轻烃地球化学特征对天然气来源和运移相态的约束——以四川盆地西部新场气田侏罗系气藏为例

Constraints of light hydrocarbon geochemistry on the source and migration phase of natural gas:A case study of Jurassic gas reservoir in Xinchang Gas Field,western Sichuan Basin

新场气田侏罗系是四川盆地川西坳陷天然气稳产的重要产层之一,但是新场气田侏罗系天然气来源和成藏特征的认识存在较大争议。为了更有效地揭示川西坳陷侏罗系天然气成藏过程和富集机理,在分析新场气田不同层位天然气轻烃化合物组成的基础上,探讨了轻烃参数判识成熟度的适用性,并揭示了轻烃地球化学特征对成烃成藏的指示意义。研究结果表明:(1)新场气田侏罗系天然气C_(5)—C_(7)轻烃组成中异构烷烃相对正构烷烃呈优势分布,C_(7)轻烃组成中甲基环己烷呈明显的优势分布,甲基环己烷指数均大于60%,为典型的煤成气;C_(6)—C_(7)轻烃组成中芳烃含量明显低于链烷烃和环烷烃。(2)上侏罗统蓬莱镇组气藏主要为上三叠统须家河组三段烃源岩生成的天然气以游离相运移聚集形成,中侏罗统沙溪庙组气藏与部分蓬莱镇组气藏主要为须家河组五段烃源岩生成的天然气以水溶相和游离相运移聚集形成。(3)蓬莱镇组气藏天然气C_(6)—C_(7)轻烃组成中芳烃含量明显偏低,异庚烷值明显偏高,主要源自不同类型化合物在溶解度、极性等方面的差异影响。结论认为,天然气在运移聚集过程中受水溶、吸附等作用的影响,运用C_(6)—C_(7)轻烃组成、异庚烷值分别进行天然气成因类型和成熟度判识时需特别谨慎。

The Jurassic in the Xinchang Gas Field is one of the important producing formations of stable gas production in the Western Sichuan Depression of the Sichuan Basin,but its gas sources and accumulation characteristics are still controversial.In order to reveal more effectively the accumulation processes and enrichment mechanisms of the Jurassic gas in the Western Sichuan Depression,this paper analyzes the light hydrocarbon compositions of gas from different horizons in the Xinchang Gas Field.On this basis,the applicability of light hydrocarbon parameters to maturity discrimination is discussed,and the implication of geochemical characteristics of light hydrocarbons for hydrocarbon generation and accumulation is clarified.The following research results are obtained.First,the C_(5)-C_(7) light hydrocarbons of Jurassic gas in the Xinchang Gas Field are dominated by isoalkanes instead of n-alkanes,and the C_(7) light hydrocarbon is obviously dominated by the methylcyclohexane(MCH)with the MCH index higher than 60%,suggesting that it is typical coal-derived gas.The content of aromatics is significantly lower than that of paraffins and cycloalkanes in the C_(6)-C_(7) light hydrocarbons.Second,the gas reservoir in the Upper Jurassic Penglaizhen Formation(J_(3)p)is mainly the accumulation of gas derived from the source rocks in the third member of Xujiahe Formation of Upper Triassic(T_(3)x^(3))and migrates in free phase,whereas the gas reservoir in the Middle Jurassic Shaximiao Formation(J_(2)s)and partial gas reservoir in the J_(3)p are the accumulation of gas derived from the source rocks in the fifth member of Xujiahe Formation(T_(3)x^(5))and migrates in both free phase and water soluble phase.Third,the C_(6)-C_(7) light hydrocarbons in the J_(3)p gas displays significantly lower aromatic content and higher isoheptane values,for different types of compounds are different in solubility and polarity.In conclusion,natural gas is influenced by water dissolution and adsorption in the process of migration and accumulation,so it is necessary to take special cautions when discriminating the origin type and maturity of gas by using C_(6)-C_(7) light hydrocarbons and isoheptane values respectively.