Modelling the thickness and distribution of the Gas Hydrate Stability Zone in the Taranaki Basin, New Zealand.

We model the theoretical Base of Gas Hydrate Stability Zone (BGHSZ) and thickness of Gas Hydrate Stability Zone (GHSZ) in the Taranaki Basin, New Zealand. This study was carried out using bathymetric data, bottom water temperature data from CTD casts, a range of geothermal gradients derived from borehole data (28°C/km, 32°C/km and 36°C/km), pore water salinity of 3.5w% and six different gas feed composition. The data were analysed using CSMHYD, ArcGIS and MATLAB software. The models show that the hydrate stability field and thickness of the GHSZ is highly variable and significantly affected by local variations in geothermal gradient, water depth, bottom water temperature and the composition of gas-forming hydrates. At 12°C bottom water temperature, structure II hydrates are found to be stable in the Taranaki Basin at 350 m water depth, assuming a thermogenic gas source for the hydrate formation containing up to 5% ethane and the presence of carbon dioxide (CO2), nitrogen (N2) and hydrogen sulfide (H2S). The structure I hydrates are stable at 527 m water depth at a bottom water temperature of 8°C. The calculated thickness of the GHSZ increases with increasing water depth, decreasing bottom water temperature, lower geothermal gradient and presence of heavier hydrocarbon gases. The structure I hydrates are calculated to be stable in sediments as deep as 680 m for 100% methane and 748 m for a gas composition feed of 90% methane and 10% ethane. Structure II hydrates are stable in sediments up to a depth of 766 m (90% methane, 8% ethane and 2% propane). A model including carbon dioxide (CO2), nitrogen (N2) and hydrogen sulfide (H2S) in the gas composition feed shows that hydrates could be stable in sediments as deep as 807 m. An increase in the percentage of propane content in the gas composition feed by 1% increases the thickness of GHSZ by more than 23 m.

This work provides a useful background for future studies which will involve the integration of sequence stratigraphy, rock physics, seismic inversion and seismic attributes to detect and characterise the distribution of hydrates in the sediments within which they have been interpreted to be stable; as well as a study of the fluid flow dynamics and its impact in the formation of gas hydrates on the basin.