Abstract:

The Qinghai-Tibetan Plateau is the largest permafrost in China with an area of 150×104 km², which is favorable for the formation and accumulation of natural gas hydrate, and the Qiangtang Basin will be the best target with a good prospect for natural gas hydrate reservoirs. The hydrocarbon gas composition and carbon isotope of the headspace gas samples from the borehole QK-1, one of the scientific drilling holes of gas hydrate in the Biluocuo area of the southern Qiangtang Basin, were determined and analyzed. The results indicate the hydrocarbon gas composition is complex, and the methane concentrations range from 3.0×10^-6 to 4,526.8×10^-6 with the average being 209.0×10^-6. The δ¹³C values of methane range from -55.9‰ to -37.8‰ with the average being -43.2‰, and the values of C₁/(C₂+C₃) are lower than 10, obviously suggesting thermogenic gas characteristics. According to the geological background and lithologic characteristics of the drilling area, it is estimated that the hydrocarbon gas comes from the deeper thermogenic gas which has migrated upward from the deep place, and may be mixed with biogenic gas in the shallow.

Key words: Qiangtang Basin, hydrocarbon gas, gas origin, distribution characteristic, natural gas hydrate