Efficiency of a faulted regional top seal, Lakes Entrance Formation, Gippsland Basin, SE Australia

Future geological CO2 sequestration in the Gippsland Basin is contingent upon an effective regional top seal; potentially provided by the late Oligocene–early Miocene Lakes Entrance Formation. This study integrates various top-seal assessment methodologies into a workflow to estimate the efficiency of the Lakes Entrance Formation as a top seal. Factors related to, for example, top-seal lithology, shale volume, carbonate content and fracture density, and factors relating to the faults that cut the top seal, fault-zone shale content, strain, slip-tendency, etc., are compared to hydrocarbon leakage and seepage indicators reported in the study area. The factors that best correlate with reported leakage indicators are combined to map the spatial risk variation.

While the study indicated that the ultimate control on top-seal efficiency is the formation’s membrane seal capacity; it also highlighted the spatial correlation between leakage indicators and some fault-related factors, suggesting that faults are key to top-seal bypass in much of the Gippsland Basin. Fault-zone shale content proved the dominant fault-related factor; as such, it can be concluded in the Gippsland Basin, at least, that a fault-zone shale content of less than 0.3 is the dominant factor with regard to faults enabling fluids to bypass top seals.