川西坳陷叠覆型油气区天然气地球化学特征与成因机制

Geochemical characteristics and genesis mechanism of natural gas in overlapped oil and gas area of Western Sichuan Depression

川西坳陷陆相领域气藏分布具有叠合性和广覆性特点,天然气地球化学特征差异明显。基于川西坳陷上三叠统须家河组及侏罗系沙溪庙组、遂宁组、蓬莱镇组1052个天然气样品,研究天然气组分、天然气轻烃和碳同位素组成,明确天然气地球化学特征分布规律,分析气源对比和运移条件,揭示川西坳陷叠覆型油气区复杂天然气藏成因机制。结果表明:川西坳陷叠覆型油气区天然气成分以甲烷为主,质量分数为65.36%~98.47%,甲烷碳同位素(δ13C 1)分布在-39.41‰~-27.10‰之间。由深层上三叠统须家河组二段(须二段)、须家河组四段(须四段)向须家河组五段(须五段),天然气甲烷质量分数降低,甲烷碳同位素减轻;自须五段向中浅层侏罗系天然气甲烷质量分数逐渐升高,甲烷碳同位素逐渐增重。研究区主要为热解成因气,其中深层须四段、须五段和中浅层侏罗系天然气为煤型气,深层须二段天然气是煤型气和油型气的混合类型。中浅层天然气主要来自须五段和须四段,下侏罗统在部分地区有贡献,在马井—什邡地区受断层沟通作用影响,须三段也有一定贡献。须四段和须二段天然气主要来自须三段、马鞍塘组和小塘子组。须五段天然气主要为自源形成。天然气地球化学特征的差异主要受母质来源和母质成熟度差异、同一气源运移过程中分馏效应及天然气藏属性差异等影响。该结果为川西坳陷叠覆型油气区致密砂岩气成藏勘探部署提供指导。

The overlapping and extensive distribution of continental tight gas reservoirs in the Western Sichuan Depression leads to obvious differences in the geochemical characteristics of natural gas.The natural gas components,natural gas light hydrocarbons and carbon isotopes of 1052 natural gas samples from the Upper Triassic Xujiahe Formation,Jurassic Shaximiao Formation,Suining Formation and Penglaizhen Formation in the Western Sichuan Depression were systematically analyzed.On this basis,the regularity of the distribution of natural gas geochemical characteristics is summarized.Combining the relationship betweenδ13C 1 andRo,the gas source correlation and migration conditions are analyzed,and the genetic mechanism of the formation of complex natural gas reservoirs in the superimposed oil and gas Western Sichuan Depression is revealed.The results show that in the superimposed oil and gas area of the Western Sichuan Depression,the natural gas composition is dominated by methane,the methane content is 65.36%98.47%,and the methane carbon isotope(δ13C 1)value is distributed between-39.41‰and-27.10‰.The methane content and methane carbon isotope decreased from the second and fourth member of Xujiahe Formation to the fifth member of Xujiahe Formation of deep Upper Triassic,and methane content and methane carbon isotope ratio gradually increase from the fifth member of Xujiahe Formation to the middle-shallow Jurassic Strata.The natural gas in the study area is mainly pyrolysis gas,among them,the fourth and fifth member of Xujiahe Formation in deep layer and the middle-shallow layers contain coal-type gas,and the natural gas of the second member of Xujiahe Formation is the mixed type of coal-type gas and oil-type gas.The gas sources in the middle-shallow layers are mainly from the fifth and fourth member of Xujiahe Formation,the Lower Jurassic have contributed in some areas,and due to the communication of faults,the third member of Xujiahe Formation also have some contribution in Majing-Shifang Area.Natural gas in the fourth and second member of Xujiahe Formation mainly comes from the third member of Xujiahe Formation and Ma'antang-Xiaotangzi Formation.The natural gas in fifth member of Xujiahe Formation is mainly self-generated self-storage.The difference of geochemical characteristics of natural gas is mainly affected by the difference of parent material source and maturity,the fractionation effect in the migration process of the same gas source and the difference of natural gas reservoir properties.The results provide guidance for deepening the theory of tight sandstone gas formation and exploration deployment in the overlying oil and gas region of the Western Sichuan Depression.