四川盆地西部致密砂岩气来源及运移地球化学示踪

通过典型钻井天然气组分、轻烃、碳同位素以及烃源岩抽提轻烃的系统测试，研究了四川盆地西部致密砂岩气的成因类型和来源。研究表明，研究区致密砂岩气以煤型气为主，须二段（T₃x²）存在部分混合气。天然气来源具有下生上储的特征，其中须二段（T₃x²）天然气主要来源于须二段（T₃x²）自身的烃源岩以及下伏小塘子-马鞍塘组（T₃m+t）烃源岩，须四段（T₃x⁴）天然气主要来源于须三段（T₃x³）烃源岩，天然气运移距离短；而侏罗系天然气则主要来源于下伏须五段（T₃x⁵）烃源岩以及须家河组早期气藏的贡献，天然气运移距离长。在此基础上，优选了3组9类天然气运移地球化学指标，分别为：①天然气运移方向和距离指标为甲烷含量［W（CH₄）］、乙烷含量［W（C₂H₆）］、非烃含量［W（CO₂）］，［W（N₂）］、甲烷碳同位素（δ¹³C₁）；②天然气运移相态指标为苯/正己烷、苯/环己烷；③天然气运移通道指标为iC₄/nC₄和iC₅/nC₅。对研究区陆相天然气的运移进行了示踪，明确了研究区致密砂岩气的运移方向、运移相态以及运移通道。结果表明：研究区上侏罗统天然气主要由下部须家河组天然气沿高速优势通道窜层渗流运移而来，断裂是其最重要的运移通道；中侏罗统部分天然气由须家河组气源沿高速运移通道运移而来，而部分天然气为下部气源以水溶相的方式运移聚集成藏；须四段（T₃x⁴）天然气成藏时，储层相对致密，运移通道发育不佳，天然气以扩散相短距离运移为主；须二段（T₃x²）天然气主要通过断裂及其伴生裂缝以游离相和水溶相短距离运移为主。

Geochemical indicators for tracing the source and migration of the tight sandstone gas in western Sichuan Basin

The genetic tight sandstone gas types and sources in the western Sichuan Basin has been analyzed by systematic testing of the components of typical drilling natural gas, light hydrocarbon, carbon isotopes and the light hydrocarbon extracted from the source rock. The tight sandstone gas is mainly composed of coal-derived gas, and mixed gas only exists in the 2^nd Member of the Xujiahe Formation(T₃x²). The natural gas source is characterized by lower generation and upper storage. Among them, the natural gas in T₃x² is mainly derived from the self-generated source rock in T₃x² and Xiaotangzi Formation (T₃t). The 4^th Member of the Xujiahe Formation (T₃x⁴) natural gas is mainly derived from 3^rd Member of the Xujiahe Formation (T₃x³) source rock, and the migration distance of natural gas is relatively short. However, Jurassic natural gas is mainly derived from 5^th Member of the Xujiahe Formation (T₃x⁵) in the Upper Triassic, and resulting from the contribution of the early gas reservoirs in the Xujiahe Formation with a relatively long migration distance. In this paper, three groups of nine natural gas migration geochemical indicators were selected to trace the natural gas migration and the migration direction, phase and channel of tight sandstone gas in the study area have been determined. The geochemical indicator groups including: (1) Migration direction and distance indicators: methane content (W_{\mathrm{C}{\mathrm{H}}_{\mathrm{4}}}), ethane content (W_{{\mathrm{C}}_{\mathrm{2}}{\mathrm{H}}_{\mathrm{6}}}), non-hydrocarbon content (W_{\mathrm{C}{\mathrm{O}}_{\mathrm{2}}} and W_{{\mathrm{N}}_{\mathrm{2}}}) and methane carbon isotope (δ¹³C₁). (2) Migration phase state indicators: benzene/n-hexane, benzene/cyclohexane. (3) Migration channel indicators: iC₄/nC₄ and iC₅/nC₅. The results show that the Upper Jurassic natural gas was mainly migrated through the channel with a high velocity from the Xujiahe Formation (underlying strata), and the fault is the most important migration channel. Part of the Middle Jurassic natural gas was migrated from the Xujiahe Formation along the faults, while part of the lower natural gas was accumulated in the form of water-soluble phase. The reservoir was relatively tightness and the migration channel was not well developed in T₃x⁴, and the natural gas was mainly migrated with diffusion phase in short distance. The natural gas was migrated through faults and associated fractures with the free phase and the water-soluble phase in short distance.