Methane Accumulation and High Concentration of Gas Hydrate in Marine and Terrestrial Sandy Sediments
Takashi Uchida, Amane Waseda, Takatoshi Namikawa, 2009. "Methane Accumulation and High Concentration of Gas Hydrate in Marine and Terrestrial Sandy Sediments", Natural Gas Hydrates—Energy Resource Potential and Associated Geologic Hazards, T. Collett, A. Johnson, C. Knapp, R. Boswell
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Gas-hydrate-bearing sand-core samples have been obtained from the Canadian Mallik wells, and the Nankai Trough wells of offshore Japan. The chloride-content anomalies in extracted pore waters, core-temperature depression, core observations, and visible gas hydrates, as well as continuous down-hole well-log data, confirm the presence of pore-space hydrateas an intergranular pore filling within sandy layers, clarifying the characteristics of subsurface natural gas hydrate in marine and permafrost zones. Gas-hydrate saturations (percent of pore volume) as high as 80% have been measured, which requires enough original pore space in the host sediment to allow the gas to accumulate. Carbon and hydrogen isotopic compositions of methane and hydrocarbon compositions in gas hydrate and gas-hydrate-bearing shallow sediments in the Nankai Trough show that methane is generated by microbial reduction of CO2 and suggest progressive decreases in microbial (biogenic) activity with depth and upward gas migration through the sediment column. In the Mackenzie delta, methane in gas hydrate is generated by thermogenic decomposition of kerogen. Based on the geochemical and geological data, gas migration is estimated to be an active flow to permeable sandy layers in the Nankai Trough, and a long migration of thermogenic gas, generated in deep mature sediments, through faults in the Mackenzie delta. Note that many similarities in appearance and occurrence between the terrestrial (Mallik) and the marine (Nankai Trough) areas exist, and this knowledge and information is crucial to the identification of other hydrate deposits and to assess their eventual energy resource potential.
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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.