四川盆地威远气田水溶气脱气成藏地球化学证据

目前,对于四川盆地威远气田的形成过程和天然气来源在认识上还存在着较大分歧。为此,针对该气田天然气的甲烷碳同位素值异常偏重的现象,首先分析了气藏的地质特征和天然气的地球化学特征:气田主力气层是震旦系灯影组,天然气以甲烷为主,含微量乙烷和痕量丙烷;气藏含水饱和度较高,普遍含有保存很好的原生水。进一步根据天然气中H_2S含量与甲烷碳同位素值的关系,判断该区天然气甲烷碳同位素值偏重并非由硫酸盐热化学还原反应(TSR)造成。最后根据该区天然气的甲烷碳同位素值和邻区的对比结果,结合构造演化背景,判断认为,威远气田的天然气主要来自水溶气,并非过去认为的自邻区经侧向运移而来。结论认为:①由于水中释放出的甲烷碳同位素值较重,水溶气的脱气成藏造成了威远气田天然气甲烷碳同位素值偏重的现象;②伴随喜马拉雅期构造运动,威远地区大幅度抬升,形成构造圈闭,在高温、高压状态下溶解在水中的天然气发生减压脱溶,释放出的气体在圈闭中成藏,进而形成了威远气田;③经计算,威远气田圈闭下的水中释放出的天然气数量与该气田的探明储量相当,印证了该气田水溶气脱气成藏的观点。

Geochemical evidence of water-soluble gas accumulation in the Weiyuan Gas Field,Sichuan Basin

At present, there are several different opinions on the formation process of the Weiyuan Gas Field in the Sichuan Basin and the source of its natural gas. In view of the fact that the methane carbon isotope of the natural gas in the Weiyuan Gas Field is abnormally heavy, the geologic characteristics of gas reservoirs and the geochemical characteristics of natural gas were first analyzed. In the Weiyuan Gas Field, the principal gas reservoirs belong to Sinian Dengying Formation. The natural gas is mainly composed of methane, with slight ethane and trace propane. The gas reservoirs are higher in water saturation, with well preserved primary water. Then, it was riminated from the relationship of H2S content vs. methane carbon isotope that the heavier methane carbon isotope of natural gas in this area is not caused by thermochemical sulphate reduction (TSR). Based on the comparison of methane carbon isotope in this area with that in adjacent areas, and combined with the tectonic evolution background, it is regarded that the natural gas in the Weiyuan Gas Field is mainly derived from water-soluble gas rather than be migrated laterally from adjacent areas. Some conclusions are made. First, since methane released from water is carbon isotopically heavier, the water-soluble gas accumulation after degasification results in the heavy methane carbon isotope of the gas produced from the Weiyuan Gas Field. Second, along with Himalayan movement, great uplift occurred in the Weiyuan area and structural traps were formed. Under high temperature and high pressure, the gas dissolved in water experienced decompression precipitation, and the released natural gas accumulated in traps, consequently leading to the formation of the Weiyuan Gas Field. Third, based on calculation, the amount of natural gas released from water which is entrapped in the Weiyuan Gas Field after the tectonic uplift is basically equal to the proved reserves of this field, confirming the opinion of water-soluble gas accumulation after degasification.