四川盆地震旦系—下古生界天然气地球化学特征及成因判识

四川盆地震旦系灯影组,寒武系筇竹寺组、龙王庙组、洗象池组的天然气组成、同位素值在不同构造位置上表现出不同的特征,造成了对其天然气成因及来源等认识上的差异。基于前人的研究成果,结合大量新钻探井资料,开展了该区天然气地球化学特征对比研究。结果表明:①天然气总体上表现为典型的干气,以烃类气体为主,甲烷含量为74.85%~97.35%,以83.0%~96.0%为主;非烃气体含量的差别主要表现在N2和He,威远—资阳地区相对高N2、高He,高石梯—磨溪地区相对低N2、低He,这种差异与烃源岩中的泥质含量有关。②不同地区天然气δ13 C1值、δ13 C2值差异大:资阳震旦系天然气δ13 C1值最轻(-38.0‰~-35.5‰),其他地区则介于-33.9‰~-32.0‰,反映了捕获阶段的不同,早期捕获的天然气同位素值较轻;威远震旦系—寒武系天然气δ13 C2为-36.5‰~-32.7‰,高石梯—磨溪龙王庙组天然气δ13 C2为-33.6‰~-31.8‰,高石梯—磨溪灯影组天然气δ13 C2则与上述天然气有较大差别,为-29.1‰~-26.8‰,主要反映了母质类型的差异。③天然气C6~C7轻烃组成均以环烷烃和异构烷烃为主,主要表现为原油裂解气特征。

Organic geochemical characteristics and origin of natural gas in the Sinian Lower Paleozoic reservoirs,Sichuan Basin

The composition and isotopic values of natural gas in the Sinian Dengying Fm and Cambrian Qongzhusi Formation, Longwangmiao Formation,Xixiangchi Fm in the Sichuan Basin exhibit unique features in different types of structures. The understandings of its origin and sources are multiple which is unfavorable for the decision making. For this purpose, the comparative study in geochemistry on natural gases composition, the isotope values and light hydrocarbon composition have been developed based on the previous research results. The natural gas is typical dry gas as a whole, which is dominated by hydrocarbon gas. The value of methane is 74.85~ 97.35%, ranging from 83.0~96.0% mostly.The contents of non hydrocarbon gas show some difference in the respects of N₂ and He. The contents of N₂ and He are relatively high in Weiyuan and Ziyang areas, while they are relatively low in the regions of Gaoshiti,Moxi.The reason of this is related to clay contents in source rocks.The carbon isotope values （δ¹³C₁ andδ¹³C₂） of natural gases in different regions differ greatly. The Ziyang Fm from the Cambrian system is the lightest with theδ¹³C₁ from -38.0 to -35.5‰. The number ofδ¹³C₁ in other regions is from -33.9 to -32.0‰. According to the research work, the divergence ofδ¹³C₁ reflects the different staged captures of natural gases which captured from early stage are relatively light. The carbon isotope values (δ¹³C₂) from Sinian and Cambrian system in Weiyuan is from -36.5 to -32.7‰;those from Longwangmiao Fm in Gaoshiti and Moxi is from -33.6 to -31.8‰. But it is -29.1~ -26.8‰ in the value ofδ¹³C₂ from Dengying Fm in Gaoshiti Moxi presenting the dramatic differences of theδ¹³C₂ from above areas which can be resulted from source rocks. Light hydrocarbon composition of C_6~C₇ mainly consists of cyclanes and isomerization alkanes which is characterized by crude oil cracking gas.