Reservoir characterization of methane hydrate bearing turbidite channel in the eastern Nankai Trough, Japan

Abstract

Abstract We have discussed reservoir architecture of methane hydrate (MH) bearing turbidite channels in the eastern Nankai Trough using 3-D seismic data and well log data. MH bearing turbidite channels exhibit complex patterns of strong reflection comprising patchy-like shape of positive and negative seismic reflectors. The groups of these reflectors obtained by picking represent the internal architecture of the channel complex that can be roughly classified into three depositional sequences. According to a seismic sequence stratigraphic analysis, each depositional sequence results in the different depositional system implying that the reservoir architecture of the turbidite channels varies corresponding to the sedimentary conditions as well as the topographic changes in the study area. Compared with well log data, the thickness of the turbidite channel at ß2 well in the southwestern part of the study area is much greater than that of ß1 well in the northeastern part. However, the depositional sequences of the northeastern part represent sand-dominated turbidite sediments ensuring that the reservoir potential is high despite the relatively smaller thickness of the turbidite channels. For constructing a geological frame model, we examined further details of reservoir characteristics of the turbidite channels around ß1 well. The identified bottom frame of the several channels is oriented along north-to-south and north-northeast-to-south-southwest directions, which coincide with the directions of paleo-current flows determined by the seismic sequence stratigraphic analysis. An interval velocity between BSR (bottom simulating reflector) and the top of the MH bearing sediments is obtained from a high-density velocity analysis. The distributions of the higher interval velocity are identified above the bottom frame of channels in the northeastern part of the study area. The turbidite sediments in the northeastern side of channels are derived from the north-northeast direction of paleo-current flows, which is different from the sediment supply system compared with those of the northern side to southwestern side of the channels. Thus, the higher velocity anomalies in the northeastern side of the channels may be related to the different coarse sediments supply system which may lead to the different reservoir architecture of the turbidite channels. Introduction The eastern Nankai Trough, extending to southwest-northeast offshore central part of Japan, has been considered to be an attractive target for the potential resource of methane hydrates (MH) from the resource assessment issue as well as the candidate of the exploitation and the production test fields of MH1, 2, 3. In this area, it is widely observed the bottom simulating reflectors (BSR) that are interpreted to represent lower boundary of MH bearing sediments4, 5, 6, 7, 8, 9. Previous studies in the " Nankai Trough?? exploratory test well acquired in 1999 report that MH is developed in the pore space of the unconsolidated turbidite sands in deep submarine-fan sediments10, 11, 12, 13. Based on this exploration result, a 2-D seismic survey in 2001 (2,802 km), 3-D seismic surveys in 2002 (a total area of three survey fields: 1,960 km2), and a multiple well drilling campaign in 2004 were intensively conducted for the purpose of MH research by Japanese Government (Ministry of Economy, Trade and Industry; METI).