Insights into the petroleum potential of the Australian North West Shelf and Arafura Sea revealed by regional 2D seismic data.
Authors: Tripathi Anand , Jones William , Rajagopal Rajesh 
Published: March 2015

This contribution discusses the interpretation of over 40,000 km of 2D seismic data acquired since 2007 and covering the Northwest Australian continental shelf. This includes the Carnarvon, Roebuck, Browse and Bonaparte basins and the Arafura Sea as far as Irian Jaya, an area of about 1.5 million km2. This data is mostly recorded to 10–12 s with an 8 km dual sensor streamer. It is of better quality than vintage seismic, providing increased resolution and improved penetration, particularly in the deeper part of the section. The new data provides continuous coverage across the Australia / Indonesia boundary allowing seismic interpretation studies to include the whole of the Australian continental margin. Individual lines are up to 600 km long and composites can be made which are up to 1,500 km long.

The interpreter can now see relationships amongst basins on a regional scale. Horizons below the top Triassic, including top Permian and often basement are mappable consistently over the whole of the Northwest Shelf, leading to a better understanding of the evolution of the continental margin. In the Exmouth Plateau area of thinned crust, the seismic has imaged reflectors possibly down to the Moho. Modelling of gravity data acquired with the seismic has helped with interpretation of the deeper horizons.

Better imaging at depth now permits the Australian continental margin sequence to be followed under the accretionary wedge in both Seram and West Timor, demonstrating that Mesozoic source and reservoir rocks may be present under the wedges, improving the prospectivity of both areas and perhaps of the poorly explored Outer Banda Arc in general.

The poster is illustrated with examples of seismic sections taken from a range of structural settings and geographical locations covering the whole Australian shelf from the Carnarvon Basin to Seram.

Introduction

Across offshore north and northwest Australia (essentially the North West Shelf) and adjoining parts of Indonesia, more than a dozen basins have been described which conservatively cover approximately 1.6 million km2. Much of the hydrocarbon exploration efforts to date have been focussed mainly on the inboard areas of the North West Shelf (abbreviated to NWS hereafter), resulting in multiple province scale oil or gas discoveries. The relatively high density of exploration drilling along the inboard part of the NWS implies that little potential exists for any new major oil or gas discoveries in this area. However, the under-explored deepwater part of the NWS is huge and entire untested basins are present along the Australian northern continental margin in the vast Timor and Arafura Seas including the eastern part of the Indonesian archipelago. In this region are basins covering hundreds of square kilometres, with as much as 10 km of sediments and evidence of active petroleum systems (that are part of the Westralian petroleum supersystem). PGS's seismic coverage in this area demonstrates the potential for multiple large drilling targets. The primary geological risks in these frontier areas are a combination of reservoir presence, source and seal effectiveness. It is clear that the thrust of future exploration will be in the untapped outboard frontier areas, as demonstrated by the recent multi-tcf Abadi gas discovery. Planned infrastructure such as floating liquefied natural gas technology will also spur more exploration activity in these relatively high-cost remote areas.