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天然气稀有气体同位素研究作为一个有别于烃类地球化学的新体系,在进入油气地质、地球化学研究领域仅20余年的时间里已取得了长足进展。元坝气田是继发现国内最大的海相气田——普光气田之后,在四川盆地内发现的又一个大型海相气田。元坝气田天然气烃源问题一直存在争议,主要原因是天然气地球化学数据揭示为源岩高—过演化阶段产物,而元坝气田能够供气的主要烃源层系均为高—过演化阶段,形成了对烃源认识的差异。研究共分析元坝气田天然气样品19个,组分分析结果表明干燥系数分布于0.98~1之间,甲烷和乙烷碳同位素组成分别为-35.6‰~-28.3‰、-34.6‰~-21.62‰;稀有气体同位素组成~3He/~4He值为(2.50~0.70)×10~(-8),~(40)Ar/~(36)Ar值为340.7~861.92,主频分布在340.7~500之间。天然气为高成熟—过成熟阶段的产物。研究认为,在元坝气田天然气有机地球化学数据展示多解而无法确定烃源层系的前提下,元坝气田天然气氩同位素特征所表征的源岩定年与发育的烃源岩不符,但又能揭示一些有意义的信息。元坝气藏烃源可能是震旦系或下寒武统筇竹寺组,燕山期早期热演化形成古油藏后,在地质历史变迁中遭到改造或破坏,于燕山中期又经历古油藏持续埋深,经历化学改造和流体调整,原油(或分散有机质)裂解生成天然气,再经过喜马拉雅期的调整改造在构造—岩性复合圈闭富集形成现今的天然气藏。原油裂解天然气中氩同位素特征表征的是古油藏裂解天然气的氩同位素特征和上三叠统须家河组高—过成熟的腐殖型烃源岩形成的天然气氩同位素特征的混源。
As a new system that is different from hydrocarbon geochemistry, natural gas rare gas isotope studies have made great strides in the recent more than 20 years in the fields of hydrocarbon geology and geochemistry. The Yuanba gas field is another large-scale marine gas field discovered in the Sichuan Basin, following the discovery of the Puguang Gas Field, the largest marine gas field in China. The source of natural gas in Yuanba gas field has been controversial for a long time. The main reason is that the geochemical data of natural gas reveal the product of high-over-evolution of source rock, while the major hydrocarbon source of gas supply in Yuanba gas field is high-over-evolutionary stage, Formed a source of understanding of the differences. A total of 19 natural gas samples from the Yuanba gas field were analyzed. The results of component analysis showed that the drying coefficients ranged from 0.98 to 1. The carbon isotope compositions of methane and ethane were -35.6 ‰ ~ -28.3 ‰, -34.6 ‰ ~ -21.62 ‰ (~ 2.50 ~ 0.70) × 10 ~ (-8), ~ (40) Ar / ~ (36) Ar values were 340.7 ~ 861.92 for rare gas isotopes, and the dominant frequencies were 340.7 ~ 500 between. Natural gas is the product of a high maturity-over-maturity stage. According to the research, under the premise that the organic geochemical data of natural gas in Yuanba gas field show multiple solutions and the source rocks can not be identified, the source rocks characterized by the argon isotopic characteristics of natural gas in the Yuanba gas field are not consistent with the developed source rocks, Reveal some meaningful information. The hydrocarbon source of the Yuanba gas reservoir may be the Sinian or the Lower Cambrian Qiongzhusi Formation. After the early Yanshanian thermal evolution formed the ancient oil reservoir, it was transformed or destroyed in the geological history change, and experienced the ancient oil in the middle of Yanshan The reservoir has been continuously burial depth. After chemical modification and fluid adjustment, the crude oil (or dispersed organic matter) is cracked to generate natural gas. After the adjustment of the Himalayan period, the natural gas reservoir is enriched in the tectono-lithologic composite trap. The characteristics of argon isotopes in the cracked natural gas of crude oil are characterized by the mixed isotopes of argon gas isotopes of natural gas from the paleo-overmature mature humic hydrocarbon source rocks in the Upper Triassic Xujiahe Formation.