fault damage zones with fracture orientations parallel to the orientation of maximum horizontal stress. As a result, there is increased permeability for wells close to the fault damage zone and with increasing depth. These findings resulted in a robust hydrogeological conceptual model of a faulted basin, which can be used in a monitoring and modelling framework for improved evaluation of risk associated with unconventional gas developments.