Elongate fluid flow structures: Stress control on gas migration at Opouawe Bank, New Zealand

Abstract

High-resolution 2D and 3D seismic data from Opouawe Bank, an accretionary ridge on the Hikurangi subduction margin off New Zealand, show evidence for exceptional gas migration pathways linked to the stress regime of the ridge. Although the ridge has formed by thrusting and folding in response to a sub-horizontal principal compressive stress (σ 1), it is clear that local stress conditions related to uplift and extension around the apex of folding (i.e. sub-vertical σ 1) are controlling shallow fluid flow. The most conspicuous structural features are parallel and horizontally-elongated extensional fractures that are perpendicular to the ridge axis. At shallower depth near the seafloor, extensional fractures evolve into more concentric structures which ultimately reach the seafloor where they terminate at gas seeps. In addition to the ridgeperpendicular extensional fractures, we also observe both ridge-perpendicular and ridge-parallel normal faults. This indicates that both longitudinal-and ridge-perpendicular extension have occurred in the past. The deepest stratigraphic unit that we image has undergone significant folding and is affected by both sets of normal faults. Shallower stratigraphic units are less deformed and only host the ridge-parallel normal faults, indicating that longitudinal extension was limited to an older phase of ridge evolution. Present-day gas migration has exploited the fabric from longitudinal extension at depth. As the gas ascends to shallower units it 'selfgenerates' its flow pathways through the more concentric structures near the seafloor. This shows that gas migration can evolve from being dependent on inherited tectonic structures at depth, to becoming self-propagating closer to the seafloor.

A Shallow Seabed Dynamic Gas Hydrate System off SW Taiwan: Results From 3‐D Seismic, Thermal, and Fluid Migration Analyses

“…A geophysical experiment was carried out in 2013 over the Four‐Way‐Closure Ridge (4WCR) near the toe of the accretionary prism off SW Taiwan to determine the influence of tectonic processes on the gas hydrate system (Berndt et al, 2019). This experiment generated a data set using a high‐resolution P‐Cable 3‐D seismic imaging system, which allows us to study fluid migration and gas hydrate dynamics in more detail on a scale from meters to tens of meters (Planke et al, 2009; Plaza‐Faverola et al, 2014; Riedel et al, 2018). Localized temperature field variations, derived from spatially dense measurements, are usually impossible to observe using a thermal probe due to technical and logistical difficulties at sea.…”

A 3‐D Model of Gas Generation, Migration, and Gas Hydrate Formation at a Young Convergent Margin (Hikurangi Margin, New Zealand)

“…Opouawe Bank, at the northeastern edge of our model (Figure ), is one of the most prominent and best studied ridges near the deformation front on this part of the margin. It is associated with focused fluid flow (e.g., Koch et al, ; Riedel et al, ), numerous active gas seeps (Greinert et al, ), and concentrated gas hydrate accumulations (Schwalenberg et al, ). However, in the eastern part of the modeled area, only open anticlines sea‐ward of the principal deformation front are fully represented in the model since the structures farther landward were too complex to map with the available data.…”

“…This is most likely achieved by focused migration of methane either from deep microbial or thermogenic sources. Near‐vertical focused migration of gas generated well below the HSZ has been imaged by high resolution 3‐D reflection seismic data (e.g., Plaza‐Faverola et al, ; Riedel et al, ) and illustrates the role that focused migration of gas plays in the formation of concentrated hydrate deposits. Pore water chemistry at Opouawe Bank is consistent with deep (1,500–2,100 m below seafloor; Koch et al, ) microbial methane generation.…”

“…Geochemistry, Geophysics, Geosystems (e.g., Koch et al, 2016;Riedel et al, 2018), numerous active gas seeps , and concentrated gas hydrate accumulations (Schwalenberg et al, 2017). However, in the eastern part of the modeled area, only open anticlines sea-ward of the principal deformation front are fully represented in the model since the structures farther landward were too complex to map with the available data.…”

Crustal fingering facilitates free-gas methane migration through the hydrate stability zone

“…in the Cascadia margin (1)(2)(3)(4)(5)(6), the Blake Ridge offshore northeast US continental margin (7), Hikurangi margin offshore New Zealand (8,9), Vestnesa Ridge offshore west Svalbard (10)(11)(12)(13)(14), and a few other locations (15)(16)(17)(18)(19), suggests that free-gas methane venting can coexist with and persist within hydratebearing formations. Such coexistence is found in nature over a wide range of pressure, temperature, and compositional conditions.…”