An analysis of gas hydrate accumulation condition in the Duck Lake area, Qiangtang Basin, northern Tibet

In recent years, the gas hydrate investigation in the permafrost region of China shows that the gas source condition is the key factor for controlling the breakthrough of gas hydrate exploration in the Qiangtang Basin. In order to further clarify the potential of gas hydrate accumulation in the Duck Lake area, the authors systematically analyzed such geological factors as permafrost, gas source, reservoir and structure based on the drilling results in recent years. The results show that there are good conditions of permafrost, geothermal gradient, gas source, reservoir, structure and water source in some areas, indicating that Duck Lake area has a certain gas hydrate accumulation potential. Finding sufficient hydrocarbon gas sources will be the main direction of the next gas hydrate investigation. In addition, the thickness of the gas hydrate stability zone (GHSZ) and the depth of bottom of GHSZ in the Duck Lake area were predicted by the data of the geothermal gradient and the gas composition obtained from the drilling and AMT results. Predictive results show that, when the methane is 85%, ethane is 9% and propane is 6%, the thickness distribution of GHSZ is basically the same as that of the permafrost. The thickness of GHSZ is between 400m and 630m, and the depth of the bottom of GHSZ is between 400m and 680m. Where methane is 98% and ethane is 2%, the thickness of GHSZ is sharply thinned, only 0~30m in most areas and only 150m thick in certain areas, and the depth of the deepest bottom of GHSZ is only 240m. Based on the results of gas logging, it is concluded that the Oligocene Suonahu Formation has more potential gas hydrate accumulation potential than the Upper Triassic Tumengela Formation in Duck Lake area, while Tumengela Formation has strong hydrocarbon generation and expulsion capability, which thus can be regarded as an important horizon for exploration of conventional oil and gas or shale gas.