PETROLEUM POTENTIAL OF THE CEDUNA SUB-BASIN: IMPACT OF GNARLYKNOTS–1A

Abstract

The Ceduna Sub-basin, located in the eastern Bight Basin, is one of the few frontier deepwater provinces in Australia whose hydrocarbon potential remains largely untested. The sediments of the sub-basin span an area of over 95,000 km2—comparable to the combined area of the Exmouth, Barrow and Dampier sub-basins on Australia’s North West Shelf. Prior to 2003, exploration wells had been drilled only on the present day shelf area of the sub-basin. The recent Gnarlyknots–1A well, drilled in May 2003 by the Woodside operated joint venture in EPP29, has provided the first calibration point in the under-explored deepwater area of the sub-basin.

The well was the culmination of a basin analysis project that integrated results from prior drilling in adjacent areas, existing seismic surveys, regional gravity and magnetics interpretations, and a newly acquired 16,000 line km 2D seismic survey. Individual play elements of reservoir, seal, and hydrocarbon charge were analysed and combined to form play maps for key stratigraphic intervals. The Gnarlyknots prospect was chosen from more than 40 leads as the best location to test multiple play levels in an area interpreted pre-drill to be favourable for reservoir, seal, and charge.

Gnarlyknots–1A confirmed the presence of several favourable play elements but failed to encounter commercial hydrocarbons. Excellent quality sandstone reservoirs, marine shale top seals and thermogenic hydrocarbon shows—indicating the presence of a hydrocarbon source rock in a mature kitchen area downdip—were all encountered in the well. The failure of the well is attributed to the absence of fault seal on the updip bounding fault of the drilled hanging wall structure. The implications of this well result for the prospectivity of the Ceduna Sub-basin have been analysed, and provide encouragement for Woodside to pursue future exploration programs in the region.