川东焦石坝地区页岩气特征及其意义

Geochemical Characteristics and Implications of Shale Gas in Jiaoshiba,Eastern Sichuan,China

焦石坝地区五峰组-龙马溪组页岩富集有大量的天然气,但针对页岩气地球化学特征研究还较薄弱,其蕴含的地质意义不甚明确.通过页岩气组分及其碳同位素特征和页岩干酪根的碳同位素特征分析,讨论了页岩气的来源、成因类型和"完全倒转"的碳同位素分布特征.五峰组-龙马溪组页岩具有很好的生烃能力、有机质丰度与含气量的关系有明显的正相关性、甲烷与干酪根相似的碳同位素特征、地层的超压特征等,综合表明研究区天然气应为页岩自生自储的页岩气;页岩气的甲烷含量均超过98%,其碳同位素平均为-29.93‰,反映了成熟度已经达到过成熟干气阶段;相对稳定的ln(C_1/C_2)和快速增大的ln(C_2/C_3)揭示其成因主要为二次裂解气;页岩气碳同位素"完全倒转"的分布特征主要受到在相对封闭环境中的原油裂解生气作用的影响,其"完全倒转"的碳同位素分布特征也反映了研究区良好的页岩气保存、富集条件.

The Wufeng-Longmaxi shales in Jiaoshiba is rich in natural gas.However,the research on the geochemical characteristic of the shale gas is weak,and its geological significance is still unclear.This article discusses the origin type and the＂full reversal＂carbon isotopic composition of shale gas from Wufeng Formation-Longmaxi Formation based on the analysis of the shale gas component and its isotope.The high hydrocarbon generation potential of Wufeng-Longmaxi shales,the positive correlation of the abundance of organic carbon and the content of gas,the similar carbon isotope distribution between methane and kerogen and the overpressure in Wufeng Formation-Longmaxi Formation suggest the shale gas should be generated from and preserved in the Wufeng Formation-Longmaxi Formation.The content of methane and its isotope indicate the stage of thermal maturity for shale gas has reached the overmature phase and the shale gas is mainly dry gas.The shale gas is mainly formed from the cracking of oil generated early based on the ratio of ln（C_1/C_2） and ln（C_2/C_3）.The ＂full reversal＂ carbon isotope distribution of shale gas resulted from the oil cracking in a relatively closed system and it also reflects a favorable preservation and accumulation conditions for shale gas.