核磁共振芳碳率作为成熟度指标的有效性讨论

Discussion on the effectiveness of ^(13)C NMR aromaticity as a maturity indicator

石油地质学中,寻找油窗范围内较为有效的成熟度指标一直是油气地球化学家致力研究的科学问题之一。随着固体核磁共振技术在干酪根地球化学上的应用及发展,芳碳率被认为是具有潜力的有效成熟度指标。为对该问题进行深入探讨,对茂名盆地油柑窝组油页岩干酪根进行了高温高压黄金管热解实验,对生烃后的干酪根残渣进行了^(13)C DP/MAS核磁共振定量分析,并与美国绿河油页岩干酪根芳碳率的热演化特征进行对比;还调研了Ⅱ型及Ⅲ型干酪根芳碳率的热演化特征。结果显示,干酪根芳碳率均随成熟度的增加而增大,但不同类型干酪根的芳碳率的增长模式不同。未成熟阶段,芳碳率可以用来区分干酪根类型;生烃高峰之前,芳碳率随热成熟度增加而显著增加,可以用来指示成熟度,但与Ro关系要因类型而异;生烃高峰之后,不同干酪根的芳碳率演化趋于一致,若用来指示成熟度对NMR测试精度要求较高。若要得到精确的成熟度数据,最好建立研究区相关类型干酪根的芳碳率热演化曲线。

In petroleum geology, the determination of an effective maturity index for the oil window is a scientific issue that geochemists have committed to solve. As solid-state nuclear magnetic resonance techniques were developed and applied to kerogen studies, aromaticity was generally accepted as the most useful maturity proxy among the NMR structural parameters. In this investigation, artificial maturation of Maoming oil shale kerogen was performed in a closed gold tube system. The insoluble kerogen residue was analyzed using quantitative 13C DP/MAS NMR spectroscopy. The relationship between aromaticity and Easy%Ro for thermally matured Maoming oil shale kerogen was established, and compared with Green river oil shale kerogen. The thermal matured characteristics of kerogen types II and III were also cited from the reported references. No matter which type of kerogen was used, aromaticity increased with maturity, but the actual relationship differed. Aromaticity can be used to classify kerogen types during the immature stage. Before the oil window peak, the kerogen aromaticity could be used to indicate the kerogen maturity since there was an obvious increase in the aromaticity, along with the maturation for each of the kerogen types. However, aromaticity measurement errors could result in the wrong indication of the maturity. Therefore, it would be better to establish the evolution curve between aromaticity and maturity for the kerogen in the investigated region.