The Greater Northern Australia-Eastern Indonesia Super Gas Province: Why so much Gas and Little Oil?

Barber, Peter M.¹; Winterhalder, Jim²
¹Basin Studies, ISIS Petroleum Consultants, Perth, WA, Australia.
²Burial Modelling, ISIS Petroleum Consultants Pty Ltd, Perth, WA, Australia.

The Northern Australia-East Indonesia Super Gas Province of Late Palaeozoic-Mesozoic age, extends from the Northern Carnarvon Basin, through the Browse Basin into the Timor Sea (collectively known as the North West Shelf), along the southern edge of the Papua-PNG obduction zone into the Coral Sea.

Numerous LNG projects along the North West Shelf (North Rankin, Wheatstone, Gorgon, Ichthys, Bayu-Undan) or in West Papua (Wiriager), and PNG, indicate the overwhelming gas-prone nature of Late Palaeozoic-Mesozoic petroleum systems along the former Northern Australian continental margin. Some 80% of all hydrocarbons discovered to date in this region comprise gas. This abstract explores the reasons why this should be the case and identifies where sweet spots exist for finding oil as well as gas.

The distribution of oil v gas-prone source rocks is controlled by successive Late Palaeozoic-Mesozoic passive margin extension, listric ramp-flat detachments, rifting and breakup, overprinted by 2nd and 3rd order global eustatic cycles of the northern Australian margin and its tectonic interaction and obduction during the Cenozoic with the West Papua-PNG foldbelt. Inherent within these tectono-eustatic cycles occur two major source rock types: Organofacies B (algal marine) and D/E (mixed terrestrial/algal)

Oil-prone Organofacies B with original TOC's and HI's of over 6% and 600 Hi respectively only occur in localised Oxfordian-Kimmeridgian rhomboid syn-rifts separated by incipient fracture zone, such as the Dampier, Vulcan and PNG foreland basins. Conversely, Organofacies D/E exhibit TOC and HI ranges of 2% and 300 respectively.and are much more widespread. These occur in lower delta plain coals throughout the pre-, syn- and post-rift, accommodated by pronounced extension during the Late Permian-Triassic along basal detachments, depositing thick Late Permo-Triassic to Jurassic fluvio-deltaic sediments with stacked coaly facies

200TCF of in-place gas has been found along the entire Northern Australian margin from the Northern Carnarvon Basin through West Papua to PNG. Yet-to-find fractal analysis suggests that substantial gas resources remain to be discovered, some of which will be reservoired in multi-TCF accumulations, offering considerable exploration incentives. Future oil discoveries will tend to be modest, being limited to existing rhomboid syn-rift basins such as the Dampier and Vulcan Sub-basins, or the obducted basins of the West Papua-PNG fold belt.

AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012