Gas Hydrate Stability Zone Offshore Nankai, Japan - Is The Gas Hydrate Stable at Present?

This work investigates the Megasplay Fault Zone (MFZ) and upper Imbricate Thrust Zone (ITZ) of the Nankai Trough, both regions of active thrust faulting where multiple BSRs are observed (Kinoshita et al. 2011). We demonstrate that gas hydrates offshore Nankai and in other offshore regions are distributed within a volume, or envelope, that does not coincide with the single numerical equations so far proposed for gas hydrate distribution on continental margins. Instead, BSRs show variable volume and surface ratios within an envelope defined by specific equilibrium equations. This study presents the concept of a Gas Hydrate Stability Envelope (GHSE), which takes into account the ratio between shallow and deep BSRs within variable sub-surface conditions. A new method of predicting hydrate stability is thus proposed: different concentrations of solid and free gas are distributed unequally within a volume of rock, which can be precisely defined using equilibrium equations for gas hydrate distribution. The proposed method also represents the nature of the stability of gas hydrate on continental margins over relatively larger scales more accurately than single equations quantifying the depths of stability of gas hydrates.