Abstract
Gas hydrates are ice-like, solid clathrates that trap natural gas molecules inside a lattice of water molecules. They are common constituents of continental margins, which has led to extensive studies exploring their resource potential, role in the climate cycle, associated slope stability and geohazard potential. A region of gas hydrate deposits occurs in the Pegasus Basin, which lies at the southwest of the East Coast Basin, east of New Zealand. Lying within the transition from oceanic subduction beneath a continental plate to a strike-slip system between two continental blocks, deformation within the basin forms migration pathways that allow focused fluid flow. This can lead to high-concentration deposits within the high-permeability sands and fractured mudstones of this region.
This project aims to develop a method to forward model gas hydrate related features observed in seismic data using synthetic seismograms produced from viscoelastic finite-difference models. The models are constructed in a manner that is stratigraphically consistent with two seismic lines, PEG09-25 and HKS02-01, which image a gas hydrate feature referred to as the “hydrate finger”.
The viscoelastic finite-difference modelling scheme presented in this thesis was found to successfully produce synthetic seismograms that resemble the real seismic data. A range of geological properties and acquisition parameters were tested in order to assess parameterisations with respect to how well they match the gas hydrate features imaged in real data. A model containing high-concentration gas hydrate and free gas was constructed to best match the “hydrate finger” feature.
Further developments to the viscoelastic code and the chosen model parameterisations are required to enable this scheme to be used with other techniques such as high-density velocity analysis and inversion. Using forward modelling techniques like those presented here in conjunction with analyses (like inversion or velocity analysis) that constrain physical properties could be useful for validating previous works and for producing quantitative assessments of gas hydrates. Using viscoelastic models in conjunction with seismic and well data allows a more comprehensive and better-constrained assessment of gas hydrate systems to be made, reducing the risk associated with future extraction.