Changes in Trap Integrity during the Evolution of Reactivated Faults in the Timor Sea, North West Shelf, Australia

Anthony Gartrell¹, Ben Clennell¹, and Yanhua Zhang^2
1CSIRO Petroleum, Perth, Australia
²CSIRO Exploration & Mining, Perth, Australia

Structural and charge histories were combined to constrain the effects of fault reactivation (negative inversion) on hydrocarbon preservation in the Timor Sea, North West Shelf, Australia. It is proposed that the fluid transport properties of reactivated faults evolved with increasing displacement and that the timing of charge events relative to this evolution resulted in significantly different effects on trap integrity for oil (early) and gas (late) systems.
Main oil charge occurred during the Late Cretaceous to Early Tertiary, accumulating in traps formed by rift phase (Jurassic) fault blocks overlain by thick Cretaceous mudrock top seal units. Oil columns are preserved against faults with lower post-rift (Neogene) fault displacements (<60 m). Conversely, oil columns bound by faults that have attained higher post-rift displacements are completely lost. Subsequent main gas charge was synchronous with, or post-dated, fault reactivation in the late Tertiary. In contrast to the oil system, preserved gas columns tend to be bound by faults with high post-rift fault displacements (>150 m).
A finite amount of displacement was apparently required before a through-going connected fault-fracture network was established (brittle-ductile behaviour). At this stage trapped oil leaked across the seal via the active faults. The seal remained intact prior to this stage allowing oil to be preserved, even though shearing of the seal occurred. As fault architectures matured within the seal at higher displacements, strain localisation and progressive milling of the fault surfaces probably resulted in a smoother, less dilatant, fault zone filled with soft clay gouge. Consequently, a reduction in vertical fault permeability occurred, even during further fault movements. Hence, late stage gas that entered traps bound by the high displacement faults was preserved.

AAPG Search and Discovery Article #90072 © 2007 AAPG and AAPG European Region Conference, Athens, Greece