NUMERICAL MODELLING OF THE AUSTRALIA – ANTARCTICA CONJUGATE MARGINS USING THE GALO SYSTEM: PART 1. THE BREMER SUB-BASIN, SW AUSTRALIA

Abstract

An analysis of variations in the tectonic subsidence of the Bremer sub-basin (offshore SW Australia) since 160 Ma using the GALO numerical basin modelling programme has made it possible both to refine previous models and to estimate the intensity of stretching and thermal activation of the lithosphere. The new model explains the rapid subsidence of the sub-basin and the deposition of the synrift Bremer 1 unit during the initial rift phase in the Late Jurassic (160 to 130 Ma). This phase of extension was accompanied by high heat flows, typical of the axial zones of continental rifts, and lithospheric stretching with a β-factor of about 1.4. Between 130 and 43 Ma, the abnormally low depositional rate and the shallow water depths suggest moderate thermal activation of the mantle and the absence of extension-driven subsidence. However during the Eocene (43 to 37 Ma), the modelling suggests that another phase of intense stretching of the sub-basin lithosphere took place with β = 1.7, explaining both the subsidence and an abrupt increase in water depth from about 50–200 m to nearer 2000 m.

The high heat flows during the initial stage of rifting and thermal activation during Cenozoic extension contributed to the early generation of hydrocarbons by source rocks in the Bremer 1 unit at the base of sedimentary cover. At the present day, these source rocks are overmature. At the same time, the modelling suggests that generation of light and heavy oil in the overlying Bremer 2 and 3 units has occurred. Source rock intervals in the upper half of the Bremer 3 unit and in the overlying successions are early mature or immature and may have generated minor volumes of hydrocarbons.