Top seal bypass risk due to fracture systems, nearshore/onshore Gippsland Basin
The extensional architecture of the Gippsland Basin was modified by a phase of contractional deformation during the Oligocene—Pleistocene postdating the main subsidence phase of the basin. This deformation caused local inversion and folding, which modified the depocentre geometry and controlled deformation of the syn-kinematic regional top-seal—the Lakes Entrance Formation. Accordingly, there is spatial variation of deformation intensity and lithofacies distribution, the latter of which possibly affected the strain accommodation behaviour of the Lakes Entrance Formation. These factors are critical and locally detrimental to seal capacity. In this study, the volume of shale distribution of the Lakes Entrance Formation was modelled and various parameters of the seismic-scale faults were computed, including shale gouge ratio, slip tendency and dilation tendency. Sub-seismic deformation was captured by strain and curvature attributes at the Latrobe unconformity, which carries the most intense imprint of the deformation phase. These parameters were correlated to known hydrocarbon seepage and leakage indicators in the basin, which could be related either to: (i) localised deformation along fault zones; or, (ii) to distributed deformation separated from the fault zones. There is generally a good match between the anomalous values of the computed parameters and the location of leakage indicators. For fault-related localised deformation zones, the match of the parameters ranks in the following order: shale gouge ratio (95%), strain (84%), curvature (84%) and slip tendency (74%). By combining these four parameters, a fault-related leakage assessment factor (FLAF) was defined and mapped across the study area. Match ratio of the parameters used to capture distributed deformation separated from the fault zones are ranked in the following order: curvature (100%), strain (100%) and volume of shale (83%). These parameters were also combined to define an ‘other’ leakages assessment factor (OLAF) and mapped across the study area. FLAF and OLAF maps are consistent with known leakage/seepage indicators in the basin and are indicative of additional areas with potential risk for top seal bypass. These risk maps provide useful input to CO2 storage and hydrocarbon exploration efforts in the basin.