FAULT ARCHITECTURE ANDTHE MECHANICS OF FAULT REACTIVATION INTHE NANCARTROUGH/LAMIN ARIA AREA OF THE TIMOR SEA, NORTHERN AUSTRALIA

Abstract

Fault-bounded Jurassic structures of the Timor Sea have in recent years been the focus of intensive oil exploration. A number of significant oil discoveries have highlighted the exploration potential of this area (e.g. Laminaria, Corallina, Buffalo, Elang, Kakatua), but the majority of tested structures are either underfilled or show evidence of a residual oil column, resulting from trap failure of previously hydrocarbon-bearing structures. Recent well results confirm that trap integrity remains the principal exploration risk in the Timor Sea.

Fault reactivation of Jurassic hydrocarbon traps is related to late Miocene-Pliocene oblique collision between the Australian plate and the SE Asian plate complex, which caused widespread transtensional faulting. The sealing potential of fault-bounded traps is, to a large degree, controlled by the orientation of the fault plane relative to the late Miocene-Recent stress field. However, the location of potential hydrocarbon leakage pathways remains difficult to define due to the complex fault architecture and a limited understanding of the interaction between Jurassic faults and Late Tertiary tectonism.

During the past few years, a wealth of new exploration wells and 3D seismic data has become available from the Laminaria High/Nancar Trough area. The use of 3D visualisation tools, seismic coherency filtering and other seismic techniques has greatly enhanced our understanding of the fault architecture of this area of the Timor Sea.

The structural architecture of the Nancar Trough/ Laminaria High is made up of several different structural intervals that are stratigraphically separated and partially decoupled along thick claystone intervals. Fault blocks at Jurassic level are typically overlain by Tertiary en-echelon graben systems, often showing characteristic 'hourglass' structures in cross-section. Detailed mapping of these fault structures on 3D seismic data has shown that the Jurassic faults and overlying Tertiary faults are

often partially decoupled.

Fault throw distributions indicate that the Mio-Pliocene faults have grown downwards instead of Jurassic faults propagating upwards during reactivation. The two fault systems are soft-linked within Cretaceous claystones, only locally linking to form through-going faults. Hydrocarbon leakage pathways are most likely located at these points where critically stressed parts of Jurassic faults link up with Tertiary faults. The position of these linkage zones in relation to structural closure is key to understanding the distribution of preserved and breached columns that have been observed to date.

The integration of 3D seismic fault plane mapping with in-situ stress analysis from borehole image and pressure test data provides a valuable tool for the evaluation of trap integrity, potential hydrocarbon leak paths and a more accurate risk assessment of exploration prospects.