FLEXURAL ISOSTATIC MODELLING AS A CONSTRAINT ON BASIN EVOLUTION, THE DEVELOPMENT OF SEDIMENT SYSTEMS AND PALAEO-HEAT FLOW: APPLICATION TO THE VULCAN SUB-BASIN, TIMOR SEA

Abstract

Although the petroleum industry is commonly interested in the upper few kilometres of the lithosphere, it is the deeper stretching events which may drive the development of regional thermal perturbations and which may overprint a significant thermal signature onto the shallower section. The Vulcan Sub-basin, which is located in the Timor Sea, northwestern Australia, has undergone a period of rifting during the Late Jurassic and shows a classic transition from intra-continental rifting to passive margin subsidence during the Late Jurassic to Early Cretaceous. A model has been developed of the Late Jurassic rifting history of the basin, which includes the flexural and stratigraphic response, and the development of the Cretaceous to Recent post- rift basin history. Quantification of the associated vertical motion of the lithosphere suggests that the transition is related to increased ductile extension in the lower crust and lithospheric mantle with little attendant upper crustal faulting to record the magnitude of this event in the structural history of the Vulcan Sub-basin. This lack of upper crustal deformation has resulted in an under- appreciation of the importance of this extensional event.

By modelling the Jurassic to Recent basin history, a thermal model may be built allowing predictions of palaeo-heat flow during the critical time of hydrocarbon generation. The model predicts that during the Jurassic and Early Cretaceous, increased lower crust and lithospheric mantle extension produced a thermal anomaly of ~20mW/m2 across the Vulcan Sub-basin. The relaxation of this thermal anomaly in the Cretaceous and Tertiary produced a rapid post-rift subsidence which allowed flooding of the margin, with increased subsidence towards the northwest. However, the evolution of this thermal perturbation beneath the upper crust resulted in a time lag between Late Jurassic rifting and maximum basin heat flow in the Early Cretaceous of up to 30 million years after Callovian breakup Therefore, the simple relationship between upper crustal faulting and total lithosphere stretching common in intra-continental rifts is predicted to break dow n immediately preceding conti nental breakup and necessitates modelling of the transition from syn-rift to post-rift stratigraphy in order to predict the thermal history of the Vulcan Sub-basin.