THE STRUCTURAL ARCHITECTURE OF THE TIMOR SEA, NORTH-WESTERN AUSTRALIA: IMPLICATIONS FOR BASIN DEVELOPMENT AND HYDROCARBON EXPLORATION

Abstract

The initial rifting in the Timor Sea, north-western Australia, took place in the Late Devonian to Early Carboniferous, with the development of the NWtrending Petrel Sub-basin. This rift system was compartmentalised by NE-trending accommodation zones which divided the sub-basin into discrete segments. In each segment, a lower plate rift margin, characterised by large displacement, low angle extensional faults, lay opposite an upper plate, or ramp, rift margin, characterised by small displacement, high angle flexural faults. Switching in the 'polarity' of the rift system took place across major, NE-trending accommodation zones.

Part of this rift system was overprinted in the Late Carboniferous to Early Permian by the Westralian Super-Basin rift system, which developed on a NE trend, orthogonal to that of the underlying Petrel Sub-basin. The entire Vulcan Sub-basin and Sahul Platform region developed as part of an upper plate rift margin, with the Vulcan Sub-basin probably forming initially as a small flexural feature in the inboard part of the upper plate rift margin. The rift margin consisted of a linked array of NW-trending accommodation zones and NE-trending normal faults; pre-existing, NW-, NE- and NS-trending ?Proterozoic fracture systems controlled, at least to some extent, the geometry of the rift system that developed. The island of Timor probably developed as a major intra-rift high, or possibly a marginal plateaux, at this time. Thermal subsidence phase sedimentation continued until the Late Triassic, resulting in the deposition of 10 to perhaps 14 km of relatively unstructured sediments.

Three major reactivation events affected the Timor Sea during the Mesozoic. These were: compression in the Late Triassic to Early Jurassic, extension in the Late Callovian to Early Oxfordian (late Middle to early Late Jurassic) and compression in the Tithonian/Berriasian (Late Jurassic/Early Cretaceous). These events all reactivated the pre-existing ?Proterozoic/ Petrel Sub-basin/Westralian Super-Basin structural architecture in a variety of ways. In the Petrel Sub-basin, reactivation was localised almost exclusively over the lower plate rift margins, leading to the formation of anticlines and ultimately, salt diapirism.

In the Vulcan Sub-basin, all of the significant hydrocarbon discoveries appear to be preferentially located either along, or at the intersection of, NW- and NS-trending fault sets with the NE/ENE-trending grain. This is probably because the intersections of these Proterozoic/Late Carboniferous-Early Permian fault sets respond in a particularly complex fashion to the varying Mesozoic stress directions. In a qualitative fashion, this observation does provide a number of largely untested exploration 'fairways' within the Vulcan Sub-basin.