Resource Assessment of Methane Hydrate in the Eastern Nankai Trough, Japan

Abstract

Resource assessment of methane hydrate (MH) in the eastern Nankai Trough was conducted through probabilistic approach using 2D/3D seismic data and drilling survey data from METI exploratory test wells " Tokai-oki to Kumano-nada??. We have extracted more than 10 prospective " MH concentrated zones?? characterized by high resistivity in well log, strong seismic reflectors, seismic high velocity, and turbidite deposit delineated by sedimentary facies analysis. The amount of methane gas contained in MH bearing layers was calculated using volumetric method for each zone. Each parameter, such as gross rock volume (GRV), net-to-gross ratio (N/G), MH pore saturation (Sh), porosity, cage occupancy, and volume ratio was given as probabilistic distribution for Monte Carlo simulation, considering the uncertainly of these evaluations. The GRV for each hydrate bearing zones was calculated from both strong seismic amplitude anomaly and velocity anomaly. Time-to-depth conversion was conducted using interval velocity derived from Seismic Vision While Drilling (SVWD). Risk factor was applied for the estimation of the GRV in 2D seismic area considering the uncertainty of seismic interpretation. The N/G was determined based on the relationship between LWD resistivity and grain size in zones with existing wells. Seismic facies map created by sequence stratigraphic approach was also used for the determination of the N/G in zone without well controls. Porosity was estimated using density log, together with calibration by core analysis. The Sh was estimated by the combination of density log and NMR log, together with the calibration by observed gas volume from onboard MH dissociation tests using Pressure Temperature Core Sampler (PTCS). The Sh in zone without well control was estimated using relationship between seismic P-wave interval velocity and Sh from NMR log at well location. Total amount of methane gas in place contained in MH within survey area in the eastern Nankai Trough was estimated to be 40 tcf as Pmean value. Total gas in place for MH concentrated zone was estimated to be 20 tcf (Half of total amount) as Pmean value. Sensitivity analysis indicated that the N/G and Sh have higher sensitivity than other parameters, and they are important for further detail analysis. Introduction Seismic data from the Nankai Trough, offshore central Japan, indicates widespread distribution of bottom simulating reflectors (BSR) 1 that are interpreted to represent lower boundary of methane hydrate (MH) bearing zones. MH in the Nankai Trough is a potential natural gas resource, however, the volume, distribution, and occurrence of MH in this area is poorly understood. In 1999, MH-bearing sand-rich intervals in turbidite fan deposits were recognized from the eastern Nankai Trough based on results of MITI (Ministry of International Trade and Industry) " Nankai Trough?? wells2, 3. Based on this exploration result, the Japanese government inaugurated a 16-year MH exploration program in 2001. As a part of this program, the Ministry of Economy, Trade and Industry (METI), Japan, drilled the " Tokai-oki to Kumano-nada?? exploratory test wells from January to May 2004, in order to obtain data for understanding the occurrence of MH and estimating resource potential4, 5. In this campaign we carried out logging-while-drilling (LWD) at 16 sites, coring at four sites, wireline logging at two sites, and long-term monitoring of formation temperature at single site5,6,7 (Figure 1). Using following data set, we conducted MH resource assessment in the survey area in 2005 and 2006. Figure 2 shows the flow chart of the resource assessment.

Hydro-bio-geomechanical properties of hydrate-bearing sediments from Nankai Trough

“…Thrust and growth faults, as well as bottom-simulating reflectors (BSRs) suggestive of the presence of hydrate, have been extensively investigated since 1971 with seismic surveys, coring, and logging Takahashi and Tsuji, 2005;Tsuji et al, 2004;Uchida et al, 2004;Waseda and Uchida, 2004a,b). The total amount of gas in place in the eastern Nankai Trough is estimated to be 1.1 trillion (10 12 ) cubic meters (Fujii et al, 2008). Isotope analyses suggest the methane gas in shallow sections of the Nankai Trough at the coring site for this study is mostly microbial, with only a slight increase of higher hydrocarbon concentrations with depth .…”

Geologic controls on gas hydrate occurrence in the Mount Elbert prospect, Alaska North Slope

“…The pressure-temperature conditions that are necessary (but not sufficient) for gas hydrate formation (see Sloan and Koh, 2008) limit the occurrence of gas hydrates to arctic sediments (typically where the geothermal gradient is impacted by permafrost), to sediments within deep lakes and seas, and to shallow oceanic sediments on outer continental shelves and slopes where water depths exceed approximately 500 m. Gas hydrate occurs in a variety of forms and a range of geologic settings. In the marine environment, gas hydrate has been documented to occur as disseminated grains filling pores within sands (Riedel et al, 2006;Fujii et al, 2008) and muds (Paull et al, 1996;Yang et al, 2008), as well as complex networks of fracture fills, veins, and nodules primarily in fine-grained sediments Collett et al, 2008a;Park, 2008;Hadley et al, 2008). In contrast, permafrost-associated gas hydrates have commonly been reported to be primarily restricted to pore-filling morphologies within sandy sediments (Dallimore and Collett, 2005).…”

Methane Hydrate Field Program. Development of a Scientific Plan for a Methane Hydrate-Focused Marine Drilling, Logging and Coring Program

“…A number of new quan ta ve es mates of in-place methane hydrate volumes (Klauda and Sandler, 2005;Frye, 2008;Wood and Jung, 2008;Bureau of Ocean Energy Management, 2012) and, for the fi rst me, technical recoverable Fujii et al, 2008) assessments, have been undertaken using petroleum systems concepts developed for conven onal oil and natural gas explora on. For example, in an assessment of methane hydrate resources on the North Slope of Alaska, Colle et al (2008) indicated that there are about 2.42 trillion cubic meters (~85.4 trillion cubic feet) of technically recoverable methane resources within concentrated, sand-dominated, methane hydrate accumula ons in northern Alaska.…”

“…Furthermore, the MMS assessment indicated that reservoir-quality sands may be more common in the shallow sediments of the methane hydrate stability zone than previously thought. Fujii et al (2008) es mated a volume of about 1.1 trillion cubic meters (about 40 trillion cubic feet) of gas exists within the hydrates of the eastern Nankai Trough, with about half concentrated in sand reservoirs.…”