Gas migration into gas hydrate-bearing sediments on the southern Hikurangi margin of New Zealand

Abstract

We present multichannel seismic data from New Zealand's Hikurangi subduction margin that show widespread evidence for gas migration into the field of gas hydrate stability. Gas migration along stratigraphic layers into the hydrate system manifests itself as highly reflective segments of dipping strata that originate at the base of hydrate stability and extend some distance toward the seafloor. The highly reflective segments exhibit the same polarity as the seafloor reflection, indicating that localized gas hydrate precipitation from gas-charged fluids within relatively permeable layers has occurred. High-density velocity analysis shows that these layer-constrained gas hydrate accumulations are underlain by thick (up to ∼500 m) free gas zones, which provide the source for focused gas migration into the hydrate layer. In addition to gas being channeled along layers, we also interpret gas migration through a fault zone into the field of hydrate stability; in this case, a low-velocity layer within the hydrate stability zone extends laterally away from the fault, which might indicate that gas-charged fluids have also migrated away from the fault along strata. At this site, where both dipping strata and faulting seem to influence fluid migration, we observe anomalously high velocities at the base of hydrate stability that we interpret as concentrated gas hydrates. Our results give insight into how shallow fluid flow responds to permeability contrasts between strata, fault zones, and perhaps also the gas hydrate system itself. Ultimately, these relationships can lead to gas migration across the base of hydrate stability and the precipitation of concentrated hydrate deposits.