OVERCOMING HISTORICAL BIAS: AN INTEGRATED GEOLOGICAL AND ENGINEERING ASSESSMENT OF THE CONISTON PROSPECT, EXMOUTH SUB-BASIN

Abstract

The discovery of heavily biodegraded oil in Novara–1 in 1982 by Esso/BHP branded the Exmouth Sub-basin as a difficult exploration province for almost two decades. A total of 24 barrels of biodegraded, 16.7o API oil was recovered from a drill-stem test conducted over a 2 m interval, indicating an uneconomic flow rate of 33 BOPD. The perception that any future oil discoveries would be similarly biodegraded and therefore highly problematic, if not impossible, to produce economically, condemned the Exmouth Sub-basin in favour of its eastern cousins— the Barrow and Dampier sub-basins.

Despite an encouraging 550 BOPD flow of biodegraded oil from a poor quality reservoir in West Muiron–5 in 1993, it was not until late 1998/9 that flows of 4,300 and 4,800 BOPD from the nearby Woodside wells Vincent–1 and Enfield–1, respectively, demonstrated that biodegraded Exmouth oils could be produced at potentially commercial rates. Concurrent reinterpretation of the Novara–1/ST results, in an integrated geological and engineering assessment, promoted the Coniston prospect adjacent to the Novara fault block.

The Novara–1/ST test was demonstrated to have failed largely due to the test parameters, rather than the nature of the oil. Geochemically, the Novara oil is similar to West Muiron–5, so any oil discovered at Coniston had the potential to flow as well as Vincent–1 if a sufficiently thick oil column was encountered in the expected similarly high quality reservoir. Coniston–1 was drilled by BHP/ Mobil in January/February 2000, and flowed biodegraded 15.6° API oil on DST at a rate of 2119 BOPD, restricted by rig and environmental constraints.

Historical bias has held back exploration of the Exmouth Sub-basin, leaving this one of the least explored of the Jurassic depocentres within the Carnarvon Basin. The Novara experience encourages explorers to challenge preconceptions and to ensure that the correct mix of technical skills is brought to bear in resolving prospectivity issues. Companies that maintain an active exploration position that allows them to respond rapidly to new concepts stand to gain most when historical biases are overturned.