--> The Gas Hydrate Petroleum System of the Northern Hikurangi Margin, New Zealand

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

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The Gas Hydrate Petroleum System of the Northern Hikurangi Margin, New Zealand

Abstract

The Hikurangi Margin east of New Zealand’s North Island is currently considered the most significant gas hydrate province in New Zealand covering over 50,000 km2. It is a subduction zone and as such displays significant small-scale variability. While the southern part of the margin is highly accretive, significantly less accretion appears to take place further north, roughly north of Hawke’s Bay. This northern part of the margin is characterized by subduction of seamounts. In the past few years, significant surveying has been conducted in the southern portion of this margin largely driven by hydrocarbon exploration. Coverage with seismic data further north is poorer; however, this part of the margin was drilled during Integrated Ocean Discovery Program’s (IODP) Expeditions 372 and 375 in 2017 and 2018. These expeditions, while primarily focusing on earthquake studies, provided new insights into the margin’s gas hydrate systems, which we will summarize. Gas for hydrate formation in the study area appears to be entirely of microbial origin. Although the evidence is less pronounced than further south, it appears that the gas is often provided along deeply rooted faults. Bottom simulating reflections (BSRs) over much of this part of the margin are unusually weak. This has previously been attributed to the presence of gas and by inference, hydrate in fractures and macropores. Drilling now indicates that gas hydrate often is present in thin, coarse-grained turbidite layers whereas finer-grained sediments contain little hydrate. If gas distribution beneath the base of gas hydrate stability is similar, it would be predicted to lead to weak BSRs in seismic data. There is increasing evidence that the gas hydrate system in the study area is dynamic, i.e., in the process of re-adjusting to pressure-temperature changes. These changes include depressurization during uplift, increase of sub-seafloor temperature following sedimentation pulses, and warming from transient fluid pulses. We here present models that predict that adjustment of the gas hydrate system to such changes may take thousands of years. These findings could have significant implications for predicting the response of gas hydrates through glacial stages and for our understanding of the formation of gas hydrate deposits.