Porewater Geochemical Assessment of Seismic Indications for Gas Hydrate Presence and Absence: Mahia Slope, East of New Zealand’s North Island

Richard B Coffin,Gareth Crutchley,Ingo Pecher,Joshu Mountjoy,Brandon Yoza,Thomas J Boyd

doi:10.3390/en15031233

Abstract

We compare sediment vertical methane flux off the Mahia Peninsula, on the Hikurangi Margin, east of New Zealand’s North Island, with a combination of geochemical, multichannel seismic and sub-bottom profiler data. Stable carbon isotope data provided an overview of methane contributions to shallow sediment carbon pools. Methane varied considerably in concentration and vertical flux across stations in close proximities. At two Mahia transects, methane profiles correlated well with integrated seismic and TOPAS data for predicting vertical methane migration rates from deep to shallow sediment. However, at our “control site”, where no seismic blanking or indications of vertical gas migration were observed, geochemical data were similar to the two Mahia transect lines. This apparent mismatch between seismic and geochemistry data suggests a potential to underestimate gas hydrate volumes based on standard seismic data interpretations. To accurately assess global gas hydrate deposits, multiple approaches for initial assessment, e.g., seismic data interpretation, heatflow profiling and controlled-source electromagnetics, should be compared to geochemical sediment and porewater profiles. A more thorough data matrix will provide better accuracy in gas hydrate volume for modeling climate change and potential available energy content.

Highlights

IntroductionAt two Mahia transects, methane profiles correlated well with integrated seismic and TOPAS data for predicting vertical methane migration rates from deep to shallow sediment
It is this shallow part of the gas hydrate system that is most susceptible to ocean warming associated with climate change, which would lead to gas hydrate dissociation and methane release into the water column [11]
Selection site was based on a review of seismic and profiles that did not show any patterns site was based on a review of seismic and TOPAS profiles that did not show any patterns indicative of of vertical vertical gas gas and and fluid fluid migration

Summary

Introduction

At two Mahia transects, methane profiles correlated well with integrated seismic and TOPAS data for predicting vertical methane migration rates from deep to shallow sediment. A critical region of marine gas hydrate systems is the upper limit of gas hydrate stability in sediments, sometimes referred to as the feather edge of hydrate stability, e.g., [7,8,9,10]. It is this shallow part of the gas hydrate system that is most susceptible to ocean warming associated with climate change, which would lead to gas hydrate dissociation and methane release into the water column [11].

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