Amorphous organic matter (AOM or bituminite) is a major oil-prone liptinite maceral in black shales that can be classified into two main types: low-reflectance AOM and micrinized AOM. In this study, we investigated the petrographic and chemical structure characteristics of these two types of AOM and discussed their origin based on 15 marine black shale samples from the Illinois Basin, including 5 samples from the Pennsylvanian shales and 10 samples from the Upper Devonian New Albany Shale. The studied samples are at early mature stage, with mean vitrinite reflectance ranging from 0.64 to 0.74%. Low-reflectance AOM is the dominant organic matter (OM) in all samples and micrinized AOM is common only in the Pennsylvanian strata, but very rare in the New Albany Shale. There are significant differences in reflectance, petrographic characteristics, and chemical structure characteristics between low-reflectance and micrinized AOM. Low-reflectance AOM has a reflectance of 0.30 to 0.57%, whereas the reflectance of micrinized AOM is typically >1.0%, which makes micrinized AOM brighter than low-reflectance AOM under an optical microscope. Both types of AOM have heterogeneous texture, occur as structureless organic streaks or lenses, and have abundant clay-sized and very fine silt-sized mineral inclusions (< 7.8 μm). Compared to micrinized AOM, low-reflectance AOM has lower aromaticity and higher aliphatic chain length (inferred from the CH₂/CH₃ ratio). Based on the petrographic and chemical functional groups characteristics, we proposed evolutionary pathways for the formation of low-reflectance and micrinized AOM in marine black shales during early diagenesis. AOM is originally derived from microbially degraded phytoplankton. Low-reflectance AOM formed in the fermentation zone through microbial degradation of phytoplankton by anaerobic bacteria. In comparison, micrinized AOM is a product of an advanced degree of microbial degradation of phytoplankton by aerobic bacteria that preferentially consume aliphatic functional groups over aromatics in the oxidation zone. Different depositional environments and subsequent diagenetic alterations resulted in the difference in micrinized AOM content between the New Albany Shale and Pennsylvanian shales. This study provides a better understanding of the origin, formation, and thermal maturation of dispersed AOM in black shale successions.

非晶质有机质（AOM或沥青质）是黑色页岩中主要的易油性锂质辉石岩，可分为两种主要类型：低反射率AOM和微粉化AOM。在这项研究中，我们调查了这两种类型的AOM的岩石学和化学结构特征，并基于来自伊利诺伊盆地的15个海洋黑色页岩样本（包括宾夕法尼亚州的5个样本和上泥盆纪的新奥尔巴尼的10个样本）讨论了它们的成因。页岩。所研究的样品处于早期成熟阶段，平均镜质体反射率范围为0.64至0.74％。低反射AOM是所有样品中的主要有机物（OM），微粉化AOM仅在宾夕法尼亚州地层中很常见，而在新奥尔巴尼页岩中非常罕见。反射率，岩石学特征，反射率和微粉化AOM之间的化学结构特征 低反射AOM的反射率为0.30至0.57％，而微粒化AOM的反射率通常> 1.0％，这使得在光学显微镜下，微粒化AOM比低反射AOM亮。两种类型的AOM均具有不均匀的质地，以无结构的有机条纹或晶状体形式出现，并且具有丰富的粘土大小和非常细的粉砂大小的矿物包裹体（<7.8μm）。与微粉化AOM相比，低反射AOM具有较低的芳香性和较高的脂族链长（从CH推断 在光学显微镜下，使微粉化的AOM比低反射AOM明亮。两种类型的AOM均具有不均匀的质地，以无结构的有机条纹或晶状体形式出现，并且具有丰富的粘土大小和非常细的粉砂大小的矿物包裹体（<7.8μm）。与微粉化AOM相比，低反射AOM具有较低的芳香性和较高的脂族链长（从CH推断 在光学显微镜下，使微粉化的AOM比低反射AOM明亮。两种类型的AOM均具有不均匀的质地，以无结构的有机条纹或晶状体形式出现，并且具有丰富的粘土大小和非常细的粉砂大小的矿物包裹体（<7.8μm）。与微粉化AOM相比，低反射AOM具有较低的芳香性和较高的脂族链长（从CH推断₂ /通道₃比）。基于岩石学和化学官能团的特征，我们提出了在成岩作用早期海洋黑色页岩中低反射率和微粉化AOM形成的演化途径。AOM最初源自微生物降解的浮游植物。通过厌氧细菌对浮游植物的微生物降解，在发酵区形成低反射AOM。相比之下，微细化的AOM是好氧细菌对浮游植物的微生物高级降解作用的产物，好氧细菌优先消耗脂肪族官能团而不是氧化区中的芳香族官能团。不同的沉积环境和随后的成岩作用改变导致了新奥尔巴尼页岩和宾夕法尼亚州页岩的微粉化AOM含量不同。