STRUCTURE AND HYDROCARBONS IN THE SHIPWRECK TROUGH, OTWAY BASIN: HALF-GRABEN GAS FIELDS ABUTTING A CONTINENTAL TRANSFORM

Abstract

As part of a regional study of the evolution of the Otway Basin, the Investigator 3D seismic survey has been structurally analysed, using 11 extracted 2D sections and 3D interpretations of key horizons. South-southwest directed extension was widespread in the Turonian forming the Shipwreck Trough, coincident with uplift of the Otway Ranges to the northeast. The Turonian extension, at ~1.5 myrs, resulted in planar faults in the northeastern part of the Trough, but large half-graben above south-southwest dipping listric master faults in the southwest, both fault sets soling into an Early Cretaceous shale detachment. The half-graben propagated north from the Mussel-Tarpwaup Hinge-Zone by footwall collapse and accommodated deposition of reservoir rocks for the known hydrocarbon accumulations. The half-graben die out along strike to the east at tip-points against an accommodation zone that developed into a continental transform (the Shipwreck Fault).

Santonian breakup in the Great Australian Bight coincided with considerable south-southwesterly extension in the Otway Basin juxtaposed against the failed Bassian rift across the Shipwreck Fault. Extension of ~1.21 km to the west of the Shipwreck Fault contrasts with ~0.42 km on the eastern side accommodated by ~0.79 km left-lateral displacement along the Shipwreck Fault. The Belfast Mudstone was deposited during this time, forming the regional seal for the known hydrocarbon accumulations.

Limited slow extension during the Campanian to Early Eocene resulted in a further 0.33 km sinistral slip along the Shipwreck Fault. Late Early Eocene Breakup in the Otway Basin ended the transitional phase, terminating extensional and Shipwreck Fault offset. The breakup caused local uplift and ~1 km erosion of Wangerrip Group sediments. The post breakup phase is characterised by prograding sequences indicating progressive-regressive cycles.

The Thylacine and La Bella gas fields occur in large tilted fault-blocks near the Hinge-Zone. These successful large structures lie along a longstanding High probably sourced from south of the Hinge-Zone. Key elements for a successful hydrocarbon play are deposition of the Turonian Waarre Formation sandstone reservoirs at rift onset and of a thick Belfast Mudstone seal during continuous Coniacian-Santonian extension. Footwall collapse north of the Hinge-Zone, bound by the deepwater Otway Basin and the continental transform, controlled the distribution of traps, regional seal and hydrocarbon maturation.