Thermal evolution of the New Zealand Hikurangi subduction margin: Impact on natural gas generation and methane hydrate formation - A model study

We present an integrated 2D model of thermal and microbial generation of methane, migration into the gas hydrate stability zone (HSZ), and formation of methane hydrates. The model reconstructs the shallow (0-20 km) thermal structure of the subduction interface between the Australian plate and the subducting Pacific plate, and the trench basin (Pegasus Basin). Modelled temperatures of less than 110 degrees C within Pegasus Basin constrain the generation of oil and gas. Whilst a cool thermal regime is predicted to limit thermogenic generation of gas to a burial depth of >10 km, it extends the interval where prolific microbial gas generation occurs. The modelled rate of microbial generation of methane increases beneath the HSZ and peaks at similar to 1600 m below seafloor. Diffusive upward migration of microbially generated methane is interpreted to lead to widespread methane hydrate formation and the presence of a semi-continuous bottom simulating reflector (BSR). Predicted average hydrate saturation within the HSZ is 0.9% for a modelled sedimentary organic matter content of 0.5% and 1.6% for 1% organic matter in fine-grained Pegasus Basin sediments. Considerably higher concentrations of methane hydrate of up to 20-70% are predicted to occur where gas migration is focussed within the frontal anticline and proto-thrust zone southeast of the modern accretionary wedge and in channel and basin floor sandstones related to the Hikurangi Channel. The Hikurangi Channel sedimentary system transported coarse clastic sediments eroded from the rising Southern Alps along the eastern margin of the Pegasus Basin since the Miocene. It provides carrier beds specifically for transport of thermogenic gas generated close to the subduction interface. A buried Mesozoic accretionary wedge originating from subduction of the Pacific Plate beneath Gondwana further focusses the migration of gas. Focussed migration of thermogenic gas leads to the highest predicted hydrate concentrations in potential channel sand reservoirs. (C) 2015 Elsevier Ltd. All rights reserved.