RECENT EXPLORATION DEVELOPMENTS IN THE NGALIA BASIN, NORTHERN TERRITORY

Abstract

Petroleum exploration of the Ngalia Basin commenced with the Pacific-American Oil Company's seismic and gravity survey in 1964, followed by geological and geophysical surveys by the Bureau of Mineral Resources (BMR) between 1967 and 1978, and then Magellan's Ngalia seismic and gravity survey in 1971.

After a ten year break, exploration resumed with the drilling of the Davis-1 well by a consortium of Australian companies which farmed-in to Magellan's Ngalia Basin Permit (OP165). The well was drilled on the flank of a large east-west trending anticlinal dome outlined by surface mapping and limited seismic coverage, and located near the northern margin of the basin. The section penetrated in the well consists of 1134 m of Carboniferous-Devonian sediments unconformably overlying 479 m of probable Cambro-Ordovician marine sediments, which in turn unconformably overlie approximately 246 m of marine ?Adelaidean sediments, including a basal sequence of dark grey marine shale. Source rock analysis indicates that this latter section may provide a significant source rock potential for the basin. A small gas flow was observed during the course of the well, which was air-drilled to a total depth of 1899 m, bottoming in metamorphics which are tentatively correlated with the Pre-Cambrian Patmungala Beds (?Arunta Block). The current exploration effort is now aimed at:

more deeply buried structures in the Naburula Fault Trough, in the western half of the basin, and

sub-surface extensions of the Walbiri-Bloodwood Fold Zone, in the eastern half.

In both cases a more extensive section of marine Cambro-Ordovician age rocks is anticipated, and the limited geochemical data available suggest that these sediments should lie within the oil window.

The Newhaven-Mt Allan Seismic Survey, consisting of 344 km of 12-fold vibroseis coverage was shot in these areas in November-December, 1981, in an attempt to define prospects and leads for evaluation by additional drilling over the next three years. The structures mapped to date include reverse faulted blocks with salt-involvement in the Newhaven area to the west, and "sled-runner" thrusts with a plane of décollement in salt in the Mt Allan area to the east.