著者

鈴木 德行 前本 健太 星野 太一

出版者

石油技術協会

雑誌

石油技術協会誌 (ISSN:03709868)

巻号頁・発行日

vol.78, no.1, pp.16-27, 2013 (Released:2015-04-03)

参考文献数

48

被引用文献数

1 2

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Change of residual hydrocarbon gases in mudstones and pelitic metamorphic rocks was investigated to evaluate the retention of methane in shale rocks during diagenesis and metamorphism. The samples were collected from borehole MITI-Mishima and outcrops in Shimanto belt, Chichibu belt, and Sanbagawa metamorphic belt exposed in Shikoku region. Concentration of residual methane normalized to total organic carbon content begins to increase when mudstone porosity reaches about 5%. The increased methane concentration at mudstone porosity less than 5% suggests a significant retention of methane in mudstone matrix. The high concentration zone of residual methane corresponds to maturity levels of vitrinite reflectance (Ro) =2 to 3%. Residual methane concentration significantly decreases in the higher maturity levels (Ro>4%), suggesting the graphitization of methane. The maturity levels of Ro=2 to 3% with the highest residual methane concentration is expected to be the favorable stage for shale gas formation, although host rock lithology such as the storage capacity and fissile nature are also related to enhanced shale gas recovery. Many parts of accretionary prism in Shimanto belt are at these maturity levels of Ro=2 to 3% favorable for shale gas formation. Accretionary prism formed under the compressional stress suggests development of gas seal structures and closed fracture systems convenient for shale gas retention, although shale rocks in Shimanto accretionary prism are comparatively poor in organic matter. Several thermogenic gas seepages in Shimanto belt would be another indication of shale gas formation in accretionary prism distributed in wide area along Pacific side of southwest Japan.