Geophysical Interpretation of the Canterbury Slope to Identify Links Between Gas Hydrate Occurrences and Slope Stability

Gas hydrate deposits have been well established globally, occurring in specific pressure-temperature conditions on both active and passive continental margins provided that a sufficient supply of methane is present. In New Zealand, gas hydrates have been identified through the presence of bottom-simulating reflections (BSRs) on both the Hikurangi and Fiordland active margins. No direct evidence for gas hydrate has yet been found in New Zealand's offshore hydrocarbon systems, including the Canterbury basin. This may be due to the fact that BSRs are masked by the generally flat-lying stratigraphy. However, the presence of gas, one of the pre-requisites for hydrate formation, on the Canterbury slope has previously been reported from both seismic profiles and borehole logs. Strong evidence links the presence of gas hydrate with increased slope instability, due to the effect of hydrate dissociation on the physical properties of the host sediment. This is especially true in a shallow-water setting like the Canterbury continental slope, where eustatic fluctuations, rapid sedimentation, and temperature shifts will promote a dynamic behaviour of the hydrate stability zone (HSZ). Here we present an alternative way of potentially predicting the occurrence of gas hydrates on the Canterbury slope. A model for the HSZ of the Canterbury slope is established using bottom-water temperatures and borehole data. The model predicts the intersection of the top of the HSZ and the seafloor at ~540 m water depth. This depth corresponds with a zone of increased slope instability, interpreted using an array of industrial and academic multichannel seismic reflection profiles and bathymetric data sets. Tentatively, this zone of shallow-based slope instability is linked to weakness planes at the top of the HSZ. The model also predicts the extent of the base of the HSZ, a prime location for finding BSRs.