鄂尔多斯盆地致密砂岩气及伴生氦气形成演化特征

鄂尔多斯盆地孕育了中国最大的超大型致密砂岩大气区及目前中国最大的特大型富/含氦气田——东胜气田，也是中国首例特大型致密砂岩富/含氦气田。运用流体包裹体地球化学方法，对比分析流体包裹体气体与现今气藏中天然气组分和同位素差异，揭示了古今天然气地球化学与成藏演化过程及氦气地球化学特征。结果表明，现今气田主要以烃类气体为主，甲烷含量多数为90%～95%；现今气田中天然气δ¹³C₁、δ¹³C₂、δ¹³C₃、δ¹³C₄值分别为-36.5‰～-28.7‰、-25.3‰～-22.1‰、-27.0‰～-21.8‰、-25.6‰～-20.7‰。气层流体包裹体中δ¹³C₁、δ¹³C₂、δ¹³C₃值分布区间分别为-42.6‰～-24.6‰、-32.7‰～-18.0‰、-27.6‰～-15.1‰。流体包裹体中...

The formation and evolution characteristics of tight sandstone gas and associated helium in Ordos Basin

The Ordos Basin is the home to China's largest grand tight sandstone gas field and the country's largest grand helium-rich gas field， Dongsheng Gas Field. It is also the first discovered great tight sandstone helium-rich gas field in China. This paper used geochemical methods to investigate the fluid inclusion， combined with the analysis of the gas isotopic composition of the present gas field， and revealed the ancient and modern natural gas geochemistry， accumulation evolution and geochemical characteristics of helium. The results showed that the current gas field was dominated by hydrocarbon gases， and the methane content was mostly from 90% to 95%. Meanwhile， the natural gas δ¹³C₁， δ¹³C₂， δ¹³C₃， and δ¹³C₄ in the current gas field were from -36.5‰ to -28.7‰，-25.3‰ to -22.1‰，-27.0‰ to -21.8‰，-25.6‰ to -20.7‰， respectively. And the distribution ranges of δ¹³C₁， δ¹³C₂ and δ¹³C₃ in gas fluid inclusions were from -42.6‰ to -24.6‰，-32.7‰ to -18.0‰，-27.6‰ to -15.1‰， respectively. The contents of hydrocarbon gas and non-hydrocarbon gas in the fluid inclusions ranged widely， and mainly originated from kerogen thermal degradation in the source rock. The hydrocarbon gas in the inclusions contains natural gases captured from three stages： （1） natural gases with low carbon isotope generated from lower mature source rocks during shallow burial； （2） At the maximum deep burial stage， natural gases with higher carbon isotope are discharged from higher mature source rocks； （3）during the uplift stage， the natural gases with low carbon isotope are secondarily adjusted and captured by natural gases pre-accumulated in the early stages. This study comprehensively indicated that during the large-scale generation， discharge and charging process of natural gas， the Paleozoic coal-derived tight sandstone gas migrated mainly in the vertical direction with some lateral direction. When entering the tight sandstone and during the secondary migration and secondary adjustment of the later stage， the general trend of short distance migration was from south to north and west to east. Tight sandstone gas fields generally contain helium， and the overall resource scale is huge， ranking first in China. Helium gas has a typical crustal origination and shows diffusive distribution with local enrichment. The distribution and enrichment of helium gas are controlled by the combination of ancient U/Th-rich basement rock series and deep large faults.