Integrated Facies Modeling Workflow for Methane Hydrate Reservoir along the eastern Nankai Trough, Japan
Authors: Aung Than Tin , Noguchi Satoshi , Oikawa Nobutaka , Kanno Takayuki , Tamaki Machiko , Akihisa Kunio 
Published: March 2015

Abstract
Turbiditic origin methane hydrate reservoirs along the eastern Nankai Trough, Japan, are primarily characterized by conventional seismic facies analysis and log interpretation. This study introduces an integrated and hierarchical object-oriented geological facies modeling workflow to model preliminarily interpreted channel patches of reservoir zones inside methane hydrate reservoirs. Five wells with gamma ray and ring resistivity logs from LWD and Wireline acquisition, one well with full core that was partly unrecoverable, a 3D seismic cube and 3D impedence cube are the primary data used to analyze the reservoir between the two Pliocene stratigraphic horizons of the Ogasa Formation (Og-b to Og-c'). The integrated workflow consists of 1) Data preparation which consists of generating horizons and facies logs as well as analysis of channel geometry, 2) domain conversion, 3) 3D structural framework building and layering, 4) Spatial analysis and verification of a correlation between the seismic impedence and the facies and 5) Hierarchical object modeling using Petrel, Schlumberger's geoscience modeling platform. Efforts were made to interpret facies in all five wells by conventional log interpretation and by applying GR and resistivity cutoffs that produced facies that matched the turbiditic stratigraphy observed in cores. An adaptive channel (AC) model was first derived by distribution, width, length and thickness of channel patches confirmed with Geobody interpretation tool with the guidance of impedence seismic. Additionally, based on spatial data, a pixel-based model generated by sequential indicator simulation (SIS) was derived. Two heiarchical models, namely, AC conditioned by SIS and vice versa are proposed.

Introduction
The occurrence of methane hydrate reserves in the subsea sediments along the eastern Nankai Trough, offshore central Japan, has been studied at the behest of METI (Japenase Ministry of Economics, Trade and Industry) and methane hydrate research consortium (MH21). 2D and 3D seimic, LWD, wireline and core data were acquired by METI for three fields from Tokai-oki to Kumano-nada within the period (1999–2004) (Figure 1). Conventional seismic attribute analysis was performed and bottom simulating reflectors (BSRs) were identified to characterize the lateral and vertical extent of methane hydrate deposits (Saeki et al. 2008, Hayashi et al. 2010). The composition of MH-concentrated turbiditic strata consisting of thin alternating sand and mud layers was confirmed from the petrophyical evaluation of LWD and Wireline logs (Fujii et al. 2008). Much research is still required to understand the physical and mechanical behavior of methane hydrate reservoirs and their implications for successful production. This study proposes an integrated workflow for geological facies modeling developed using the Schlumberger Seismic to Simulation tool, Petrel. The workflow was applied to the Beta region, a candidate for production in the eastern Nankai Trough.

Geologic Setting
The Nankai Trough is situated along the the subduction zone where the Phillipine sea plate has been subducting beneath the Eurasian plate. Along the eastern Nankai Trough, the tectonic setting is strongly influenced by the arc-arc collision between the Izu-Ogasawara Arc and the Honshu Arc. Methane hydrate has been widely observed in the Tokai-Kumano forearc basins along the Nankai Trough. The stratigraphy of the Tokai-Kumano forearc basins is comprised of the Pleistocene Kakegawa Group and the Ogasa Group (Moore et al. 2001, Takano et al. 2009). Based on seismic stratigraphic analysis, 2D/3D surveys, and well data (core and logs), four key depositional stages are classified and subdivided into 17 sequence units of the submarine fan turbidite sediments of the Pleistocene Kakegawa and Ogasa groups. In this study, a single sequence is added as Og-c' between Og-c and Og-d sequences. Table 1 summarizes the depositional sequence, sediment characteristics and tectonic events that relate to the Kakegawa and Ogasa Groups. This modeling work flow focuses on the three sequences Og-b, Og-c and Og-c' of the middle Pleistocene age.