Section 1: Descriptions and Observations of Seeps
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.
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With the increased resolution power of many geophysical methods, we are seeing direct evidence of seeps on a wide variety of data, including conventional seismic. New methods and technology have also evolved to better measure and detect seeps and their artifacts and reservoir charge and to map migration and remigration routes. In addition, detection of seepage is important for minimizing the risks associated with shallow gas drilling hazards, ensuring platform stability, and preventing well blow-outs.