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ABSTRACT: Overpressure in the Barrow sub-basin, North West Shelf, Australia

Tingate, P. R.1, A. Wrightstone2, D. Dewhurst2, K. Dodds2, A. Khaksar1, and P. van Ruth1
(1) National Centre for Petroleum Geology and Geophysics, Australian Petroleum Cooperative Research Centre, Adelaide, Australia
(2) CSIRO Division of Petroleum Resources, Australian Petroleum Cooperative Research Centre, Bentley, Australia

The Barrow Sub-basin is a major offshore Previous HithydrocarbonNext Hit province in Australia. It formed primarily as a result of Jurassic extension culminating in Early Cretaceous break-up. Thermal subsidence declined with time following break-up, but renewed tectonic subsidence occurred in the Late Miocene. Mesozoic sediments are predominantly siliciclastic but Neogene deposits are carbonate-rich.

To obtain a quick regional overview, over 400 well completion reports were examined and those wells with mud weights greater 1.25 S.G. were considered likely to be overpressured and became the focus for further work. A subset of these wells was investigated with more reliable pressure Previous HitindicatorsNext Hit such as kicks or Previous HitdirectNext Hit pressure measurements. Depth-pressure profiles across the region are variable and often show pressure compartmentalisation. Whilst highly overpressured sandstones are not very common, they do represent a serious concern for explorationists. Using a range of overpressure Previous HitindicatorsNext Hit the following observations were made:

Overpressured reservoirs in the Barrow Sub-basins -occur over a wide depth range (3000 to 4000+ mbsl), -occur over a wide stratigraphic range (Late Triassic to Early Cretaceous) -are not regionally limited by major structural boundaries. -occur within sequences dominated by fine grained sediments and occur in depositionally, or structurally, isolated sandstones. -occur to the west of the Barrow Sub-basin around the Alpha Arch and Rankin Trend, coinciding with thickest Tertiary deposition.

Previous work in the study area has tended to support Previous HithydrocarbonTop generation as the primary cause of overpressure but more recent publications have emphasised compaction disequilibrium. The contribution of these mechanisms is currently under investigation.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia