--> Gas Hydrate Reservoirs of the Southern Hikurangi Subduction Margin: Insights From Geophysics And Numerical Modelling

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Gas Hydrate Reservoirs of the Southern Hikurangi Subduction Margin: Insights From Geophysics And Numerical Modelling

Abstract

New Zealand’s southern Hikurangi margin is characterized by highly oblique convergence and subduction of the Pacific Plate beneath the Australian Plate. Compression leads to widespread folding, faulting, and dewatering processes that play key roles in fluid migration into the gas hydrate stability zone (GHSZ). Large submarine canyons and smaller gullies dramatically change the seafloor topography, influencing gas hydrate stability in the underlying sediments. Canyons also deposit coarse-grained sediments down their axes and onto slope basins, probably providing good quality reservoir rocks for gas hydrate formation. In this presentation we will summarize key research results over the last five years related to gas hydrate formation style on the southern Hikurangi margin. We will describe a range of processes, including: • Gas hydrate formation in response to canyon incision. By cutting down into the substrate, canyons lead to a pronounced downward shift of the base of the GHSZ. This downward shift can capture free gas beneath the hydrate system and convert it into gas hydrates. With time, this process can lead to concentrated gas hydrates beneath incising canyons. • Gas migration into the GHSZ along dipping layers. Dipping layers of alternating lithologies (e.g. sands and shales) are known to play an important role in fluid migration and gas hydrate formation. The Hikurangi margin has many examples that highlight this process. Recent research has shown that concentrated gas hydrate formation tends to favor landward-dipping strata over seaward-dipping strata. This is probably due to relationships between asymmetric sedimentation in accretionary wedges and pronounced gas hydrate recycling on the landward sides of thrust ridges. • Faulting, fluid flow and hydrate formation. Dipping strata in accretionary wedges are inextricably linked to deeply-rooted underlying thrust faults. Shallower in the system, normal fault systems develop in response to flexural folding and gravitational collapse at the seafloor. Both types of faults have been linked to pronounced fluid flow and hydrate formation in anticlinal ridge systems of the margin. These processes have resulted in a broad array of gas hydrate accumulations on the margin. Although the deposits have not been drilled, their expressions in seismic data are similar to concentrated deposits elsewhere around the world, for example in the Nankai Trough and the Gulf of Mexico.