A NEW MODEL FOR THE HUTTON/BIRKHEAD RESERVOIR/SEAL COUPLET AND THE ASSOCIATED BIRKHEAD-HUTTON(!) PETROLEUM SYSTEM

Biomarker analysis of source rocks and oils from the Permian and Jurassic of the central Patchawarra Trough and the Gidgealpa area, reveal that much of the oil in the Eromanga Basin may have a significant lateral migrational component and be of Jurassic (i.e. intra-Eromanga) origin. Differences in hopane signatures can be used to discriminate between palaeo-oil and presently migrating live oil, and to constrain migration pathways. Thus, in some locations the identification of new source kitchens has been made possible by a combination of seal and biomarker analysis taking into account stratigraphic inheritance on conventional structural drainage maps. 3D seismic, sequence stratigraphy, dipmeter interpretation and neodymium model age dating together with conventional correlation techniques, have provided a new model for the deposition of the Hutton Sandstone to Birkhead Formation transition in the Eromanga Basin. Analysis of seal and carrier bed properties through time, in combination with hydrocarbon geochemistry and thermal modelling, indicates that the Birkhead-Hutton (!)' petroleum system has produced significant quantities of oil in the Cooper Basin sector of the Eromanga Basin. A disconf ormity near the base of the Hutton/Birkhead transition has controlled the location of oil-prone source rocks within the Birkhead Formation and stratigraphically focussed migration along palaeo-topographic ridges. A diachronous influx of volcanic-arc-derived (VAD) sediment within the Birkhead Formation has been traced right across the productive part of the Eromanga Basin. This influx of VAD sediment is associated with the main seal to underlying accumulations within both the lower Birkhead Formation and Hutton Sandstone. Sands comprising VAD sediment, which are juxtaposed, form the weak link within the main seal. The sediments between the VAD influx and the underlying unconformity in many locations constitute a waste zone. Palaeo-oil columns are common beneath extant, live oil accumulations. This indicates that a possible decrease in seal potential of the VAD sediment has occurred over time. The main seals to underlying accumulations were originally static, water-wet capillary seals which, mostly through an alteration of wettability, changed to simple permeability seals for currently migrating oil. Seal analysis, biomarker studies and geothermal modelling indicate that a double migration pulse has occurred in some areas of the Eromanga Basin. Palaeo-oil columns are related to a Late Cretaceous charge, and live oil accumulations to presently migrating oil.