Yanhua Zhang

Three-dimensional coupled deformation and fluid-flow numerical modeling, charge-history analysis, and seismic imaging of inferred leakage-related geobodies are integrated to investigate the response of a complex set of Jurassic trap-bounding normal faults to extensional reactivation and to assess hydrocarbon upfault seepage on the Laminaria High (Timor Sea, Australian North West Shelf). Fluid inclusion data are consistent with the presence of paleo-oil columns below the current accumulations in the Laminaria and Corallina fields. Evidence for other partially breached (current and paleo-oil column) as well as breached (dry with paleo-oil column) closures across the region implies that active and widespread seepage took place after the time of initial oil charge. The distribution of current and paleo-oil zones, and the location of inferred hydrocarbon leakage indicators defined on 3D seismic data, correlates with the prediction of fault-seal effectiveness based on modeled strain distribution. Within the geologic framework of the Laminaria High area, this distribution suggests that when sufficient reactivation shear strain is accumulated by reservoir faults, ductile deformation might give way to brittle failure in the top seal, allowing active flow pathways to develop and upfault seepage to take place from the reservoir to thief zones or the seafloor. The observations emphasize that strain and upfault fluid-flow partitioning is constrained by prereactivation fault size, lateral fault-tip distributions, and the presence of fault jogs inherited from successive episodes of growth processes. These elements can explain the complex distribution of paleo- and preserved oil columns in the study area and further support Cenozoic tectonic activity as being the first-order control on trap breaching and hydrocarbon seepage in this region.

Laverton is an important region in the Yilgarn Craton for gold mineralization. Previous studies suggested that the premineralization structure was dominated by a fault restraining step‐over structure with the Wallaby fault and Sunrise Shear Zone as the key fault splays hosting the world‐class Wallaby and Sunrise Dam gold deposits, respectively. Two major gold mineralization phases occurred during two of a series of tectonic events with different far‐field stress orientations. 3D coupled deformation and fluid flow modelling was used to investigate the effects of varying far‐field stress orientations on reactivation of the fault structure in the region. The results show that structural reactivation is sensitive to regional shortening directions. Two shortening directions are identified to be favourable for reactivation of the Wallaby fault and Sunrise Shear Zone: (i) NW–SE shortening and (ii) ENE–WSW to E–W shortening. The reactivated segments exhibited localization of shear strain and dilation as well as fluid focusing at locations corresponding to the Wallaby and Sunrise Dam deposits. This is in contrast to the models with shortening direction between NNW–SSE and NE–SW, which showed little fault reactivation at both locations. These results support previous independent interpretations of the controls on gold mineralization at Laverton. The two shortening directions favourable for structural reactivation inferred from the models are consistent with the kinematics of the two main gold mineralization events in the region. Our results suggest that strain localization, dilation and fluid focusing during structural reactivation could be some of the key factors governing gold mineralization at Laverton.