四川盆地中侏罗统沙溪庙组天然气地球化学特征及成因

四川盆地中侏罗统沙溪庙组是低油价形势下四川盆地天然气勘探的重要领域，但盆地内不同地区天然气来源尚不明确，影响下一步勘探部署决策，为此开展侏罗系沙溪庙组天然气地球化学特征及成因研究。结果表明：①沙溪庙组天然气属于干酪根降解气，甲烷含量>84%，含少量乙烷、丙烷等烃类气体及少量的氮气、二氧化碳等非烃气体，不含硫化氢，不同区域的天然气成熟度存在差别；②天然气δ¹³C₁值为-39.2‰~-31.2‰、δ¹³C₂值为-32.8‰~-22.3‰、δ¹³C₃值为 -28.7‰~-19.5‰，天然气碳同位素未发生倒转，川西地区为煤成气，川中地区为油型气，川东地区为煤成气和油型气混合气，以油型气为主；③不同区域天然气δ¹³C₁值、δ¹³C₂值的差异，与其来源于不同类型烃源岩贡献比例大小有关。川西、川西南地区主力烃源岩为须五段煤系烃源岩，川中地区为下侏罗统湖相烃源岩，川东地区天然气来源于须五段和下侏罗统烃源岩。研究结果对四川盆地侏罗系沙溪庙组下一步天然气勘探部署决策具有重要的指导意义。

Geochemical characteristics and genesis of natural gas in Shaximiao Formation of Middle Jurassic in Sichuan Basin

Shaximiao Formation of Middle Jurassic in Sichuan Basin is an important field of natural gas exploration in Sichuan Basin under the situation of low oil price. However， the source of natural gas in different areas of the basin is not clear， which affects the next exploration deployment decision. The geochemical characteristics and genesis of natural gas in Jurassic Shaximiao Formation are studied. The results show that： （1） The natural gas of Shaximiao Formation belongs to kerogen degradation gas， with methane content more than 84%， containing a small amount of ethane， propane and other hydrocarbon gases and a small amount of nitrogen， carbon dioxide， and other non-hydrocarbon gases， without hydrogen sulfide. The maturity of natural gas in different regions is different. （2） The natural gas δ¹³C₁ values are from -39.2‰ to -31.2‰， δ¹³C₂ values are from -32.8‰ to -22.3‰， and δ¹³C₃ values are from -28.7‰ to -19.5‰. The carbon isotope of natural gas is not reversed. The west of Sichuan is dominated by coal-derived gas， central Sichuan is characterized by oil-type gas， and eastern Sichuan is dominated by mixed gas between coal-derived gas and oil-type gas， mainly oil-type gas. （3） The difference of δ¹³C₁ and δ¹³C₂ values in different regions is related to the contribution ratio of different source rocks. The main source rocks in western Sichuan Basin and southwest Sichuan Basin are the coal bearing source rocks of the 5^th Member of Xujiahe Formation， the Lower Jurassic lacustrine source rocks in central Sichuan Basin， and the source rocks of natural gas in eastern Sichuan Basin are from the 5^th Member of Xujiahe Formation and Lower Jurassic. The research results have important guiding significance for the next natural gas exploration and deployment decision-making of Jurassic Shaximiao Formation in Sichuan Basin.