Upward‐Doming Zones of Gas Hydrate and Free Gas at the Bases of Gas Chimneys, New Zealand's Hikurangi Margin. Issue 9 (21st September 2021)

• Record Type: Journal Article
• Authors: Crutchley, G. J.; Mountjoy, J. J.; Hillman, J. I. T.; Turco, F.; Watson, S.; Flemings, P. B.; Davy, B.; Woelz, S.; Gorman, A. R.; Bialas, J.
• Is Part Of: Journal of geophysical research. Volume 126:Issue 9(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-09-21
• Subjects: gas hydrate -- gas chimney -- hydraulic fracturing -- mass transport deposit -- seal -- capillary pressure -- Hikurangi margin; Geomagnetism -- Periodicals; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 551.1
• DOI: 10.1029/2020JB021489
• Languages: English
• ISSNs: 2169-9313

Abstract

Abstract: Focused gas migration through the gas hydrate stability zone in vertical gas conduits is a global phenomenon. The process can lead to concentrated gas hydrate formation and seafloor gas seepage, which influences seafloor biodiversity and ocean biogeochemistry. However, much is unknown about how gas and gas hydrate co‐exist within and around gas conduits. We present seismic imaging of the gas hydrate system beneath a four‐way closure anticlinal ridge at New Zealand's southern Hikurangi subduction margin. Gas has accumulated beneath the base of gas hydrate stability to a thickness of up to ∼240 m, which has ultimately led to hydraulic fracturing and propagation of a vertical gas conduit to the seafloor. Despite the existence of an array of normal faults beneath the ridge, these structures are not exploited as long‐range gas flow conduits. Directly beneath the conduit, and extending upward from the regional base of gas hydrate stability, is a broad zone characterized by both negative‐ and positive‐polarity reflections. We interpret this zone as a volume of sediment hosting both gas hydrate and free gas, that developed due to partial gas trapping beneath a mass transport deposit. Similar highly reflective zones have been identified at the bases of other gas conduits, but they are not intrinsic to all gas conduits through gas hydrate systems. We suggest that pronounced intervening sealing units within the gas hydrate stability zone determine whether or not they form. Plain Language Summary: Gas hydrates are ice‐like substances composed of natural gas and water. They form between sediment grains underneath large regions of the Earth's seafloor. An important reason to study gas hydrates is that they partly control the way that methane gas flows through sediments and out of the seafloor. It is this flow of methane that sustains some diverse biological communities on the seafloor and affects the chemistry of the oceans. In this study, we use reflected sound waves to explore how gas flow beneath the seafloor depends on the way in which sedimentary layers are folded and fractured. Our data reveal a 240‐m thick reservoir of gas that is trapped in a large sedimentary fold, ∼500 m beneath the seafloor. The buoyancy of the gas has caused a vertical fracture zone to propagate upward to the seafloor, where gas bubbles are venting into the ocean. Further, our data suggest that a broad accumulation of gas hydrates (together with gas) has formed beneath the vertical fracture zone. This gas hydrate deposit may grow larger with time, and it will continue to influence the way that gas flows through the sediments. Key Points: 240 m thick free gas column accumulated beneath the base of hydrate stability, which led to hydraulic fracturing and gas chimney formation Broad zone of free gas and gas hydrate formed beneath the gas chimney, extending upward from the regional base of hydrate stability Such zones of hydrate and free gas likely form due to pronounced lithological contrasts (sealing layers) within the hydrate stability zone