OFFSHORE GIPPSLAND BASIN FIELDS

Abstract

The offshore Gippsland Basin, underlies the continental shelf and slope between eastern Victoria and Tasmania.

The basin is filled with up to 25,000' of sediment, varying in age from Lower Cretaceous to Recent. The Lower Cretaceous section is represented by at least 10,000' of nonmarine greywackes of the Strzelecki Group. The overlying sediments of Upper Cretaceous to Eocene age comprise the interbedded sandstones, siltstones, shales and coals of the Latrobe Group, with a cumulative thickness of about 15,000'. Offshore, the Latrobe Group is overlain unconformably by up to 1500' of calcareous mudstones of the Lakes Entrance Formation and up to 5000' of Gippsland Limestone carbonates. Pliocene to Recent carbonates, reaching a maximum thickness of about 1000', complete the sedimentary section of the basin.

Australia's first commercial offshore field, the Barracouta oil and gas field, was discovered in the Gippsland Basin in February 1965. Further exploratory drilling over the following two and a half years led to the discovery of the Marlin gas field and the Kingfish and Halibut oil fields.

The principal hydrocarbon accumulations are reservoired by sediments of the Latrobe Group within closed structural highs on the Latrobe unconformity surface. Seal is provided by the mudstones and marls of the Lakes Entrance Formation and Gippsland Limestone.

A field development programme was initiated immediately after Barracouta had been confirmed as a commercial gas reservoir. By the end of 1967, the Barracouta 'A' platform had been erected. Construction and positioning of the Marlin, Halibut and the two Kingfish platforms followed.

To date development drilling has been completed on the Barracouta and Halibut fields, while development of the Marlin field has been temporarily suspended following completion of four wells. Development of the Kingfish oil field which commenced in March 1970, is still in a relatively early stage.

The Barracouta field has been producing gas and oil since March and October, 1969 respectively. The Marlin gas field was put on stream in November, 1969 and the Halibut oil field in March 1970. As yet no wells drilled in the Kingfish oil field have been completed for production.

The four fields provide a major source of hydrocarbons for the Australian market. By the end of September, 1970 cumulative production of sales quality gas from the Barracouta and Marlin fields was almost 23 BCF. Cumulative production of stabilised oil from Barracouta was 2 million barrels and over 26 million barrels from Halibut.