鄂尔多斯盆地致密砂岩气及伴生氦气形成演化特征

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

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δ^(13)C_(1)、δ^(13)C_(2)、δ^(13)C_(3)、δ^(13)C_(4)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δ^(13)C_(1)、δ^(13)C_(2)、δ^(13)C_(3)值分布区间分别为-42.6‰~-24.6‰、-32.7‰~-18.0‰、-27.6‰~-15.1‰。流体包裹体中烷烃气与非烃气含量变化区间范围较大,主要为气源岩干酪根热降解气。流体包裹体中的烃类气,包含3个阶段捕获的天然气:①浅埋藏过程中,较低成熟烃源岩生成的轻碳同位素天然气;②最大深埋阶段,较高成熟源岩生烃排出的重碳同位素天然气;③抬升阶段,先期聚集的天然气次生调整捕获的轻碳同位素天然气。综合研究认为,上古生界煤系致密砂岩气,在天然气大规模生成、排放和充注过程中,运移方向以垂向运移为主,兼有侧向运移;进入致密砂岩后的二次运移及后期次生调整阶段,具有由南向北、由西向东短距离运移的总体趋势。致密砂岩气田中普遍含氦,总体资源规模巨大,位居中国多个盆地之首。鄂尔多斯盆地氦气为典型壳源成因,呈现弥散式分布、局部富集,氦气分布和富集受古老富U、Th元素基底岩系和深大断裂复合控制。

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 heliumrich 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 δ^(13)C_(1),δ^(13)C_(2),δ^(13)C_(3),and δ^(13)C_(4) 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 δ^(13)C_(1),δ^(13)C_(2) andδ^(13)C_(3) 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.