The Source of Oil and Gas Accumulations in the Browse Basin, North West Shelf of Australia: A Geochemical Assessment

Abstract

The Browse Basin located offshore on Australia's North West Shelf hosts considerable, but as yet undeveloped, petroleum resources with 36 Tcf EUR (Estimated Ultimate Recovery) of gas and 1148 MMbbl of condensate. It is poised to become Australia's next major conventional liquefied natural gas (LNG) province with the Ichthys, Prelude and Concerto fields expected to be in production by the end of 2016. Significant gas accumulations are also found along, and to the northeast of, the Brecknock-Scott Reef Trend (Calliance, Brecknock, Torosa and Poseidon) and in the Heywood Graben (Crux). Despite the economic importance of these fields and the extensive ongoing exploration activity, the origin of hydrocarbons remains ambiguous and a thorough geochemical evaluation of reservoir fluids and source rocks was carried out to redefine the petroleum systems of the Browse Basin. Geochemical data reveal that the gas-prone source rocks occurring throughout the Lower to Middle Jurassic Plover Formation have pervasively charged reservoirs of the Browse Basin at numerous stratigraphic levels. On the other hand, oil-prone source rocks within the Upper Jurassic lower Vulcan and Lower Cretaceous Echuca Shoals formations appear to be charge limited. The fluvio-deltaic sediments of the Plover Formation are the primary source for the dry gas found in the Plover reservoirs of the Brecknock-Scott Reef Trend and Ichthys fields. The Plover source rocks have also contributed to the wet gas accumulations reservoired within the Upper Jurassic Brewster Member of the Ichthys and Prelude/Concerto fields with additional inputs from the lower Vulcan Formation. Gases from the Crux field in the Heywood Graben are isotopically more enriched in 13C than any gases generated from the Caswell Sub-basin depocentre suggesting derivation from coal-rich facies within thick Jurassic syn-rift sediments. The few sub-economic oil discoveries made in the Browse Basin are confined to the central Caswell Sub-basin (Caswell) and to the Yampi Shelf (Cornea, Gwydion and surrounds) where oil, together with some gas, is found in Cretaceous reservoirs. Molecular and carbon isotopic data show that the oil, and the gas to some extent, is derived from marine organic matter within the Echuca Shoals Formation. However, accumulations on the Yampi Shelf also contain gases sourced from Plover source rocks, emphasising the migration of multiple hydrocarbon charges towards the basin margins.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015