Fluid Migration at Opouawe Bank, offshore New Zealand
This thesis presents new insights into fluid flow at Opouawe Bank, on the Hikurangi margin off New Zealand. Three case studies highlight different aspects of the gas migration system. The first study shows that seepage is driven by biogenic gas migration. Accumulating free gas underneath the base of gas hydrate stability is generated at a maximum depth of 1500-2100 m below the seafloor (mbsf). The flow is mostly focused along vertical conduits with some stratigraphically controlled flow along dipping horizons. Case study 2 links the vent systems with the tectonic framework. Gas migrates along extensional fractures systems that are elongated perpendicular to the ridge axis of Opouawe Bank. They are parallel to the main direction of compression induced by subduction and result from secondary longitudinal extension of the ridge. With declining overburden stress they develop into more pipe-like conduits in the shallow sediment at < 100 mbsf. Case study 3 investigated shallow gas migration. Differences in migration structures indicate different evolutionary stages of seep formation, from channeled gas flow, to gas trapping, and doming. Based on geo-mechanical quantifications, gas column heights necessary to create domes were calculated. Contrary to ongoing discussions, this study demonstrated that these doming features can evolve into seep sites without pockmark formation. The combined results of these case studies suggest that the exceptional observation of extensional gas migration structures may represent a common phenomenon at convergent margins. This has implications in the overall understanding of the stress regime of the accretionary prism. Furthermore, the study of seafloor doming yields a better understanding of gas migration and the stability of the seabed. This is relevant to improve the safety of oil and gas exploration operations and provides an additional tool to understand the mechanics of upward migrating gas through marine sediments.
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