Occurrence and distribution of gas hydrates on the Puysegur continental margin and slope-stability aspects on Ritchie Banks, Hikurangi Margin, New Zealand

Fohrmann, Miko.

Offshore gas hydrate occurrences represent a possible economical opportunity to New Zealand but they may also demonstrate a hazard since they are linked to submarine slope failure, which can trigger tsunamis. Occurrences of bottom simulating reflections (BSRs), which can be related to the presence of gas hydrates have been previously observed over a wide-spread zone on the active continental margin associated with the incipient Puysegur subduction zone, south-east of the South Island of New Zealand. However, u'nlike New Zealand's other large gas hydrate province located on the active Hikurangi Margin, east of the North Island, no effort has been made to date to delineate the Fiordland-Puysegur gas hydrate province and to describe it in terms of its seismic characteristics. Five seismic reflection datasets (> 5400 km) are analysed for the presence of BSRs. , The BSRs identified exhibit classic characteristics that are indicative of a reflection caused by free gas at the base of the gas hydrate stability zone. The BSRs predominantly reveal a negative polarity - implying a decrease in acoustic impedance, they cross-cut strata, and they exhibit variable amplitude-versus-angle (A V A) responses. A reduced interval velocity distribution below the BSR additionally points to the presence of free gas. Based on the - . distribution of BSRs it is concluded that the Fiordland-Puysegur gas hydrate province extends over 2200 km2 • Since the sedimentary properties of the Fiordland Margin are not well constraint, an amplitude-versus-angle (A V A) simultaneous inversion that converts seismic angle stacks to rock property slices of P- and S-impedance and vrJvs is chosen to predict the presence and extent of gas hydrates across Dusky Ridge. This inversion technique is chosen because it is a proven technique that obtains reliable results, especially in a deep-water environment like Fiordland where no a priori well information is available. The inversion results suggest that gas hydrate is present in a broad zone that extends over 250 m above the BSR. This result, in combination with the observation that a significant proportion (16%) of the mapped region contains structural and stratigraphic features that may be linked to regions of increased hydrate concentrations, leads to the estimate that the Fiordland-Puysegur gas hydrate province may contain 1.688 tcf of gas, which relates to nearly half the amount of the Maui gas field in the Taranaki Basin prior to production. The dissociation of gas hydrate caused by observed temperature fluctuations at the seafloor is finally modelled to test a possible mechanism for submarine slope failure. A potential hazard site at Ritchie Banks, located at the Hikurangi Margin, reveals an 8-km-long V BSR running parallel to the inclined seafloor. This site is treated as a critical Coulomb wedge on the verge of failure to obtain estimates of 1) pore-fluid pressure and 2) coefficients of friction, which are used as initial conditions in a 2D finite element code (FLAC20 ). The simulation accounts for increasing pore fluid caused by the postulated dissociation of gas hydrate and demonstrates that slope failure is possible, even though the results indicate that only the top of the ridge fails.

xxix, 245 leaves : ill. (some col.), maps ; 30 cm. + 1 CD-ROM (4 3/4 in.)

2009Fohrmann

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