Abstract:

Through observation and statistical analysis of gas-hydrate drilling cores in Sanlutian mining area, the characteristics of fractures were discussed from the point of linear density, dip angle and filling degree. The occurrence type of gas hydrates and the basic feature of structural fractures in the study area were confirmed. The influence of fractures on the formation of gas hydrate was also discussed. The results showed that the gas hydrate mainly occurred in siltstone, dark mudstone and oil shale, and about 62.5% of gas hydrate occurred in structural fractures,which are mainly low-dip angle fractures and unfilled fractures with proportion of 50.37% and 73.68%, respectively;in addition, the density of fractures in the overlying strata reduced to some extent. It is concluded that the development of fractures has important effect on the formation of gas hydrate in the study area: firstly, the unfilled fractures have provided migration passages and accumulation spaces for hydrocarbon gases;secondly, the fractures with low-dip angles make the strata have low permeability vertically and formations have poor connectivity, greatly limiting the upward diffusion of hydrocarbon gases; finally, the reduction of fractures in the overlying strata also inhibits the upward diffusion of hydrocarbon gases, playing the role of cap rocks.

Key words: Sanlutian mining field, natural gas hydrate, fracture