Reservoir Characterization of Methane Hydrate Bearing Turbidite Channel in the Eastern Nankai Trough, Japan

摘要

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 132 well in the southwestern part of the study area is much greater than that of Bl 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 Bl 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.