Quantitative characterization of methane hydrate reservoir in the eastern Nankai Trough, Japan
Authors: Noguchi Satoshi , Furukawa Toshiko , Oikawa Nobutaka , Aung Than Tin 
Published: May 2016

Abstract
Quantitative evaluation of methane hydrate (MH) bearing turbidite reservoir in the eastern Nankai Trough, offshore Japan is conducted using a combination of 3-D seismic and well data. For constructing 3-D facies model, we have interpreted internal sand body and stacking patterns of the channels to discuss reservoir heterogeneities of the turbidite channels. Geobody using seismic volume rendering shows that the extracted body is distributed toward northeast to southwest directions in good agreement with the direction of the channel-filling sediments according to the seismic sequence stratigraphic analysis. Stacking patterns of channel facies are identified around the ß1 well. The identified channels exhibit low sinuosity channels consisting of various magnitudes of channel widths ranging from tens to several hundred meters. Orientation of these channels typically exhibits north-to-south and north-northeast-to-south-southwest directions, which are consistent with Paleo-flow current directions. Internal property of the stacking patterns of the channel sediments is examined using a seismic acoustic inversion analysis. The distribution patterns of the velocity between the BSR and the top of the MH-bearing sediments show that higher velocities are particularly identified along north-northeast direction, which coincides with the sediment supplies of the turbidite channels. Assuming a simple rock physics model, distribution of hydrate saturation is also correlated well with the facies distributions. This fact suggests that reservoir architecture of the MH-bearing turbidite channels is primary controlled by sedimentary conditions (i.e., depositional systems) of the turbidite channels.

Introduction
The eastern Nankai Trough, which is located offshore of central Japan, is considered an attractive potential resource of methane hydrates (MHs) and a candidate for the exploitation and production of MH test fields (Fujii et al., 2008, 2009; Yamamoto et al., 2009). Previous works conducted various studies of seismic, well logging, and coring operations such as "Nankai Trough" exploratory test well (which was acquired in 1999), a 2-D seismic survey in 2001 (2,802 km), 3-D seismic surveys in 2002 (the total area of three survey fields was 1,960 km2), and a multiple well-drilling campaign in 2004 by the Ministry of Economy, Trade and Industry (METI). According to the results, MHs are developed in the pore spaces of an unconsolidated sandy sediment within the deep submarine-fan turbidite system (Nakamizu et al., 2004; Takahashi et al., 2004; Tsuji et al., 2004; Uchida et al., 2004). In some locations in the eastern Nankai Trough, MHs are selectively accumulated in the turbidite formations; such zones are called "MH-concentrated zones". These MH-concentrated zones are the best targets for recovering MHs for future exploitation and for the commercialization of MHs. Therefore, our primary target is the MH-concentrated zones to characterize reservoir potentials of MHs.
In this paper, we focus on the submarine channel types of the MH-bearing sediments (ß MH-concentrated zone, hereafter ß site) in one of the MH-concentrated zones in the eastern Nankai Trough. In particular, we discuss quantitaive evaluation of facies distributiona and internal properties for characterizing the reservoir heterogeneities in the MH-bearing turbidite channels.