OS4A-5:SEISMIC AMPLITUDE ANOMALIES AND VELOCITIES ASSOCIATED WITH THE GAS HYDRATE SYSTEM ON NEW ZEALAND’S HIKURANGI MARGIN: IMPLICATIONS FOR HYDRATE DEPOSITION STYLES

Gareth CRUTCHLEY1*， Douglas FRASER2， Andrew GORMAN2， Guy MASLEN1, Stuart HENRYS1， Ingo PECHER1,3
1. GNS Science, NEW ZEALAND; 2. Department of Geology, University of Otago, NEW ZEALAND; 3. School of Environment, University of Auckland, NEW ZEALAND

    2D seismic data were acquired in the Pegasus Basin on the southern end of New Zealand’s Hikurangi subduction margin in 2010 to stimulate interest in petroleum exploration in the area. These data contain source-receiver offsets of up to 10 km, which enable a range of quantitative seismic methods that are relevant to gas hydrate research. In order to investigate the hydrate system in different parts of the basin, we have re-processed the data with an emphasis on 1) higher-resolution, to image shallow geological relationships in good detail, and on 2) pre-stack time migration (PSTM) so that detailed velocity analysis can be carried out on 2D profiles.

    Prominent bottom simulating reflections (BSRs) marking the base of gas hydrate stability are widespread in the basin and beneath thrust ridges towards the north of the basin. We observe the commonly-reported high reflectivity beneath BSRs due to free gas accumulations, but also note high-reflectivity focused along particular layers above the regional BSR level. The latter seem to be the result of focused gas injection into the gas hydrate stability zone, as opposed to localized BSR uplift due to a strong thermal perturbation like has been interpreted elsewhere on the Hikurangi margin. Gas injection often appears to occur along stratigraphic horizons that cross-cut BSRs, but there is also evidence in the study area for sub-vertical injection across sedimentary layering. Differences between these two styles of gas invasion likely have implications for the distribution of gas hydrates at these sites.

    To generate velocity models of the gas hydrate system, we pick stacking velocities from common reflection point gathers and then convert them to interval velocities. We have tested several workflows for extracting stacking velocities, which include manual, horizon-based, and fully-automated picking. Our methods resolve low velocities associated with free gas beneath BSRs, and also high velocity zones above BSRs. On-going work is aimed at determining the origin of high-velocity zones with respect to lithological relationships and high gas hydrate concentrations, since both can result in significant velocity increases. Separating these two effects is critical for understanding where concentrated hydrate deposits are likely to have formed.