Characterising structural variation in the shallow subsurface at the Otway Basin, Australia: Challenges for environmental monitoring of geological CO ₂ storage

Effective measurement, monitoring and verification (MMV) strategies for Geological Carbon Storage (GCS) and other geological engineering technologies must consider subsurface uncertainties when determining the spatial coverage and resolution of a monitoring programme. MMV of any future leakage to surface or near-surface receptors requires understanding uncertainties in the top 0-100 metres, and it is possible that optimal sites for storage, may well have non-optimal near-surface geology for characterisation of shallow leakage risk. Here we present a multiscale case study of the difficulties in site characterisation at such a site. In this paper, we compare field observations from a 45km section of coastal outcrops and an inland limestone quarry; to core and well logs from the Otway International Test Centre (OITC) CO 2 injection project. We find that the type of faulting varies strongly with location, changing from reverse at the coast, to strike-slip at the OITC. The observed variability in fault type across this region highlights the need to be cautious when using nearby faults which outcrop at the surface as indicators of local fault architecture, as any variability will impact shallow fluid migration and leakage. This work highlights the need for better understanding of near-surface processes affecting fault zones, and better methods of geophysical characterisation of the shallow subsurface, particularly as there is a scale up of GCS projects worldwide.