四川盆地中二叠统天然气地球化学特征及成因判识

Natural gas geochemical characteristics and genetic type identification of Middle Permian in Sichuan Basin

 董才源 谢增业 裴森奇 张璐 杨春龙 尹瀚翔

中国石油勘探开发研究院，河北 廊坊 065007 中国石油天然气集团公司天然气成藏与开发重点实验室，河北 廊坊 065007 中国石油西南油气田分公司川西北气矿，四川 江油 621700

 四川盆地中二叠统是继震旦—下古生界之后的又一勘探热点领域，但对其天然气地球化学特征及成因类型尚不清楚。为此，通过剖析最新气样天然气组分、烷烃同位素等地球化学参数，结合前人研究成果，总结四川盆地中二叠统天然气地球化学特征，论证天然气成因类型。研究结果表明：1）四川盆地中二叠统天然气为典型干气。以烃类气体为主，CH4体积分数高，C2H6，C3H8体积分数低，干燥系数大；非烃气体体积分数低，含微量N2、中低量CO2，中低质量浓度的H2S。2）四川盆地中二叠统天然气甲、乙碳同位素大多发生倒转。δ13C2分布范围较广，为-36.7‰～-25.2‰，δ13C1相对较重，介于-35.6‰～-27.7‰；δ2H1主要分布于-141‰～-125‰。3）多数天然气δ13C2轻于-29.0‰，甲基环己烷/正庚烷和（2￣甲基己烷+3￣甲基己烷）/正己烷2项比值分别为大于1.0和0.5，ln（C1/C2）值介于6.19～7.87，ln（C2/C3）值介于3.00～4.76，碳同位素、轻烃、组分特征综合判识中二叠统天然气类型为原油裂解气。

 The Middle Permian in Sichuan Basin is another important exploration field after the Sinian?鄄Lower Paleozoic, but its natural gas geochemical characteristics and genetic types are not clear. To achieve this purpose, by analyzing natural gas component and alkane isotope of the latest gas samples, combined with the previous research results, the geochemical characteristics of the Middle Permian natural gas in the Sichuan Basin are summarized and the natural gas genesis type is demonstrated. The results show that: （1）the natural gas of Lower Permian is typical dry gas composited mainly in hydrocarbon with high CH4, low C2H6 and C3H8, big dry coefficient, little N2, middle-low CO2 and H2S. （2）the isotopes of CH4 and C2H6 reverses in most gas samples; the isotope value range of C2H6 is mainly distributed in -36.7‰￣-25.2‰, while the isotope value of CH4 is relatively heavy from -35.6‰ to -27.7‰, and δ2H1 from -141‰ to -125‰. （3）the C2H6 isotope of most nature gas is lighter than -29‰; methyl cyclohexane/n-heptane and (2￣methylhexane+3￣methylhexane) / n-hexane are heavier than 1.0 and 0.5 respectively; the ln(C1/C2) value of nature gas in Middle Permian locates in 6.19￣7.87 and ln(C2/C3) value locates in 3.00￣4.76. The above three evidence suggests that the genetic type of natural gas is oil cracking gas.

10.6056/dkyqt201804009