COUPLED MECHANICAL/FLUID FLOW MODELS OF TRAP INTEGRITY AND FAULT REACTIVATION: APPLICATION TO THE NORTH WEST SHELF OF AUSTRALIA

Abstract

In the Timor Sea, northwestern Australia, convergence of the Australian and Eurasian plates in the latest Miocene induced fault reactivation which resulted in significant rupture of seals and reduced trap integrity, resulting in the partial to complete loss of hydrocarbons from many traps. In addition, this margin-scale tectonism produced a major fluid flow event which involved the flowage of hot, saline brines up major faults and through the Mesozoic and Tertiary sequences.

In an attempt to provide a better understanding of the process of fault reactivation and trap breach, a number of coupled mechanical/fluid flow models have been developed for the Vulcan Sub-basin. These models consider the effect of fault permeability and fluid overpressures on fluid migration and seal rupture in an extensional tectonic environment. Modelling has shown that rupture of the seal is more likely where an overpressured region at depth within the sub-basin leads to overpressuring beneath the seal. Models in which the seal permeability is a function of material failure result in transient failure along the whole length of the fault, with fluid flow being directed upward through the seal.

In addition, two specific accumulations have been modelled using a purely mechanical approach, namely the commercial Jabiru oil field and the breached East Swan accumulation. Fault reactivation and volume strain in the Jabiru and East Swan accumulations under an imposed stress field predict volume increase, associated breaching of the seal, and the development of Hydrocarbon-Related Diagenetic Zones (HRDZs) which map zones of trap leakage. For the Jabiru field, modelling predicts that volume strain occurs at the southern tips of the major Mesozoic faults. In the East Swan region, trap breach is predicted to occur in the SW of the field for stress states where σ1, is oriented between 15°E and 45°E. Volume strain is greater for E/W and NE/SW extension than for N/S extension. Both models show good agreement with seismic mapping of HRDZs.