THE STRATIGRAPHY OF THE OFFSHORE GIPPSLAND BASIN

Abstract

The Gippsland Basin in south-eastern Australia mainly underlies the continental shelf between eastern Victoria and Tasmania. It is filled with Lower Cretaceous-Recent sediment and has become a major source of hydrocarbons for the Australian market.

Forty-two wildcat and stepout wells, additional development wells and over 7,000 miles of seismic lines provide a framework on which to build the region's geological history. The time-stratigraphy of the basin is derived from extensive use of spore-pollen assemblages in the mainly non-marine Cretaceous-Eocene and foraminifera in the marine Oligocene-Pliocene, largely complemented by seismic and to a lesser extent electric log correlations. Ten Cretaceous and five Paleocene-Eocene spore-pollen zones and fourteen Oligocene-Pliocene foraminiferal zonules are recognized. Only broad-scale lithostratigraphic units, initially recognized along the northern, onshore margin of the basin are traceable offshore. The Lower Cretaceous is represented by at least 10,000 feet of non-marine greywacke of the Strzelecki Group. The Upper Cretaceous-Eocene, with a cumulative thickness of 15,000 feet is termed the Latrobe Group and consists mainly of lacustrine and fluviatile elastics. Channels dissected the top of the Latrobe Group during the Eocene and were filled with sediments recognizable as distinct sequences within the group and termed the Flounder and Turrum Formations. A destructive marine phase during latest Eocene time left the glauconitic Gurnard Formation as the youngest member of the Group.

Subsequent marine inundation of the basin resulted in deposition of up to 1,500 feet of calcareous mudstone referred to the Lakes Entrance Formation and up to 5,000 feet of marl, calcarenite and limestone of the Gippsland Limestone during the Oligocene and Miocene. Up to 1,000 feet of Pliocene- Recent calcarenite, micrite and marl complete the sedimentary sequence.