川西坳陷中段中、浅层天然气来源与碳同位素地球化学特征

川西坳陷中段是四川盆地主要的天然气产区之一，但目前对该区中、浅层侏罗系天然气的成因和 气源普遍存在争议。该文在对孝泉-新场-合兴场、洛带-新都和马井等气田56 个天然气样品的C₁-C₄ 碳同位素特征分析的基础上，结合天然气组分特征和烃源岩地球化学特征，分析了川西坳陷中段中、浅 层天然气的来源。研究表明：天然气碳同位素的总体分布显示出δ¹³C₁＜ δ¹³C₂＜ δ¹³C₃＜ δ¹³C₄ 的正序特征；碳同位素值在垂向上由深层须4 段至中、浅层沙溪庙组、遂宁组和蓬莱镇组直至下白垩统气藏，总体显示出 减小趋势。利用δ¹³C₂-δ¹³C₁判别天然气成熟度，表明由深至浅天然气的成熟度越来越高。中、浅层侏罗系 蓬莱镇组、遂宁组和沙溪庙组天然气表现出热裂解气的特点。研究区中、浅层天然气主要来自须5 段和 须4 段，部分来自侏罗系自流井组。

Origin and carbon isotope geochemistry characteristics of middle-shallow natural gas in the middle part of Western Sichuan Depression

The middle part of Western Sichuan Depression is one of the major gas-producing areas in Sichuan Basin. However, there still exist some different opinions on the genesis and origin of middle-shallow natural gas in this area. In order to get a better understanding of the issues, the authors selected 56 gas samples and analyzed their carbon isotope compositions of C1 -C4, which were collected from producing wells in Xiaoquan-Xinchang-Hexingchang, Luodai-Xindu and Majing gas fields, and analyzed the origin of the middle-shallow natural gas combining with the natural gas composition and the source rock geochemistry characteristics. The population distribution of the methane carbon isotopes indicates a normal pattern of δ13C1 δ13C2 δ13C3 δ13C4. And the carbon isotopes show an obvious decrease vertically from the deeper Xu 4 member, Shaximiao, Suining and Penglaizhen formations until the shallower Lower Cretaceous gas reservoirs, reflecting the origin and migration characteristics of the natural gas. By means of calculating differences between δ13C2 and δ13C1, it could be concluded that the gas maturity increases from deep to shallow. In addition, the paper illustrated that the natural gas of Shaximiao, Suining and Penglaizhen formations in the study area behaves a type of pyrolysis gas and are mainly from Xu 5 and Xu 4 members, partly from Ziliujing Formation.