Assessment of gas hydrate reservoir from inverted seismic impedance and porosity in the northern Hikurangi margin, New Zealand

Hikurangi margin, New Zealand, the shallowest subduction zone on Earth is the unique place to study the slow slip creep-like deformation and seafloor failure, and their probable link to the presence of gas hydrate, cold seeps and fluid migration. International Ocean Discovery Program expedition 372 discovered gas hydrate within thin intervals (ranging from ~5 to 25 m) of the deformed sediments in the Tuaheni landslide complex area of the northern Hikurangi margin.

 

To characterize the reservoir, both acoustic impedance (product of velocity and density) and porosity were inverted, along two perpendicular seismic profiles crossing the well, using a model-based post-stack seismic inversion technique. Results show the layer structure of alternate high-impedance with low-porosity and low-impedance with high-porosity sediments. The gas hydrate in this region is distributed in low impedance and high porosity clay dominated layers, while, high impedance observed here is because of low porosity and deformed sediments having no gas hydrate. These high impedance layers with alternate low impedance thin beds are the probable fluid migration pathways and hence may be one of the reasons for seafloor collapse.

 

The correlation between inverted impedance and porosity indicates that the high impedance layers are due to low porosity consolidated sediment without significant gas hydrate concentration. Gas hydrate is distributed in relatively high porosity and low impedance layers along the seismic profiles. Our result shows that the amount of gas hydrate concentration is low in this region and sediments are highly deformed/disturbed in the presence of many fluid/gas migration pathways. The decollement of the landslide, the base of the debris flow and the fluid migration pathways are clearly modeled. Seven sedimentary units are observed based on the acoustic impedance and porosity along the seismic profiles.

 

Citation

Uma Shankar, Maheswar Ojha, Ranjana Ghosh, Marine and Petroleum Geology, https://doi.org/10.1016/j.marpetgeo.2020.104751