Abstract
A resource assessment of methane hydrate (MH) in the eastern Nankai Trough was conducted through a probabilistic approach using 2D/3D seismic reflection data and drilling survey data obtained from Ministry of Economy, Trade and Industry (METI) Tokai-oki to Kumano-nada exploratory test wells. We extracted more than 10 prospective MH-concentrated zones characterized by high resistivity in well log, strong reflectors and high velocity on seismic profiles (data), and turbidite deposits delineated by sedimentary facies analyses.
The amount of methane gas contained in MH-bearing layers was calculated using a volumetric method for each zone. Each parameter, such as gross rock volume (GRV), net-to-gross ratio (N/G: ratio of sandy layer divided by gross thickness), MH pore saturation (Sh), porosity, cage occupancy, and volume ratio, was given as a probabilistic distribution for a Monte Carlo simulation, considering the uncertainty of these values.
The GRV of each hydrate-bearing zone was calculated from both strong seismic amplitude and velocity anomalies. The N/G was determined from the relationship between LWD resistivity and grain size in zones with existing wells. A seismic facies map created by a sequence stratigraphic approach was also used for determining the N/G in zone without well controls. Sh was estimated from a combination of density and NMR logs, together with calibration by gas volume measured from onboard MH dissociation tests using a pressure temperature core sampler (PTCS). Sh in a zone without well control was estimated from the relationship between seismic P-wave interval velocity and Sh from NMR log at a well site.
A total area of 4687 km2 of Bottom Simulating Reflectors (BSRs) was interpreted within the survey area in the eastern Nankai Trough (12000 km2). The total amount of methane gas in place contained in MH in the survey area was estimated to be 40 tcf as a Pmean value (10 tcf as P90, 83 tcf as P10). Total gas in place at the MH-concentrated zone (767 km2) was estimated to be 20 tcf (Half of the total amount) as Pmean value. Sensitivity analysis indicated that N/G and Sh have higher sensitivities than other parameters.
