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Fault seal analysis in ‘branched’ fault traps, eastern Gippsland Basin

Summary The impact of small-scale and ‘branched’ faults in the assessment of fault-trap integrity often remains unknown or unresolved when risking/evaluating a lead or prospect in regards to its petroleum prospectivity. The following study considered 40 leads in the eastern Gippsland Basin and assessed the overall impact of branch lines on fault trap integrity. Faults and surfaces were interpreted within a 30X50 km area using a 3-D seismic volume, depth-converted using purpose-programmed software and, a fault seal analysis carried out on the top three ranked leads. The fault seal analysis was underpinned by logs from 13 wells located in and adjacent to the leads. It was found that the likelihood of fault reactivation increased for 32% of the branch-line cases considered, reduced for 26% of cases while the change being minimal for the remaining 42%. Overall, modelling of the volume of shale attribute indicated that it is unlikely that sand-onshale windows can be maintained along fault-strike lengths that range up to 20 km. The study conclusively demonstrates that the primary factor affecting fault-trap integrity in the Halibut Subgroup is the high proportion of across-fault sand-onsand windows; by comparison, the contribution of fault tips, ‘branched’ faults and presence of shale smear is secondary. One implication is that the fault-trap integrity for leads located in the eastern Gippsland Basin is more likely to be enhanced closer to the depocentre where the shale content is anticipated to be higher.