Sequence Stratigraphic Distribution Analysis of Methane-hydrate-bearing Submarine-fan Turbidite Sandstones in the Eastern Nankai Trough Area: Relationship between Turbidite Facies Distributions and BSR Occurrence

Abstract

Since previous research revealed that most of the methane hydrates in the eastern Nankai Trough area occur in matrix pores of turbidite sandstones, the facies distribution of turbidite sandstones may be one of the important keys to evaluate the distributions and actual volume of methane hydrates in the eastern Nankai Trough area. This paper attempts to reconstruct depositional processes of submarine-fan turbidites, and examines the relationship between turbidite facies distributions and bottom simulating reflector (BSR) occurrence as a proxy of methane hydrate using sedimentologic and sequence stratigraphic methodology. First, 2D/3D seismic survey data and well data including cores and logs were used to identify turbidite facies, seismic facies, and depositional sequences. The targeted Plio-Pleistocene Kakegawa and Ogasa Groups can be divided into 17 depositional sequences, and include six seismic facies indicating submarine-fan elements and surrounding slope to basin-floor environments. Next, facies maps for each depositional sequence unit were created by plotting all information on seismic facies, 3D seismic geomorphology, and well facies data. The obtained facies maps reveal that 11 major submarine canyons functioned as positionally fixed sediment supply systems from main land Japan, along which submarine fans were formed in the forearc basins. Submarine-fan depositional styles changed through Plio-Pleistocene from a braided channel type, through small radial fan, troughfill fan, and muddy sheet fan types, to a channel-levee system type. Finally, the facies maps of each depositional sequence were overlaid with the BSR distribution. The overlaid maps indicate that the BSRs occur on feeder channels, distributary channels, and proximal lobes of submarine fans, suggesting that methane hydrates selectively occur in coarser grained portions of a submarine fan. Because the lower part of the Kakegawa Group is mainly composed of braided channeltype submarine fan turbidites, the lower Kakegawa horizon serves one of the major horizons bearing methane hydrates in the eastern Nankai Trough area.

Bulk sediment mineralogy of gas hydrate reservoir at the East Nankai offshore production test site

“…1) (e.g., Baba and Yamada, 2004;Takano et al, 2009Takano et al, , 2013. The results of the ODP and IODP operation revealed that these basins are filled with submarine turbiditic sediments, which are commonly rich in quartz and feldspar (Underwood et al, 2005;Kinoshita et al, 2009).…”

“…The targeted basin fills, the Pleistocene Ogasa Group, cover the northern flank of the Daini Atsumi Knoll east of the Kumano forearc basin, and consist of confined deepwater turbidite system (Egawa et al, 2013;Noguchi et al, 2011;Takano et al, 2009). Seismic stratigraphy of this Ogasa turbidite system is subdivided into nine sequences from Og-a to Og-i in ascending order , among which sequence Og-b is defined as the gas hydrate reservoir sequence in the study area (Noguchi et al, 2011;Komatsu et al, 2015).…”

“…Widespread occurrence of bottom simulating reflectors (BSRs) in the deepwater subsurface seismically indicates the distribution of gas hydratehosted sediments (e.g., Ashi et al, 2002;Baba and Yamada, 2004). Recent three-dimensional (3-D) seismic survey and coring/logging operations have revealed a methane hydrate concentration zone (MHCZ) situated immediately above the BSR if the conditions of gas-hydrate petroleum systems are fulfilled (e.g., Fujii et al, 2009;Noguchi et al, 2011;Takano et al, 2009). In the early spring of 2013, the world's first offshore production test of gas hydrates was conducted for the MHCZ at Daini Atsumi Knoll, east of the Kumano forearc basin (Fig.…”

3-D internal architecture of methane hydrate-bearing turbidite channels in the eastern Nankai Trough, Japan

“…A large number of active faults have been recognized by the KAIKO-Tokai survey (the Research Group for Active Submarine Faults off Tokai, 1999;Soh and Tokuyama, 2002). The stratigraphy of the forearc basin of the eastern Nankai Trough is comprised of the Plio-Pleistocene Kakegawa and Ogasa Groups (Hiroki et al, 2004;Arai et al, 2006;Takano et al, 2008Takano et al, , 2009). Takano et al (2008Takano et al ( , 2009) conducted a seismicsequence stratigraphic analysis using 2-D/3-D seismic surveys and well data (including cores and logs) to reconstruct the depositional processes of submarine-fan turbidites in the eastern Nankai Trough area.…”

Evolutionary process of Beppu Bay in central Kyushu, Japan: a quantitative study of the basin-forming process controlled by plate convergence modes

“…The eastern part of the arc was under a continued compressive regime, which provoked formation of a regional unconformity in the eastern forearc around 3.0~2.5 Ma (e.g., Kameo et al 2010;Kameo and Sekine 2013). Watershed mountains simultaneously emerged along the MTL in the central part of SWJ (Oka 1978;Oka and Sangawa 1981;Mizuno 1987), which was succeeded by extensive deformation of the adjoining forearc basin (Takano et al 2009). As depicted in the isochron map of the yellow horizon (Figure 6), the tension graben around Beppu Bay in the western part of the island arc became inactive and the basin architecture changed to be under a transtensional/transpressional regime.…”