Marine Gas Hydrates in New Zealand
Methane hydrates are abundant on the two active continental margin regions of offshore New Zealand. The New Zealand crust consists of a rifted fragment of Gondwana that now straddles the Pacific / Australian plate boundary. Along the east coast of the North Island, oceanic Pacific crust subducts under a continental block of the Australian Plate along the Hikurangi margin; to the west of Fiordland on the South Island, oceanic Australian crust subducts under a continental block on the Pacific Plate (Figure 1). Gas hydrate accumulations along these margins occur over well-constrained pressure and temperature ranges in the upper 500 m or so of seafloor sediments and in water depths of at least 600 m. Over the past few years, researchers at the University of Otago and GNS Science (New Zealand's geologically oriented crown research institute) have made use of existing industry and government seismic data sets to assess the extent of gas hydrates on New Zealand's margins and to estimate volumes of methane associated with them. This has primarily been done by the identification and analysis of bottom simulating reflections (BSRs) in the seismic data.
A BSR is generally produced by seismic energy reflecting from the top of gas-filled sediments lying below the zone of hydrate stability. In active margins, such as the subduction related Hikurangi and Fiordland margins, high fluid flux often results in the maintenance of a free-gas reservoir under any significant hydrate accumulation. As a result, regions of high concentrations of hydrate will often correspond to zones of significant BSRs. Such features currently are the targets of several energy-related gas hydrate research projects around the world.
BSRs have been observed on seismic data from the Fiordland Margin for over 20 years (Townend 1997). Systematic study of the data in this region has identified a small hydrate province covering 2200 km2 and containing an estimated volume of 1110 km3 of solid hydrate (Fohrmann et al. 2007) – which would contain the equivalent of ~Â 6400 TCF of methane at standard temperatures and pressures (STP). Our knowledge of the Fiordland gas hydrate province is limited by the sparse dataset here – and our lack of supporting geological information on seafloor geology. There are few samples of sediment from this region and no wells. Hydrate accumulations in this region appear to be associated with failures of the slope. One particular submarine landslide appears to correspond to a location where the BSR appears to outcrop on the seafloor (Figure 2) (Crutchley et al. 2007). Seismic inversion methods have been used to constrain our quantifications of hydrate in Fiordland (Figure 3) (Fohrmann et al. in review). These analyses depend on assumptions of sediment properties, but confirm that significant accumulations of natural gas are present in the hydrate and underlying free gas reservoirs.
A comparative abundance of seismic and geologic data off the east coast of New Zealand has led to a better understanding of the hydrate system in this region (Henrys et al. 2003). The hydrate province in this area is over 20 times the size of the Fiordland province at 50,000 km2, possibly containing as much as 23,000Â km3 of solid hydrate (~ 133,000 TCF of methane at STP). Such huge quantities of methane deserve attention for their resource potential, and current research is examining methods to high-grade sweet-spots in the hydrate system at optimal locations (e.g., Figure 4) where geological structures and active fluid flow result in a dynamic system where hydrates and associated free gas can be concentrated (Pecher et al. 2004; Henrys et al. 2007; Crutchley et al. in prep). Other significant effort is examining the relationship between seafloor stability and hydrate occurrences on the Hikurangi Margin (Pecher et al. 2005; Faure et al. 2006) which may have ramifications for global climate studies if a mechanism can be found for the cycling of methane between the shallow seafloor and atmosphere.
Non-seismic gas hydrates
Recent analyses of several European and North American offshore hydrocarbon provinces, have shown significant quantities of methane hydrate – without a corresponding BSR signal. In these cases, hydrates are often detected by wireline logs in locations where strata are oriented parallel to the seafloor – and would therefore mask a BSR character. In these low-fluid-flux passive margin settings, high concentrations of hydrate are often detected without a corresponding free-gas zone underlying them (Senger et al. in prep). In fact, if the lithology of the sediments in such an environment is suitable, economically significant deposits of hydrate are possible. Current work is aimed at the analysis of well logs from the Canterbury and Great South Basin petroleum provinces to determine the potential of these regions for hydrate deposits.
Ingo Pecher (now at Heriot-Watt University), Stuart Henrys, and the management of GNS Science have facilitated much of the gas hydrate research undertaken in recent years. Funding (e.g. for the acquisition of the TAN0607 data shown in Fig. 4) has come from the New Zealand Foundation for Research, Science and Technology, and Marsden Fund grants to the gas hydrate team led by scientists at GNS Science. Additional funding has come from the University of Otago Research Committee. Seismic lines from the Fiordland margin shown in this abstract have been made available through the generosity of the New Zealand Ministry for Economic Development. Seismic processing has made use of an academic licence for Globe Claritas. This work would not have been possible without the dedication of my students, particularly Miko Fohrmann, Gareth Crutchley and Kim Senger.
Barnes PM, Sutherland R, Delteil J 2005. Strike-slip structure and sedimentary basins of the southern Alpine Fault, Fiordland, New Zealand. GSA Bulletin 117(3/4): 411-435.
Crutchley GJ, Gorman AR, Fohrmann M 2007. Investigation of the role of gas hydrates in continental slope stability west of Fiordland, New Zealand. New Zealand Journal of Geology and Geophysics 50(4): 357-364.
Crutchley GJ, Gorman AR, Pecher IA, Henrys SA in prep. Full-waveform inversion applications to sites hosting potentially high gas hydrate accummulations: Hikurangi Margin, New Zealand.
Faure K, Greinert J, Pecher IA, Graham IJ, Massoth GJ, de Ronde CEJ, Wright IC, Baker ET, Olson EJ 2006. Methane seepage and its relation to slumping and gas hydrate at the Hikurangi margin, New Zealand. New Zealand Journal of Geology and Geophysics 49: 503-516.
Fohrmann M, Gorman AR, Pecher IA 2007. Seismic characterization of the Fiordland gas hydrate province, New Zealand. In: Collett T, Johnson A, Knapp C, Boswell R ed. Natural Gas Hydrates: Energy Resource Potential and Associated Geological Hazards: AAPG Special Publication. Pp. in press.
Fohrmann M, Gorman AR, Jarvis KDG, Pecher IA in review. Gas hydrate reservoir characterization offshore Fiordland, New Zeland, using seismic amplitude-versus-angle (AVA) waveform inversion. Geophysics.
Henrys SA, Ellis S, Uruski C 2003. Conductive heat flow variations from bottom-simulating reflectors on the Hikurangi margin, New Zealand. Geophysical Reserach Letters 30(2): doi:10.1029/2002GL015772.
Henrys SA, Woodward DJ, Pecher IA 2007. Variation of bottom-simulating reflection (BSR) strength in a high-flux methane province, Hikurangi margin, New Zealand. AAPG Special Publication in press.
Pecher IA, Henrys SA, Zhu H 2004. Seismic images of gas conduits beneath vents and gas hydrates on Ritchie Ridge, Hikurangi Margin, New Zealand. New Zealand Journal of Geology and Geophysics 47(2): 275-279.
Pecher IA, Henrys SA, Ellis S, Chiswell SM, Kukowski N 2005. Erosion of the seafloor at the top of the gas hydrate stability zone on the Hikurangi Margin, New Zealand. Geophysical Research Letters 32(24): L24603.
Senger K, Gorman AR, Fohrmann M in prep. Defining the has hydrate stability zone in low geothermal gradient systems.
Townend J 1997. Estimates of conductive heat flow through bottom-simulating reflectors on the Hikurangi and southwest Fiordland continental margins, New Zealand. Marine Geology 141: 209-220
Andrew Gorman is a geophysicist at the University of Otago in Dunedin on the South Island of New Zealand. He graduated from the University of Calgary in 1987 with a BSc in geophysics and spent six years in the Western Canadian oil patch with Chevron and Suncor before heading off to the University of British Columbia for a PhD in crustal seismology as part of the Lithoprobe project. In 2000, he moved on to the University of Wyoming for a post-doc in marine seismology focussing on gas hydrate accumulations on the Blake Ridge off the coast of the Carolinas. Gas hydrate and shallow marine research has continued to be one of his major research efforts since his move to New Zealand in 2003. His work has been facilitated by substantial academic marine research facilities within the country, which includes the University of Otago's own research vessel, Polaris II, equipped for coastal marine geophysical surveys.