PETROLEUM EXPLORATION IN THE PROTEROZOIC BEETALOO SUB-BASIN, NORTHERN TERRITORY

Exploration over Roper Group outcrop in the McArthur Basin prompted Pacific Oil & Gas to investigate the petroleum potential of the Beetaloo Sub-basin, a 15 000 km2 Proterozoic depression concealed beneath Phanerozoic cover. Since 1989 drilling and seismic has identified a broadly flat-lying sequence with uplifted, eroded margins. A 3 500 m composite Proterozoic section consisting of three sandstone-to-mudstone sequences has been drilled. The lower two sequences comprise conformable units of the Mesoproterozoic upper Roper Group and unconformably overlying them is a previously unknown sequence comprising the informally labelled 'Jamison Sandstone' and 'Hayfield Mudstone', probably of Neoproterozoic age. Organic-rich intervals in the Roper Group mudstones range one to three per cent TOC in the Kyalla Member, and two to seven per cent TOC in the 'Middle' Velkerri Formation. Across most of the sub-basin the oil window lies within the Kyalla Member around 900–1350 m, while the 'Middle' Velkerri Formation is around 2 500 m and well into overmaturity, but may still have potential for gas. Potential reservoirs in the Bessie Creek, Moroak and 'Jamison' Sandstones, and in sandy units within the mudstones, are compromised by diagenesis, but porosities of up to 20 per cent and permeabilities of tens and rarely hundreds of millidarcies have been measured. Encouraging shows were observed in many of these intervals, and small quantities of oil and gas have been recovered in drill stem tests. With only a few targets drilled to date, this frontier area requires more exploration before its hydrocarbon potential can be adequately assessed.