未固結堆積物のメタンハイドレート胚胎層の岩石物理モデル

Abstract

It is well known that the methane hydrate exists below the sea floor over 500 m or the polar region under the frozen ground in the Earth. We made the cross-plot of the methane hydrate and Vp, Vs, or Vp/Vs from the well log data at the eastern Nankai Trough in Japan and the Mackenzie Delta in Canada. We found that as the methane hydrate saturation increases, Vp and Vs increase, and Vp/Vs decreases from these plots. We discussed the rock physics model of methane hydrate bearing layers. We calculated Vp, Vs, and Vp/Vs as the function of the methane hydrate saturation, based on four imaginary rock physics model of methane hydrate bearing layers. From the comparison between the real well sonic data and the calculated data from four models, we inferred that the rock physics model of methane hydrate bearing layers was matrix-supporting model at both the eastern Nankai Trough and the Mackenzie Delta areas. However, there were some error and difference with the Vp, Vs and Vp/Vs from the matrix-supporting model and real well sonic data between the eastern Nankai Trough and the Mackenzie Delta. We estimated the effect of Vp, Vs, and Vp/Vs for the change of clay content of matrix-supporting model. As the clay content decreases, the Vp and Vs increase, and Vp/Vs decreases. The clay content from well core data corresponds with the calculated Vp, Vs, and Vp/Vs as the function of the methane hydrate saturation and the clay content from the matrix supporting model. In case of the estimation of methane hydrate saturation from Vp, Vs, and Vp/Vs, we need to consider the geology, especially sand/clay ratio of the methane hydrate bearing layers.