Gas hydrates on New Zealand's Hikurangi Margin : the importance of focused fluid flow for highly-concentrated deposits, methane seepage and sea floor erosion

Crutchley, Gareth James.

New Zealand's Hikurangi Margin hosts a large gas hydrate province that is receiving increased research attention on a global scale. Recent efforts have targeted 1) the potential of the province as an alternative energy resource, 2) methane cold seeps and 3) slope stability issues. Seismic Line 05CM-38 was used for identification and characterisation of potentially highly-concentrated deposits of gas hydrate ("sweet spots"). Two potential sweet spots within anticlines were inspected with a lD fullwaveform inversion routine to determine fine-scale velocity structure. At Western Porangahau Ridge, a high-amplitude feature within the gas hydrate stability zone (GHSZ) was characterised as a high velocity zone overlying a low velocity zone. High and low velocities are interpreted as being caused by gas hydrate and free gas, respectively. At Eastern Porangahau Ridge, results revealed low velocity zones associated with the bottom simulating reflection (BSR) that are much more pronounced near the centre of the ridge compared with further west in the limb of the anticline. At both anticlines, inversion results suggest a localised increase in fluid flow into the GHSZ - a process that is likely to encourage the deposition of relatively highly concentrated hydrate deposits. A network of cross-cutting seismic lines were analysed to reveal plumbing systems that supply gas to three general areas where methane seepage has been observed on the sea floor of the uplifted Rock Garden ridge. At the first area ("1M3"), seismic data reveal gas migration through the G HSZ that appears to be influenced by faulting in the hanging wall of a major thrust fault. At the second area ( "Weka"), data show a distinct convergence of the BSR with the sea floor. Gas supply to this seep is predicted to be focused upwards along the underside of the base of the GHSZ. The third area (the "Faure" site) is associated with a prominent arcuate slump feature. Gas migration appears to be channelled along dipping sedimentary layers that link the BSR with the sea floor. Collectively, these seep sites reveal a range of geological phenomena that are important in establishing fluid flow pathways between gas reservoirs and the sea floor. Regional erosion of the Rock Gar;den ridge top has previously been linked to the gas hydrate system. 2D numerical fluid flow simulations were conducted to investigate the potential influence of gas hydrate-induced permeability reductions and shallow gas pockets on hydro-fracturing _,a process that could lead to regional erosion. Steady-state simulations indicate that anomalous fluid pressure can develop close to the sea floor in response to lower-permeability hydrate-bearing sediments and the underlying gas pockets. Transient simulations suggest that large-scale cycling of fluid overpressure may be occurring on time scales of years to tens of years. It is predicted that regions of intense hydro-fracturing preferentially develop beneath the ridge top rather than beneath the ridge flanks, due to more pronounced overpressure generation and migration of gas through the GHSZ.

xvi, 196 leaves, [7] leaves of plates (folded) : ill. (some col.), maps ; 30 cm. + 1 CD-ROM (4 3/4 in.)

2009Crutchley

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