四川盆地五峰组-龙马溪组页岩气成因与碳同位素倒转机制--来自热模拟实验的认识

Origin and carbon isotope reversal of shale gas in Wufeng-Longmaxi formations, Sichuan Basin: implication from pyrolysis experiments

四川盆地上奥陶统五峰组-下志留统龙马溪组是国内目前唯一实现页岩气商业性开发的层系,但目前对页岩气成因仍有一定分歧,主要原因是缺乏直接的实验证据。通过对国内外上奥陶统-下志留统低、中成熟度页岩和笔石开展黄金管生烃热模拟实验研究发现:(1)五峰组-龙马溪组页岩气主体以浮游藻类等富氢、富脂质有机质生成的油进一步滞留裂解为主,笔石生油能力较差,高-过成熟演化阶段有一定的生气能力,最高可达浮游藻类等富氢、富脂质有机质生气能力的20%左右;(2)五峰组-龙马溪组页岩下部层位由于富集浮游藻类、疑源类等富氢、富脂质有机质等生烃能力更强的有机质,从生气量上就优于上部以笔石为主的层位,这是下部为商业性页岩气层的一个重要原因;(3)单纯的热演化分馏、原油裂解气和干酪根裂解气的混合未造成烃气碳同位素倒转,地层抬升作用、地层水、矿物、金属等对页岩气中烃类的后期改造可能是造成页岩气同位素倒转的重要原因,碳同位素倒转可能更多体现了页岩气后期保存过程的影响。

The Wufeng-Longmaxi shale in the Sichuan Basin is the only stratum in China to realize commercial development of shale gas. There are some differences in shale gas generation and a lack of direct experimental evidence of the reasons for these differences. Hydrocarbon generation experiments using pyrolysis in a gold tube with low and medium maturity shale and graptolite of O3-S1 were carried out. Gas from remaining oil produced by hydrogen-rich and lipid-rich organic matter such as planktonic algae is the main contributor of shale gas in the Wufeng-Longmaxi formations. The oil generation ability of graptolite is poor, and it can produce gas during the high-maturity and over-mature stages, up to about 20% of the hydrogen-rich and lipid-rich organic matter such as algae. The lower layer of the Wufeng-Longmaxi shale in the Sichuan Basin is superior to the upper layer, which is mainly composed of graptolite, because of abundant hydrogen-rich and lipid-rich organic matter such as phytoplankton. Pure thermal evolution fractionation and the mixture of primary(from kerogen) and secondary(from oil) gas do not cause hydrocarbon gas isotopes to "reverse". Formation uplift,formation water,minerals,metals and other late transformation of hydrocarbons in shale gas may be an important cause of shale gas isotopic "inversion". Carbon isotope "inversion"may be more of a reflection of the later shale gas preservation process.