Abstract
AbstractFocused 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.
Highlights
A faint Bottom simulating reflections (BSRs) is apparent in places, but for the most part we interpret the upper limit of strong reflectivity as the seismic manifestation of the base of the gas hydrate stability zone (BGHSZ) (Figures 4a and 4b), which correlates with the depth of BSRs in the low-frequency data
We suggest that future modeling of gas conduit propagation might consider the role of pronounced lithological contrasts within the hydrate stability zone, and how such heterogeneity influences free gas flow and gas hydrate formation
We have presented seismic reflection images of a classic gas conduit through the gas hydrate stability zone (GHSZ), amid a network of normal faults
Summary
Gas migration through the gas hydrate stability zone (GHSZ) is a widespread global phenomenon, with many examples having been documented from seismic reflection data, scientific drilling and modeling studies (Brooks et al, 1994; Flemings et al, 2003; Fu et al, 2020; Gorman et al, 2002; Liu & Flemings, 2006; Plaza-Faverola et al, 2011; Tréhu, Flemings, et al, 2004; Wood et al, 2002). The driving force behind gas conduit formation is overpressure accumulation, where gas buoyancy from interconnected free gas columns beneath the base of the gas hydrate stability zone (BGHSZ) drives the system toward mechanical failure (e.g., Flemings et al, 2003; Tréhu, Flemings, et al, 2004). It is often unclear what extent focused free gas migration makes use of existing planes of weakness (shallow faults) versus the generation of new pathways formed by hydraulic fracturing (e.g., Finkbeiner et al, 2001; Flemings et al, 2003; Hornbach et al, 2004). Investigate the possible co-existence of gas hydrate and free gas in a zone of high reflectivity that extends upward from the regional BGHSZ, at the base of the conduit
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