The geological and geophysical characteristics of structural?stratigraphic plays in the Offshore Gippsland Basin

Abstract

Recent developments in the Offshore Gippsland Basin have suggested that not all large oil accumulations are necessarily restricted to four-way dip, closed structures at Top Latrobe level; the discovery of the Fortescue oil field in September 1978 indicated that further substantial oil accumulations may be trapped by a mechanism of combined structural-stratigraphic closure. Integrated well log correlations, micropalaeontologicaI dating, stratigraphic interpretation, geochemistry and reservoir pressure analyses have all been cited to indicate that the Fortescue oil accumulation consists of a series of discrete sandstone reservoirs lying on the western flank of, but separated from, the giant Halibut oil field. Seismic data have been incorporated with detailed geological data to develop a model by which such structural-stratigraphic plays can be recognised. Truncated seismic events, associated with the stratigraphic truncation of Upper Palaeocene to Lower Eocene delta-top coal swamp deposits, can be mapped across the area of the Fortescue field such that the existence of permeability barriers (basal and lateral seals) can be recognised and predicted. By combining the geometry of such permeability barriers with partial structural reversal at the level of the Top Latrobe unconformity, the geometry of the Fortescue oil field can be predicted. Palaeogeographical studies suggested that similar Palaeocene-Eocene coal swamp deposits might provide adequate permeability barriers to oil accumulations elsewhere in the Gippsland Basin. The structural-stratigraphic model was therefore extrapolated to other parts of the basin in an attempt to recognise such potential plays. It is suggested that the West Kingfish and Yellowtail oil fields, perhaps not fully defined at Top Latrobe level by complete four-way dip closure, could be further examples of the structural-stratigraphic concept. The Pisces No. 1 well, recently drilled by Union Texas Australia Incorporated and its Joint Venture Partners in Permit Vic P/12 on the southern margin of the Gippsland Basin, was a further test of the structural-stratigraphic play model. While the well proved to be dry, probably due to the lack of adequate vertical seal, the structural-stratigraphic model was essentially vindicated.