Background and Objectives
The Queensland Office of Groundwater Impact Assessment (OGIA) is responsible for assessing cumulative groundwater impacts arising from coal seam gas extraction (CSG) in the Surat cumulative management area (CMA). A key component of the assessment is the characterisation of faults. Faults have the potential to influence aquifer connectivity and are therefore key inputs to the construction of both geological and numerical models.
Significant advances were made in 2016 by OGIA to better represent regional fault systems in a groundwater-modelling framework. Since then, OGIA has focused on the characterisation of a large number of smaller faults.
As such, the main objectives of this project were to document the current understanding of faults in sedimentary basins and to assess fault-induced connectivity between CSG reservoirs and adjacent aquifers.
Methodology
OGIA’s approach to conceptualising faults comprises of three steps:
This involved a review and interpretation of available seismic data to identify faults of interest. Once faults were identified, attributes such as displacement, stratigraphic and lithological contact and fault permeability were estimated utilising several fault seal methods.
Case studies were conducted in each aquifer of interest. This was done in order to understand the likely processes occurring at each site and to validate the fault parameterisation approach. Detailed numerical modelling was also utilised to test the parameterisation approach.
Following validation of the approach, the fault seal analysis is carried out at all known fault locations in the Surat basin in order to identify faults, which may increase the connectivity between formations of interest.
Results and Conclusions
Mapping and characterisation of a large number of faults in the Surat Basin have improved the conceptual understanding of groundwater flow, aquifer connectivity and analysis of monitoring trends. Overall, there are some faults that are likely to affect aquifer connectivity particularly between the Springbok Sandstone and the Walloon Coal Measures. However, the assessment so far provides no basis to expect that faulting will have implications for the propagation impacts at regional scale.
As part of this study, several key attributes have been estimated for all known faults in the Surat basin including, location, displacement and key parameters pertaining to fault permeability. These datasets can be used in future numerical modelling workflows to estimate the impacts of faults on groundwater flow.