Abstract Thermal maturity of source rocks indicated by vitrinite reflectance (Ro) is an important and reliable parameter to determine the petroleum potential of sedimentary basins. Originally, Ro was used to characterize the degree of coalification of coals and thus works well for coal seams, but it has limitations when used to determine the thermal maturity of black shale successions. A suite of six coal-shale pairs from the Upper Devonian black shale formations in the eastern United States (New Albany Shale, Ohio Shale, Dunkirk Shale, and Rhinestreet Shale) was selected to study the applicability of Ro in assessing the thermal maturity of black shale successions. The results show that vitrinite in the studied coal samples is dominated by collotelinite, whereas vitrinite in black shales occurs as small dispersed particles (~5 μm) in the mineral matrix. When comparing the size and morphology of dispersed vitrinite particles in shales and collotelinite fragments in coals, vitrinite in shales and adjacent coals should have the same origin. The measured mean random Ro of vitrinite in coals ranges from 0.51–0.68%, and is 0.04–0.11% (average 0.07%) lower than that of dispersed vitrinite particles in enclosing shales. This observation contrasts with previously reported Ro suppression in liptinite-rich black shales. A high standard deviation of Ro measurements indicates a highly heterogeneous nature of vitrinite in black shales. Three mechanisms can be envisioned to contribute to the higher Ro of dispersed vitrinite particles in black shales relative to enclosed coals. First, small vitrinite particles may become more oxidized because small particles are more frequently suspended during transport to the site of deposition and reworking of bottom sediments after deposition than large pieces of driftwood that will turn into enclosed coal lenses during burial diagenesis. Second, misidentification of zooclast (e.g., chitinozoan) fragments as vitrinite due to loss of diagnostic morphology. Third, various macerals in the vitrinite group (e.g., corpogelinite and collotelinite) may have had different original reflectance values. Based on the petrographic characteristics of vitrinite and vitrinite-like particles, caution should be applied when using the Ro values of dispersed vitrinite and vitrinite-like particles in black shales as indicators of thermal maturity.

中文翻译：

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使用镜质体反射评估黑色页岩的热成熟度：来自美国东部泥盆纪黑色页岩的见解

摘要 镜质体反射率（Ro）指示的烃源岩热成熟度是确定沉积盆地石油潜力的重要可靠参数。最初，Ro 用于表征煤的煤化程度，因此适用于煤层，但在用于确定黑色页岩层序的热成熟度时存在局限性。来自美国东部上泥盆世黑色页岩地层（新奥尔巴尼页岩、俄亥俄页岩、敦刻尔克页岩和莱茵斯特里特页岩）的一组六个煤-页岩对被选择来研究 Ro 在评估黑色页岩热成熟度方面的适用性。页岩层序。结果表明，所研究煤样中镜质体以胶块体为主，而黑色页岩中的镜质体在矿物基质中以分散的小颗粒 (~5 μm) 的形式出现。在比较页岩中分散的镜质体颗粒和煤中的镜质体碎片的大小和形态时，页岩和相邻煤中的镜质体应该具有相同的来源。测得的煤中镜质体的平均随机 Ro 范围为 0.51-0.68%，比封闭页岩中分散的镜质体颗粒低 0.04-0.11%（平均 0.07%）。这一观察结果与先前报道的富含脂铁矿的黑色页岩中的 Ro 抑制形成对比。Ro 测量值的高标准偏差表明黑色页岩中镜质体的高度异质性。可以设想三种机制导致黑色页岩中分散的镜质颗粒相对于封闭煤具有更高的 Ro。第一的，小镜质颗粒可能会变得更加氧化，因为小颗粒在运输到沉积地点和沉积后重新加工底部沉积物的过程中更频繁地悬浮，而不是在埋藏成岩过程中将变成封闭的煤透镜的大块浮木。其次，由于失去诊断形态，动物碎屑（例如几丁质动物）碎片被误认为是镜质体。第三，镜质组中的各种微晶（例如，corpogelinite 和collotelinite）可能具有不同的原始反射率值。基于镜质体和类镜质颗粒的岩相特征，使用黑色页岩中分散的镜质体和类镜质颗粒的 Ro 值作为热成熟度的指标时应谨慎。