The Fortescue-1 well drilled in the Gippsland Basin in June 1978 was a dry hole. However, results of detailed stratigraphic analysis together with seismic data provided sufficient information to predict the possible occurrence of a stratigraphic trap on the flank of the giant Halibut structure.Three months later the West Halibut-1 well encountered oil in the Latrobe Group 16 m below that depth carried as the original oil-water contact for the Halibut field. Following wireline testing in both the water and oil-bearing sandstone units, two separate pressure systems were recognised in the well. Three additional wells, Fortescue-2, 3 and 4, were drilled to define further the limits of the field, the complex stratigraphy and the hydrocarbon contacts.Integration of detailed well log correlations, stratigraphic interpretations and seismic data indicated that the Fortescue reservoirs were a discrete set of units stratigraphically younger and separated from those of Halibut and Cobia Fields. Analysis of pressures confirmed the presence of two separate pressure systems, proving none of the Fortescue reservoirs were being produced from the Halibut platform. Geochemical analysis of oils from both accumulations supported the above results, with indications that no mixing of oils had occurred.Because the Fortescue Field is interpreted as a hydrocarbon accumulation which is completely separated from both Halibut and Cobia Fields, and was not discovered prior to September 17, 1975, it qualified as "new oil" under the Federal Government's existing crude oil pricing policy. In late 1979, the Federal Government notified Esso/BHP that oil produced from the Fortescue Field would be classified as “new oil”.

The Permian Gidgealpa Group contains major reserves of gas and minor amounts of oil in fluvial to marginal marine sandstone reservoirs within coal measures in the Southern Cooper Basin. Lithofacies analysis of a sequential set of palynologically defined time-rock units through the Gidgealpa Group has clarified depositional trends, identified regions of maximum channel development, located positions of shore-lines and shown directions of marine transgressions and regressions.Braided streams deposited the oldest unit, the Tirrawarra Sandstone, on a possibly glacially scoured land surface. As topographic gradients declined, rivers began to meander, and sandstone, shale and coal deposits of the Patchawarra Formation encroached over Tirrawarra Sandstone. Northwards flowing rivers gradually filled pre-Permian valleys with detrital sediments.During uppermost Patchawarra Formation deposition, the Cooper Basin was invaded from the east by a restricted sea, which then deposited the Murteree Shale. As this sea retreated, shoreline sediments of the Epsilon Formation prograded towards the east. A second transgression inundated the land, and the Roseneath Shale was deposited. Finally, delta deposits forming the Daralingie Beds prograded eastwards in the wake of the retreating sea.A period of erosion followed, during which time the present structural grain was imposed on the basin. Subsequently, eastward flowing meandering rivers deposited sandstone, shale and coal of the Toolachee Formation on an essentially flat topography, with only minor hills. The Toolachee Formation grades upwards into non -coal bearing beds, which perhaps register a climate change.

A lithofacies study on the Upper Permian Toolachee Formation has been conducted in the Gidgealpa-Moomba-Big Lake area to determine the suitability of the technique in the reconstruction of depositional environments and palaeogeographic trends throughout the Cooper Basin. The Toolachee Formation is one of the main gas producing intervals in the basin, especially in the area of study, which is approximately 2,000 square kilometres. Thirty-one wells drilled in this region indicate that the formation ranges in thickness from 35 metres to over 115 metres.The Toolachee Formation, taken as a whole, is too thick to show any significant features on a lithofacies map over the limited area of investigation. However, lithofacies maps of three approximately chronostratigraphic subdivisions of the same formation show both vertical and lateral trends. Vertically, the percentage of sandstone decreases from the lowermost subdivision to the uppermost subdivision; coal percentages show the opposite trend; and core material shows fining upwards sequences. Laterally, isopachous thin areas (depositional highs) in most cases correlated with an increase in shale or coal lithologies. Histograms of coal cycles show that the lower and middle parts have similar composite sequences of, from the base upwards, sandstone mixture of sandstone and shale-shalecoal.The depositional model proposed is an aggradational flood-plain which, prior to the commencement of deposition, had been eroded to a peneplain. Sediments were deposited from rivers of gradually declining flow gradient until marsh and lacustrine conditions prevailed for long periods of time. Deposition ceased at the sediplain stage.

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