渝东南地区五峰组—龙马溪组页岩气地球化学特征及其成因分析

Geochemical Features and Genesis of Shale Gas of Wufeng-Longmaxi Formation in Southeastern Chongqing

在对渝东南地区五峰组—龙马溪组页岩现场解吸气样的气体组分和稳定碳同位素分析的基础上,对页岩气的成因类型和烷烃碳同位素倒转原因进行了探讨。渝东南地区五峰组—龙马溪组页岩气的甲烷含量高(94.33%~98.96%),非烃组分(主要包括N_2、CO_2)含量较少,干燥系数大于0.98,为典型的干气。甲烷、乙烷碳同位素值的范围分别为-49‰^-24.4‰、-39.4‰^-29‰,含气性较好的武隆地区Y1井气样烷烃气体呈现δ^(13)C_1>δ^(13)C_2>δ^(13)C_3的碳同位素"完全倒转"特征,含气性较差的酉阳地区Y2井气样烷烃气体基本具正碳同位素系列特征。天然气成因类型判识标志和图版分析表明,渝东南地区五峰组—龙马溪组页岩气为有机质高温裂解的油型气,是早期生成的干酪根裂解气和后期原油裂解气的混合气,这也是引起武隆地区Y1井气样碳同位素倒转的主要原因,同时这也可能与页岩气高产有关。

Wufeng Formation-Longmaxi Formation is an important sequence in Sichuan and Chongqing area for shale gas exploration.Based on the analysis of chemical component and stable carbon isotope composition of the shale gas acquired by desoption experiments on Wufeng-Longmaxi Formation shale cores from the Souteastern Chongqing,it is discussed for the genetic type of shale gas and the reason for alkane carbon isotopic reversal.The results showed that the methane content is high with a value from94.33%to98.96%in Longmaxi Formation shale gas,and the non-hydrocarbon content is low,mainly including N2and CO2.The drying coefficient is more than0.98,indicating typical dry gas.The values of carbon isotopes of methane and ethane are-49‰^-24.47‰and-39.4‰^-29‰respectively,and the carbon isotopes of C1to C3alkane in Well Y1with good gas-bearing presents complete reversal,but Well Y2presents normal.According to the statistical criteria,natural gas in the Southeastern Chongqing area is derived from the sapropelic source rocks,which is a result of high-temperature pyrolysis.It is a mixing product of pyrolysis of kerogens and secondary pyrolysis of crude oil,with obvious features of secondary pyrolysis of oil.The reason for complete reversal of carbon isotopic in Well Y1is the mixing of two gases.Perhaps,it has some relationships with high production of shale gas.