The Otway GeoCquest Field Validation experiment involved the injection of approximately 10,000 tonnes of supercritical CO₂-rich gas (80 mol% CO₂ and 20 mol% CH₄) into the lithologically heterogeneous Paaratte Formation Parasequence 2 within the onshore Otway Basin at a depth of approximately 1.5 kilometres. One of the major objectives of this field-scale test was to study the role of small-scale geological heterogeneities, such as petrophysical thin beds composed of low-permeability, low-porosity intraformational baffles. We assessed how these features control the vertical and lateral distribution of the CO₂ plume across different geological layers and influence overall plume-migration dynamics.

To monitor plume behaviour, a high-frequency pulsed-neutron logging (PNL) program was conducted in the dedicated passive monitoring well, CRC-8, over a five-month period, with multiple passes acquired daily during both injection and post-injection phases. A time-lapse differencing approach applied to baseline and monitoring datasets enabled detection of subtle changes in log responses and their spatiotemporal evolution with high fidelity. An integrated thermodynamics-based petrophysical framework was developed that combines different PNL measurements to quantify changes in saturation across the reservoir while accounting for measurement noise, depth mismatches, and borehole environmental effects.

Results indicate rapid migration and early gas breakthrough along preferential high-permeability streaks, together with clear evidence that small-scale heterogeneities strongly influence CO₂ plume distribution. Geological features such as heterolithic intervals and grain-size variability were found to control capillary entry pressures, resulting in capillary-heterogeneity-driven flow patterns and non-uniform saturation distributions at the field scale. Additionally, sedimentary structures including clay-rich thin beds, cross-bedding, and low-angle sandstone beds with carbonaceous laminae exerted further control on plume geometry during both injection and post-injection phases. Overall, these findings highlight the dominant role of small-scale heterogeneities in governing plume migration and provide critical insights for advancing carbon sequestration monitoring technologies and improving the reliability of predictive models.