川西坳陷高成熟煤系烃源岩分子成熟度参数的异常分布

The Abnormal Distribution of Molecular Maturity Parameters of Coal Measure Source Rocks with High Maturity in West Sichuan Depression

川西坳陷上三叠统及下侏罗统高成熟煤系烃源岩分子成熟度参数具有明显的"逆转"特征。其演化模式表现为"两段式"的特点,上部层位(J1z和T3x5)和下部层位(T3x3和T3t)烃源岩成熟度参数的变化规律具有明显差异,同时部分参数随深度的增加逐渐变小,表现出较为明显的"逆转"现象。上部层位(J1z和T3x5)和下部层位(T3x3和T3t)沉积环境(如盐度、pH值、Eh值等的变化)的差异应是其分子成熟度参数出现"两段式"演化特征的重要因素;差向异构体热稳定性差异是造成部分分子成熟度参数"逆转"的主要原因;煤系地层自身特点可能也是导致其"逆转"的原因之一。不同的分子成熟度参数具有不同的热反应过程,且其对环境的敏感程度不一,因此导致不同参数具有不同的演化趋势。这一结论表明,在利用分子成熟度参数进行油源对比及成熟度评价时,沉积环境及源岩自身的热演化程度是必须考虑的重要因素,这对煤系地层的油气勘探具有一定的指导意义。

The high molecular maturity parameters of source rocks in the Upper Triassic and Lower Jurassic of West Sichuan Depression presented the character of obvious "reversion".Its evolutional mode had the feature of "two intervals",the changing rules of maturity parameters of source rocks had obvious differences between upper formation(J1z and T3x5) and lower formation(T3x3 and T3t),and some maturity parameter decreases with the increase of depth which appear the "reversion".The differences of sedimentary environment(such as salinity,Ph,Eh) between upper formation(J1z and T3x5) and lower formation(T3x3 and T3t)are dominant factors for molecular maturity parameter with appearance of "two intervals",the difference of heat stability of isomers was the main reason causing the "reversion" for some of the molecule maturity parameters,and the self-feature of coal measure strata might be one of factors causing its "reversion".Different molecular maturity parameters presented different thermal reaction processes,and it had the different sensibilities to environment.So it caused the different parameters with different evolutionary trends.The result indicates that the sedimentary environment and the thermal evolution degree of source rocks are important factors,when the molecular maturity parameters are used to make the comparison of oil source and the evaluation of the maturity.It provides a guidance for coal measure strata exploration.