DISCOVERY OF THE WANAEA AND COSSACK OIL FIELDS

Abstract

Exploration of the Dampier Sub-basin on the North West Shelf of Australia commenced with a reconnaissance seismic survey in 1965. In 1969 Madeleine-1, the first well drilled on the Madeleine Trend, encountered water bearing Upper Jurassic sandstones. Following acquisition of a regional grid of modern seismic in 1985 and 1986, and comprehensive hydrocarbon habitat studies, the Wanaea and Cossack prospects were matured updip from Madeleine 1. They were proposed to have improved reservoir development and an oil charge.

The Wanaea Oil Field was discovered in 1989 when Wanaea-1 encountered a gross oil column of 103 m in the Upper Jurassic Angel Formation. The well flowed 49° API oil at 5856 BPD (931 kL/d) with a gas-oil ratio of 1036 SCF/STB. Two appraisal wells were drilled in the field in 1990.

The Cossack Oil Field was discovered in 1990 when Cossack-1 encountered a gross oil column of 54 m also in the Angel Formation. The oil-water contact is 18 m deeper than in Wanaea-1. Cossack-1 flowed 49° API oil at 7200 BPD (1145 kL/d) with a gas-oil ratio of 98 SCF/STB.

The Angel Formation reservoir consists of mass flow sandstones interbedded with bioturbated siltstones. Sandstone porosities average 16 to 17 per cent for both the Wanaea and Cossack Fields. Permeabilities average about 300 mD at Wanaea and about 500 mD at Cossack.

An extensive 3-D seismic survey was conducted over the Wanaea and Cossack Fields in 1990. Final reserves calculations await interpretation of this survey, but it is clear that the combined Wanaea and Cossack oil reserve is the largest outside Bass Strait.