Distribution and Seismic Characterisation of Gas Hydrates in Fiordland, New Zealand.

We have compiled the distribution of gas hydrates on the Fiordland margin based on the occurrence of BSRs in an extensive database of seismic reflection data, acquired by academic, government and petroleum research cruises over the last 30 years. Bottom simulating reflections (BSRs) are thereby found in an area of approximately 2200 sq km on the continental slope south-west. Main characteristics of the BSRs include: 1) predominately negative polarity compared to the seafloor reflection, which infers to a decrease in acoustic impedance, 2) the reflections follow the morphology of the seafloor, thereby crosscutting strata, and 3) strong variations of amplitudes with offset (AVO), indicating the presence of free gas below the reflection. In addition, the existence of free gas is supported by a reduced interval velocity distribution below the BSR, with velocity decreases of up to 30% compared to the overlaying layers. Because seismic reflection data is the only available data source in the Fiordland region (other a priori information like well information is not available) a simultaneous AVO inversion algorithm is used to obtain rock property attributes of P-impedance, vp/vs, and density, respectively. This information is used to verify the quantities of gas hydrate and their potential as a gas reserve in this region.