OCCURRENCE OF METHANE HYDRATE AND BSRS IN THE NANKAI TROUGH, OFFSHORE JAPAN

As a part of a project assessing resource potential of methane hydrate which is expected to be associated with widely distributed BSRs (Bottom Simulating Reflectors) in the Northwest Pacific, offshore Central Japan, the MITI Exploratory test well "Nankai Trough" drilling campaign was carried out from late 1999 to early 2000, at the water depth of 945m. During the drilling campaign, a total of six wells were drilled within a 100m radius of the main hole: two pilot holes, the main hole, and three post survey wells. Distances among each hole are 10m to 100m. Wireline logging, LWD and/or coring were carried out and logging records and core analyses data were obtained from the wells. Based on the results of Resistivity, Sonic, FMI, CMR, and other logging data which are very effective for methane hydrate evaluation, it was determined that there were four methane hydrate bearing sand rich intervals, and total thickness of the methane hydrate rich beds is 12 to 14m. Such intervals consist of turbidite fan deposits. The intervals were correlated between the wells, and wireline logging data showed that the methane hydrate saturation was 20 to 80% in the layers. The bottom depth of high hydrate concentration was correlated with the depth of BSR by VSP. Methane hydrate occurrence to fill pore spaces among grains, an advantage for resource point of view, was visually confirmed by cores from two wells. It has been revealed that BSR has various features, that is, variety in amplitude and continuity, which is presumably influenced by methane hydrate concentration and occurrence in the sediments. To obtain the reliable volume for methane hydrate resource, it is necessary to correlate and calibrate the subsurface geological information from wells and BSRs. The characteristics of BSRs and their distribution were examined using 2D and 3D seismic data, and plan for multi-well drilling campaign in early 2004 were settled to verify methane hydrate occurrences with related to BSRs.