祁连山多年冻土区天然气水合物的形成条件

Formation conditions of gas hydrates in permafrost of the Qilian Mountains, Northwest China.

祁连山多年冻土区面积约10×104km2,年平均地表地温为-1.5～-2.4℃,冻土层厚度为50～139m。区内侏罗纪小型含煤盆地广布,产有丰富的煤层气。南祁连盆地是一潜在的油气盆地,存在下石炭统臭牛沟组、下二叠统草地沟组、中三叠统大加连组、上三叠统尕勒得寺组4套烃源岩,具有良好的生油生气潜力。在木里煤田33号钻孔的冻土层内发现有连续逸出的高含量烃类气体,井口点燃即可燃烧,简易采气分析结果表明甲烷含量高达38.07%～75.9%。根据该钻孔的气体组分、年平均地表地温、冻土层厚度、地温梯度等数据分析,这里基本具备形成天然气水合物的温压条件,计算结果显示水合物稳定带的顶界和底界埋深分别为171m和574m,稳定带厚度为403m。

Permafrost in the Qilian Mountains covers an area of about 100000 km^2 with an annual mean ground surface temperature ranging from -1.5 to -2.4℃ and permafrost thickness ranging from 50 to 139 m. Small Jurassic coal-bearing basins are widespread in the Qilian Mountains, which contain abundant coal bed methane. The South Qilian basin is a potential petroleum-bearing basin, where there are four suites of hydrocarbon source rocks, namely, the Lower Carboniferous Chouniugou Formation, Lower Permian Caodigou Formation, Middle Triassic Dajialian Formation and Upper Triassic Galedesi Formarion, which all have a high genetic potential. Continuous escape of hydrocarbon gases was found inside the permafrost interval at hole 33 in the Muli coal field. When we ignited the wellhead, the escaping gas burned. Simple gas sample analysis shows that it contains as high as 38.07-75.9% methane. Based on the gas composition, annual mean ground surface temperatures, permafrost thickness and thermal gradients of hole 33, the permafrost in the main has the temperature-pressure conditions for the formation of gas hydrates. Calculation indicates that the hydrate stability zone probably occurs in the 171-574 m interval with a thickness of 403 m.