Thermal maturation is traditionally evaluated based on vitrinite reflectance (VR_o) measurements and its relationship to oil and gas generation and diagenetic transformation are ingrained in many basin modeling tools. However, vitrinite derives from higher land plants that evolved in Devonian. In pre-Devonian rocks graptolite reflectance (GR) is the most significant thermal index for establishing thermal maturation. Currently, conversions of GR to VR_o equivalent rely on several established linear relationships. This study investigates a continuous thermal evolution of GR during artificial maturation of the Lower Ordovician (Tremadocian) Alum Shale of Estonia. We observe an anomalous breakdown in the gradient of GR versus thermal maturity. The anomaly trend is characterized as a suppressed GR gradient throughout the entire gas window (VR_o: 1.0–2.0%). We attribute the suppressed measured GR trend to surface imperfection caused by the generation and evasion of hydrocarbon gases that generate nano-porosity vacuolation of the graptolite tissues. After the gas window, GR resumes its increasing trend with a similar gradient as observed in the pre-gas window, due to continued aromatization and condensation of the organic molecules. The GR anomaly indicates a potential underestimation of thermal maturity up to 0.52%VR_o when applying a linear conversion formula between random GR and VR_o directly. Therefore, a significant maturity correction should be applied to all legacy GR-based maturity measurements that indicate a gas or post-gas maturity rank.

传统上基于镜质体反射率 (VR _o ) 测量来评估热成熟度，它与油气生成和成岩作用的关系在许多盆地建模工具中根深蒂固。然而，镜质体来源于泥盆纪进化的高等陆地植物。在前泥盆纪岩石中，笔石反射率 (GR) 是确定热成熟度的最重要的热指数。目前，将 GR 转换为 VR _o等效依赖于几个已建立的线性关系。本研究调查了爱沙尼亚下奥陶统（Tremadocian）明矾页岩人工成熟过程中 GR 的持续热演化。我们观察到 GR 与热成熟度梯度的异常分解。异常趋势的特征是整个气体窗口（VR _o: 1.0–2.0%)。我们将抑制的测量 GR 趋势归因于由碳氢化合物气体的产生和逃逸引起的表面缺陷，这些气体产生笔石组织的纳米孔隙空泡化。在气体窗口之后，由于有机分子的持续芳构化和冷凝，GR 以与前气体窗口中观察到的相似梯度恢复其增加趋势。当直接应用随机 GR 和 VR _o之间的线性转换公式时，GR 异常表明热成熟度可能被低估高达 0.52%VR _o。因此，应该对所有基于 GR 的传统成熟度测量值进行显着的成熟度校正，这些测量表明气体或气体后成熟度等级。