--> Gas hydrate characterization on New Zealand’s southern Hikurangi margin: Glendhu and Honeycomb ridges

AAPG Asia Pacific Region Geosciences Technology Workshop:
Gas Hydrates – From Potential Geohazard to Carbon-Efficient Fuel?

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Gas hydrate characterization on New Zealand’s southern Hikurangi margin: Glendhu and Honeycomb ridges

Abstract

The Hikurangi margin on New Zealand’s east coast contains a world-class gas hydrate province. These clathrate compounds of natural gases, trapped within a water molecule lattice, could represent a relatively low-carbon energy source for the future. However, the presence of highly concentrated gas hydrate in shallow sediments creates a very complex natural system that is not yet fully understood. The study of such systems is fundamental to the assessment of geohazards such as seafloor stability as well as the biology of seafloor environments and ocean acidification. Gas hydrates are widespread in shallow sediments across the Hikurangi subduction margin. At the southern end of the margin, the Pacific plate is subducting obliquely beneath the Australian plate, creating a series of prominent thrust ridges that extend for several hundred kilometers. Glendhu and Honeycomb ridges are two parallel four-way closure systems that measure about 100 km and 60 km along the longitudinal direction, respectively, and lie at the toe of the deformation wedge in water depths ranging from 2100 to 2800 meters. A synthesis of recently acquired multi-channel seismic reflection data with existing 2D industry seismic surveys allowed the identification and mapping of high amplitude anomalies occurring between the seafloor and the bottom-simulating reflector. These highly reflective features are interpreted as concentrated hydrate accumulations within high permeability layers. A quantitative characterization of these features is being carried out in order to estimate the amount of gas that is trapped in form of hydrates within the hydrates stability zone. To this end, a first reliable Vp velocity model needs to be obtained from the industry data through 2D tomography and pre-stack depth migration; the velocity model is needed to constrain both pre- and post-stack stochastic seismic inversion. Information about reservoir sediments derived from porosity analysis of dredged samples and reflectivity analysis of the high amplitude anomalies will provide additional constraints for stochastic inversion.