Arafura Basin

Basin Details and Geological Overview

The Neoproterozoic to Palaeozoic Arafura Basin is located on the northern Australian margin. It is underlain by the Proterozoic McArthur Basin and the Archean to Proterozoic Pine Creek Inlier, and overlain by the Mesozoic to Recent Money Shoal Basin. The structure of the Arafura Basin is dominated by the highly deformed Goulburn Graben.

The Goulburn Graben is a northwest trending asymmetric feature, over 350km long and up to 70km wide, and contains a sedimentary section in excess of 10km thick. The region to the north of the Goulburn Graben forms the basin depocentre and contains a sedimentary succession up to 15km thick. South of the Goulburn Graben a north-dipping relatively undeformed ramp contains up to 3km of sedimentary rocks.

The Arafura Basin is located on the northern margin of Australia in the Arafura Sea and extends from the onshore Northern Territory to offshore northern Australia and beyond the Australian-Indonesian border, covering approximately 200 000km2 in Australian territory.

The Arafura Basin formed in the Neoproterozoic as a result of NW-SE upper crustal extension producing a series of NE-SW trending half graben across much of the basin. During this time the dominantly clastic sediments of the Wessel Group were deposited. Subsequent periods of subsidence in the Cambro-Ordovician, Late Devonian and Late Carboniferous to Early Permian were separated by long, tectonically quiescent periods of non-deposition and erosion. The predominantly marine carbonates of the Cambro-Ordovician Goulburn Group are overlain by shallow marine to non-marine clastics and carbonates of the Devonian Arafura Group and fluvio-deltaic clastics of the Late Carboniferous to Early Permian Kulshill Group equivalent.

The Goulburn Graben formed in the Late Carboniferous to Early Permian in response to oblique extension associated with the break-up of Gondwana. Subsequent contraction in the Triassic resulted in oblique inversion of the Goulburn Graben, uplift and erosion of up to 3.5km of sediment. The deformational event affected areas to the north and south of the Goulburn Graben to a lesser extent. Subsequent erosion resulted in formation of a peneplain across the basin upon which the sediments of the Money Shoal Basin were deposited.

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Petroleum Systems and Hydrocarbon Potential

The Arafura Basin, despite its thick sedimentary succession and shallow waters depths, is largely a frontier region. Previous exploration in the region has been limited to the Goulburn Graben, where the thick sedimentary succession and large inversion structures provided the exploration targets. Drilling activity occurred in 3 phases - the early 1970s, early 1980s and the late 1980s to early 1990s. To date, a total of nine wells has been drilled, with the best result being an oil and gas show at Arafura-1. This exploration also confirmed the presence of essential petroleum system elements including potential Palaeozoic and Mesozoic source and reservoir rocks and a thick regional seal in the overlying Money Shoal Basin sediments.

The lack of commercial success has been attributed to a combination of poor quality reservoirs in the Palaeozoic and/or restricted fluid movement, absence of hydrocarbon charge, timing of events and breach of structure. However, despite the current lack of success, there are numerous indicators for untested petroleum potential, particularly in the undrilled northern region of the basin:

• The petroleum systems elements found in the Goulburn Graben are also likely to be present in the northern region
• Many of the risks identified in the Goulburn Graben are reduced in the northern region (shallower reservoirs, reduced charge risk and reduced risk of breached structures)
• A wide range of play types are present, ranging from faulted anticlines and sub-unconformity plays in Palaeozoic strata to fault block plays and numerous stratigraphic plays in Mesozoic strata
• Direct evidence that hydrocarbon generation and expulsion has occurred in the Arafura Basin is provided by oil shows/indications and gas indications in the majority of wells drilled, and the presence of interstitial solid bitumens in many samples. Seismic data show bright amplitudes at various stratigraphic levels and these may indicate hydrocarbons in the section
• A 2005 Geoscience Australia survey to investigate potential hydrocarbon seepage in the Arafura Basin collected a range of datasets, including sub-bottom profiles, side-scan sonar and echosounder data. These, together with available deep-seismic data, show a range of features that are indicative of the presence of hydrocarbons in the northern and northeastern basin.

Key References