The Late Jurassic Springbok Sandstone in the Surat Basin is highly heterogeneous in terms of lithology and hydrogeological properties. This heterogeneity is poorly defined in well logs, due - in part - to clay phases that do not exhibit a prominent gamma ray signature. The resulting uncertainties in the hydrogeological properties are transferred to uncertainties in the groundwater models of the Springbok Sandstone. Further, only a small amount of porosity and permeability data is publically available and no petrophysical model of the Springbok Sandstone has been published in the peer-reviewed literature. At the same time, accurately predicting the potential groundwater impact due to coal seam gas production from the underlying Walloon Subgroup is of significant societal and economic importance.
Design and Methodology
We present new porosity and permeability data from more than 50 core samples from the Springbok Sandstone alongside a review of existing data. Based on this dataset and wireline data from five study wells a new petrophysical model for the formation is proposed.
Original Data and Results
The results show that (a) the Springbok Sandstone is highly variable in terms of hydrogeological parameters, (b) this variability can be captured with a petrophysical model that draws on a full log suite (i.e. triple-combo) and (c) electrofacies classifications based on gamma ray and bulk density log cut-offs do not reflect this variability.
Ultimately our results can be utilized in combination with 3D geological models to predict the presence of rock units that have the sufficient transmissivity to constitute aquifers. This work also forms the platform, which allows in combination with future detailed (sequence) stratigraphic analyses to define geobodies and their facies affiliations to predict aquifer properties and their spatial distribution in the Springbok. Our results therefore provide key inputs into a potential regional aquifer characterisation of the Springbok.