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The Effect On Seismic Data Of The Phanerozoic Of The Beetaloo Sub-Basin, Mcarthur Basin, Northern Territory.

Donley, James A.

Honours Degree, 1994

University of South Australia


The McArthur Basin of the Northern Territory contains the Mid-Proterozoic sediments of the Tawallah, McArthur, Nathan and Roper Groups. The former three Groups are thickest within the central Batten and Walker Troughs. The Roper Group attains its maximum thickness within the Beetaloo Sub-basin, a fault bounded rift basin which is concealed beneath Phanerozoic sediments in the south-west. Since 1988, the Beetaloo Sub-basin has been the focus of petroleum exploration by Pacific Oil and Gas. The Jamison and Moroak Sandstones of the Roper Group have been the main reservoir targets.

The Phanerozoic sediments of the north-western Georgina Basin unconformably overlie the Proterozoic in the Beetaloo Sub-basin. Following deformation of the Roper Group sequences, the land surface was eroded and dissected by a Late Proterozoic drainage system. Seismic mapping of the Base Phanerozoic unconformity has revealed the presence of three main drainage features; the Altree Channel, the Burdo Channel and the Chanin Channel. These channels were effective in controlling the distribution of the Antrim Plateau Volcanics when they were extruded in the Early Cambrian. A link has been established between Proterozoic structural highs and the Late Proterozoic Channels; it appears that these anticlinal features were preferentially eroded. Therefore in effect, Proterozoic structural highs have localised thick accumulations of the Antrim Plateau Volcanics.

Beneath anomalous thicknesses of Volcanics, Proterozoic reflectors suffer velocity pull-up, causing exaggerated or false structuring. Pacific Oil and Gas have developed a method which seeks to integrate seismic time data with E-M, gravity and magnetic information, to replace depth estimates previously calculated solely from seismic data. The resultant depth estimates will be compensated for any pull-up or push-down in travel time due to thickness or velocity variations in the Phanerozoic sequence. The method relies on at least one tie between E-M/gravity/magnetics data and either the Top Volcanics, Base Volcanics or Unconformity seismic interpretations.

A Sierra Geophysics seismic interpretation package was used to establish seismic control within the Phanerozoic section. The Top Cambrian Limestone, the Near Top Antrim Plateau Volcanics and the Base Phanerozoic Unconformity were mapped. To tie these reflectors, angular discordance, volcanic edge interpretation and check shot data was used. As an additional aid to the interpretation, a geological/geophysical model was used.

Australian School of Petroleum



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