Variation of Bottom-simulating-reflection Strength in a High-flux Methane Province, Hikurangi Margin, New Zealand
Stuart A. Henrys, Derek J. Woodward, Ingo A. Pecher, 2009. "Variation of Bottom-simulating-reflection Strength in a High-flux Methane Province, Hikurangi Margin, New Zealand", Natural Gas Hydrates—Energy Resource Potential and Associated Geologic Hazards, T. Collett, A. Johnson, C. Knapp, R. Boswell
Download citation file:
Bottom-simulating reflectors (BSRs) represent the base of a gas-hydrate zone underlain by widespread free gas. On the southern Hikurangi margin offshore of the east coast of New Zealand, multichannel seismic data reveal that the gas-hydrate province extends from about 600-m (1968-ft) water depth to the Hikurangi Trench and covers an area of about 50,000 km2 (19,305 mi2). We analyzed BSR strength in a grid of seismic data across this area. Simplified rock-physics models were used to estimate the reflection coefficient of BSRs with a gas concentration above which compressional wave velocity is mostly insensitive to gas saturation. This reflection coefficient was found to be —0.20, resulting from at least 8-10% gas saturation. Four percent of the gas-hydrate stability zone on the southern Hikurangi margin is underlain by strong BSRs with reflection coefficients that are —0.20 or stronger. Mapped variations in BSR and sea-floor reflection amplitude ratios and reflection coefficients reveal a strong correlation, on a regional scale, between the amplitude of BSRs and structures that promote fluid flow. Isotope, geochemical, and geophysical data from previous studies onshore point to a thermogenic origin for methane and suggest that New Zealand east coast fluids are derived from accreted, organic-rich, sedimentary sources overlying the subducting slab and that these sources must have an age of about 70 Ma. We therefore speculate that BSR formation on the Hikurangi margin is supported by the long-term recycling of fluids along faults that penetrate through the oldest sediments in the forearc and sole at the plate interface, as mapped in crustal seismic sections elsewhere on the Hikurangi margin. Squeezed subducting sediments at the plate interface may provide a rich source of water driving fluid recycling.
Figures & Tables
In September 2004, the American Association of Petroleum Geologists (AAPG) convened a Hedberg Research Conference in Vancouver, British Columbia, Canada titled "Natural Gas Hydrates: Energy Resource Potential and Associated Geologic Hazards." As a continuation of the Hedberg Research Conference in Vancouver, the conveners of the conference and the editors of this Memoir have worked with more than 150 authors and coauthors to prepare this Memoir on gas hydrates. This publication follows the goals of the Hedberg conference; however, the contents of this Memoir were expanded to include all aspects of gas hydrates in nature. This Memoir contains 39 individual contributions, ranging from long topical summaries to shorter focused research papers. This Memoir has been published in two parts, with digital versions of all the complete research papers included on the enclosed CD. The hardcopy portion of the Memoir includes abstracts and several key figures for each of the contributions along with a complete copy of a gas hydrate technical review. The digital portion of this Memoir has been organized into a series of topical sections consisting of review articles, marine gas hydrate papers, and gas hydrate laboratory and modeling studies. Because of the rapidly emerging worldwide interest in gas hydrates, this comprehensive treatise on the geology of gas hydrates will be valuable to both the gas hydrate research community and exploration/development geologists working in arctic and deep marine environments.