高、中、低氢指数芦草沟组干酪根生烃动力学差异性研究

Comparative analysis of kerogens hydrocarbon generation kinetics in the Lucaogou Formation source rocks with high,medium,and low hydrogen indices

芦草沟组油页岩在吉木萨尔盆地内广泛分布,具有很强的非均质性。为探究烃源岩非均质性对生烃动力学的影响,选取大龙口剖面芦草沟组高(724 mg/g)、中(555 mg/g)、低(219 mg/g)3个不同氢指数(HI)的干酪根样品,进行生烃动力学热模拟实验对比研究。结果表明:HI控制烃源岩的生烃潜力, HI越高,烃源岩的生烃能力越强,倾油性越强,生油气转化率越快,气油比(GOR)越低,生成的油芳香度越小,生成的气态烃干燥系数越小;HI越低,生烃能力越弱,倾气性越强,生油气转化率越慢,GOR越高,生成的油芳香度越大,生成的气态烃干燥系数越大。3个干酪根生油气动力学参数计算结果表明:HI越高,生油气活化能分布越集中;反之,生油气活化能分布越分散。生烃动力学参数反演生烃史(5℃/Ma)结果表明:HI越高,烃源岩生油量越大,排油越早,生气期越短;反之,生油量越小,排油越晚,生气期越长。根据烃源岩排油门限(100mg/g)和排气门限(20mg/g)研究,将芦草沟组干酪根生烃动力学结果应用于吉木萨尔凹陷烃源岩生烃评价,结果表明:吉木萨尔凹陷芦草沟组成熟度为低熟–生油窗阶段,大部分已达到排油门限但未至排气门限,具有很高的页岩油开采潜能,不具有天然气开采潜力。烃源岩非均质性对油气差异聚集和相态变化的启示为:HI高的烃源岩生烃占优势时产物以油相为主, HI低的烃源岩生烃占优势时产物以气相为主,二者搭配可形成“油溶气、气溶油”的运移模式;此外,在高演化阶段,低HI烃源岩的高产气能力可以对之前充注的油藏产生次生改造,形成次生凝析气藏。

Oil shales from the Lucaogou Formation are widespread in the Jimusar Sag,exhibiting significant heterogeneity.We conducted pyrolysis experiments on three kerogens isolated from source rocks with high(724 mg/g),medium(555 mg/g),and low(219 mg/g)hydrogen indices(HI)within confined gold tubes to investigate hydrocarbon generation kinetics.Results indicate that hydrocarbon generation potential is closely associated with HI,with larger HI values corresponding to higher hydrocarbon generation potentials.Such source rocks are predisposed to generate oil with more aliphatics and fewer aromatics,resulting in lower gas oil ratios(GOR)and dryness coefficients,and faster conversion of oil and gas generation.Conversely,lower HI values lead to lower hydrocarbon generation potentials,more gas generation,and oil with more aromatics and fewer aliphatics,resulting in higher GOR and dryness coefficients,and a slower conversion of oil and gas generation.Kinetic parameter calculations revealed that higher HI values correspond to a more concentrated activation energy distribution,while lower HI values exhibit a more dispersed distribution.Hydrocarbon generation histories were reconstructed under a natural geological heating rate of 5℃/Ma using kinetic parameters.The results indicate that higher HI values result in increased oil generation,earlier oil discharge,and shorter gas generation times.Conversely,lower HI values are associated with reduced oil generation,delayed oil discharge,and extended gas generation periods.Applying previously established hydrocarbon expulsion thresholds for oil(100 mg/g)and gas(20 mg/g),our calculated kinetic results were used to evaluate source rock hydrocarbon generation in the Jimusar Sag.Findings suggest that the Lucaogou Formation has reached the low maturity stage in the oil window,with most source rocks exceeding the oil expulsion threshold but not reaching the gas expulsion threshold.This implies a high prospecting potential for shale oil exploration without significant gas exploration potential.Differential hydrocarbon accumulation and phase change in strongly heterogeneous source rocks during hydrocarbon generation and evolution stages can be attributed to their distinct characteristics.Source rocks with high HI values predominantly yield oil-phase hydrocarbon products,while Source rocks with lower HI values favor gas-phase products.The combination of these characteristics result in the expulsion pattern of“oil dissolved in gas and gas dissolved in oil”.During the high thermal evolution stage,the high gas generation potential of low HI source rocks may exert a strong gas intrusion effect on preceding filled oil reserves,eventually leading to the formation of secondary condensate gas reservoirs.