AN HISTORIC GEOLOGY OF THE GIPPSLAND BASIN

Abstract

The Mesozoic and Tertiary rocks of the Gippsland Basin form a simple natural sequence as the deposits of a main regressive - transgressive cycle, followed by a regressive phase of lesser magnitude.After a period of early Mesozoic erosion in Gippsland, a river flowing from the north-west built extensive deposits which drove the sea back well beyond the present coast line. Strzelecki Formation is the name applied to the flood-plain sediments of this period. The channel belt deposits have been given various names and of these the Childers Formation appears to be the most valid. However, with the deltaic plain deposits, it can be regarded as forming a part of the Latrobe Valley Complex.About the end of Mesozoic time the sea began to transgress, also at this time there was some vulcanism in the western part of the basin. As transgression proceeded, the delta became digitate in form, similar to today's Mississippi delta. Silty and muddy sediments accumulated beyond the distributaries and between them, whilst the distributaries were areas of dominantly sand desposition. Marine agencies reworked some of the sand to form beaches and occasional barrier islands.As the sea transgressed the prodelta silty muds (Lakes Entrance Formation), covered the submerged distributaries and these in turn became covered by the cleaner water calcareous deposits of the Lower Gippsland Limestone. Meanwhile onshore, the flood-plain and earlier channel-belt deposits became overlain by later channel-belt deposits, and then by deltaic plain deposits with abundant coal. Transgression reached its zenith, probably during Miocene time, and a final regressive phase culminated in the situation as we see it today.The formation water in the main body of the Latrobe Valley complex is very fresh, and calculations show a static situation for both the Latrobe Valley and the contiguous glauconitic sandstone member, and thus the existence of down-dip escape is most unlikely. The oil at Lakes Entrance has not been flushed down-dip, rather it has moved up-dip.It follows from a consideration of the depositional history in Gippsland that producing structures at the top Latrobe Valley level are basically stratigraphic traps. They lie along ancient distributary channel trends and are flanked by contemporaneous muddy facies. Separate closures along the trends have been brought about by the development of saddles due to differential compaction and probably normal faulting at depth.The oil and gas pools today are more or less at their greatest depth of burial. An association is obvious between depth and hydrocarbon content for the top Latrobe Valley pools. A progression is noted from dry gas at Golden Beach (2,000') to a thick oil i Halibut (8,000'). Coal rank increases with depth. It is concluded that coal is the main hydrocarbon source in Gippsland. Thus all the Latrobe Valley section is prospective. The top Latrobe Valley horizon is probably the most productive to date because it has very effective cappingBarracouta 1, intersected a barrier island sand. There is likely to be more of these parallel to ancient shorelines. They could prove productive.