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.

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

“…The pronounced trend of predicted high gas hydrate concentration (trend I) along the principle deformation front of the Hikurangi Subduction Margin (Barnes et al, ; Figure ) is consistent with strong BSRs and gas related anomalies observed beneath deforming ridges along the margin (Crutchley et al, , , ; Henrys et al, ; Pecher et al, ; Figures and a) and increased velocities within the HSZ (Crutchley et al, , , ). 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.…”

“…Geochemistry, Geophysics, Geosystems 2010; Figures 9 and 10a) and increased velocities within the HSZ (Crutchley et al, 2015. Opouawe Bank, at the northeastern edge of our model (Figure 9), is one of the most prominent and best studied ridges near the deformation front on this part of the margin.…”

“…Scenario B, assuming more permeable carrier beds in the subducting plate and hence a greater contribution of thermogenic gas, predicts average volumes of methane stored in gas hydrates in Pegasus Basin that are very similar to estimated gas hydrate related gas in the Gulf of Mexico, where a probabilistic resource assessment estimated an average of 46.8 Bcf/km 2 (Frye, ). Predicted widespread and often highly concentrated hydrate deposits are consistent with widespread BSRs, seismic velocity and amplitude anomalies, and electromagnetic anomalies (Crutchley et al, , , ; Schwalenberg et al, ). Although geologically fundamentally different, the Gulf of Mexico and southern Hikurangi Margin hydrate provinces have in common that they are charged largely from beneath the HSZ (Boswell et al, ; Burwicz et al, ).…”

Experimental Investigation of Gas Flow and Hydrate Formation Within the Hydrate Stability Zone

“…Approximately, 10% of natural hydrate occurs in coarse‐grained material (Boswell & Collett, ), which have favorable production characteristics (Boswell & Collett, ; Moridis et al, ; Yamamoto et al, ). Some of these reservoirs consist of thick, dipping, sand layers, bounded by low permeability material, that contain gas hydrate above and free gas below the hydrate stability zone (Boswell, Collett, et al, ; Boswell et al, ; Crutchley et al, ; Tréhu et al, ). Hydrate exists in these sand layers far above the base of hydrate stability at saturations ranging between 60 and 90% (Collett et al, ).…”

The elastic wave velocity response of methane gas hydrate formation in vertical gas migration systems

“…In the finer sand used in the study of Hu et al (2012), the hydrate may cement grains and easily bridge across the pore space, and both of these modes have a greater impact on velocity. On the other hand, as we know, in the system of gas migration into gas hydrate-bearing sediments in field, the hydrate stability zone has high velocities, and the low-velocity zone is free gas-charged (Bunz and Mienert, 2004;Crutchley et al, 2015). And in the hydrocarbon leakage system, the low-velocity gas-charged zones always represent the leakage process (Løseth et al, 2009).…”