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ABSTRACT: Assessing fault seal risk for the Wild Dog Road prospect, Otway Basin, Australia

Ryan, Lisa1, Boult Peter2, Richard Jones3, and John Kaldi1
(1) National Centre for Petroleum Geology and Geophysics, Adelaide, Australia 
(2) Boral Energy, Adelaide, Australia
(3) National Centre for Petroleum Geology and Geophysics, University of Adelaide, Adelaide, Australia

The Wild Dog Road (WDR) prospect was estimated to contain 3.6 BCF of gas within a fault dependent footwall play. The main reservoir is the basal Upper Cretaceous Waarre Sandstone, which produces gas from 7 other fields in the Port Campbell Area. The top seal is the Belfast Mudstone, the thickness of which is controlled by growth faults. Risk of fault juxtaposition leakage in the area is high as throws are of similar magnitude to seal thickness. However, risk of leakage associated with recent tectonic activity and fault reactivation was considered to be low for the WDR fault.

WDR prospect is covered by 3D seismic and several neighboring fields provide good well control. The Belfast Mudstone is overlain by a potential thief sand (Nullawaarre Greenstone) and underlain by a potential waste zone (Flaxmans Formation). Modelling of the WDR fault using FAPS showed overlap of the thief sand and waste zone creating a potential leak window. Shale gouge ratio's (SGR's) were generated for this leak window and cross-plotted against potential across fault pressure difference for comparison with worldwide seal v's non-seal analogues. A maximum hydrocarbon column height of 135 m was predicted showing that the field could be full to structural spill point.

Drilling revealed a hydrocarbon column height of 20m, which is less than structural spill. This suggests that either the trap was not charged to spill, capillary migration had occurred through the potential leak window, or that recalibration of the SGR attribute is needed for assessing fault seal risk this area.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia