Thermogenic Petroleum Potential of the Nankai Subduction Zone, Offshore Sw Japan

Abstract

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.

Upper-plate conduits linked to plate boundary that hosts slow earthquakes

“…2d–h ). These seismic reflection features are typical of mud volcanoes 29 and suggest the occurrence of active upwelling flows inside the low-velocity columns. The seafloor bathymetry of the area shows several circular mounds (with steep slopes) close to the seismic profile, indicating another typical feature of mud volcanoes (Fig.…”

Seismic reflection imaging of deep crustal structures via reverse time migration using offshore wide-angle seismic data on the eastern margin of the Sea of Japan

“…On the other hand, imaging shallow structures, such as sedimentary layers in the Yamato Basin and deformations of the fault-fold belt in the Sado Ridge to the Mogami Trough (Sato et al 2014), is difficult by using only primary reflections because of spatial aliasing of migrated reflections from wide-spacing OBSs (e.g., Zelt et al 1998). RTM with surface-related multiples (e.g., Grion et al 2007;Liu et al 2011;Shiraishi et al 2017Shiraishi et al , 2019 could be useful for imaging internal structures of the thick crust and sedimentary layers. Updating velocity models by waveform inversion involving reflection phases may also be helpful for further improvement of the reflection profiles (e.g., Sirgue et al 2010;Shiraishi et al 2019) with better spatial resolution and reliability of velocities within the thick crust than that by traveltime tomography, especially in the thick crust with the shallow complex fault-fold belt (the eastern side of Figs.…”

“…RTM with surface-related multiples (e.g., Grion et al 2007;Liu et al 2011;Shiraishi et al 2017Shiraishi et al , 2019 could be useful for imaging internal structures of the thick crust and sedimentary layers. Updating velocity models by waveform inversion involving reflection phases may also be helpful for further improvement of the reflection profiles (e.g., Sirgue et al 2010;Shiraishi et al 2019) with better spatial resolution and reliability of velocities within the thick crust than that by traveltime tomography, especially in the thick crust with the shallow complex fault-fold belt (the eastern side of Figs. 3 and 4).…”

“…RTM is based on numerical modelling with the wave equation to simulate forward wavefields from the source with a function and backward wavefields from receivers with observed seismic records (e.g., Chang and McMechan 1990). By adopting the source-receiver reciprocity on the relation between airgun shots and OBSs, the downgoing wavefields are extrapolated forward in time from OBS locations, and the upgoing wavefields are extrapolated backward in time from airgun shot locations (Shiraishi et al 2019). The forward-extrapolated wavefield F i (t, x) is simulated forward in time ( t : 0 → T ) based on a scalar wave equation with a migration velocity c(x) and a source function s(t) from the i th OBS location…”

“…This method is now widely used for better imaging in complex models than conventional ray-based seismic migration methods (e.g., Etgen et al 2009;Zhou et al 2018). In a crustal-scale study, Shiraishi et al (2019) applied the RTM-based technique for using primary and multiple reflections observed by 1-km spacing OBSs to investigate the subducting plate and internal structures of the accreted sediments in the Nankai Trough.…”