THERMOGENIC PETROLEUM POTENTIAL OF THE NANKAI SUBDUCTION ZONE, OFFSHORE SW JAPAN

K. Shiraishi^1,*, Y. Yamada¹ and T. Nibe^2,3

¹ Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 3173-25 Showa-machi, Kanazawa-ku, Yokohama, 236-0001, Japan.

² JGI, Inc., Tokyo, Japan.

³ Present address: Japan Petroleum Exploration Co. Ltd. (JAPEX), Tokyo, Japan.

^* corresponding author, kshiraishi@jamstec.go.jp

Key words: Nankai Trough, Kumano forearc basin, Japan, plate subduction, accretionary prism, seismic reflection imaging, thermogenic hydrocarbons, biogenic gas, gas hydrates, ocean-bottom seismograph, multi-channel seismic.

In the Kumano forearc basin, offshore SW Japan, the potential occurrence of hydrocarbons has been inferred from both geophysical surveys which point to the presence of natural gas hydrates, and geochemical analyses of gas samples recovered from boreholes and submarine mud volcanoes. The forearc basin is located in the inner part of the Nankai Trough where the Philippine Sea plate is subducting northwards beneath SW Japan. The basin succession unconformably overlies a deformed accretionary prism complex. Hydrocarbons with both microbial and thermogenic origins have been recorded in the Kumano Basin, but the thermogenic petroleum potential is poorly constrained because the regional-scale geological architecture of the Nankai subduction zone is not well understood.

In this paper, the regional-scale petroleum potential of the Nankai Trough is investigated based on the geological interpretation of new seismic reflection images. The images were derived from legacy seismic survey data using advanced processing techniques such as reverse time migration, which make use of both primary and multiple reflections in wide-angle seismic data collected with ocean-bottom seismographs. In the study area, a seaward-dipping reflector was identified which terminates at the plate boundary décollement and which is interpreted as part of the boundary between previously-identified older (>14 Ma) and younger (<6 Ma) accretionary prisms. A triangle body at the base of the younger accretionary prism is interpreted to consist of underthrusted sediments including thick hemipelagic mudstones with source rock potential which were scraped off the down-going oceanic plate. These tentatively identified, deeply-buried mudstones may have the potential to generate thermogenic hydrocarbons which may then migrate upwards along dipping carrier beds or faults and fractures to the unconformity at the base of the Kumano forearc basin succession. The presence of hydrocarbons in the accretionary prism may be inferred from positive amplitude anomalies which have been identified in the reprocessed multi-channel seismic data.

The new reflection interpretations therefore allow the potential for thermogenic petroleum to be proposed in the study area. The hydrocarbon potential at this subduction margin may therefore rely not only on shallow microbial methane, but may also involve deeper, thermogenic petroleum generated within the underlying accretionary prism.

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