Wang Haidong, Su Long, Qin Zhen, Gao Zhanbin, Zhao Zhongtang, Ma Fuhai, Peng Xiaofei, Ma Yonghui, Li Conglin, Yang Jun, Han Shaojia
Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences, Lanzhou 730000, China.
University of Chinese Academy of Sciences, Beijing 100049, China.
ACS Omega. 2025 Aug 5;10(32):35497-35511. doi: 10.1021/acsomega.4c11593. eCollection 2025 Aug 19.
The Middle Jurassic coal seams in the Beishan region represent an important target for hydrocarbon exploration; however, research on the associated coal-measure source rocks remains limited. The coupling between depositional paleoenvironment and hydrocarbon generation potential among various coal-measure lithologies (including coal, associated dark mudstone, and carbonaceous mudstone) within the same or similar sedimentary facies belts is still poorly understood. In this study, the paleoenvironment was analyzed using major and trace element geochemistry to assess its influence on source rock development. Subsequently, organic geochemical methods were employed to evaluate the hydrocarbon generation potential of the source rocks. The results indicate the following: (1) Major and trace element data show that the coal-measure source rocks are enriched in CaO, Sr, and U, with Sr contents decreasing in the order of dark mudstone > coal > carbonaceous mudstone. (2) The Middle Jurassic lacustrine basin experienced cyclic water-depth fluctuations (shallow-deep-shallow), with paleosalinity dominated by saline to brackish conditions, a warm and humid paleoclimate, and predominantly reducing paleo-redox conditions. (3) Organic geochemical analyses reveal that dark and carbonaceous mudstones have higher organic matter abundances than coal. The organic matter is primarily Type II kerogen, with minor contributions from Type I and Type III. Vitrinite reflectance (Ro) values range from 0.54% to 0.84%, and the average value is 437 °C, indicating a low-maturity to early mature thermal evolution stage. These findings demonstrate that, within the same coal-measure source rock system, lacustrine-swamp facies dark and carbonaceous mudstones interbedded with coal represent high-quality hydrocarbon source rocks. Their enhanced hydrocarbon generation potential is attributed to elevated paleosalinity, which promotes the preservation of hydrogen-rich macerals such as desmocollinite. Currently at a low to early mature stage of thermal evolution (Ro: 0.54-0.84%), these source rocks possess significant potential for generating low-maturity oil and gas. Notably, their hydrocarbon generation potential surpasses that of the coal layers, offering critical theoretical support for Middle Jurassic coal-measure source rock exploration in the Beishan region.
北山地区中侏罗统煤层是油气勘探的重要目标；然而，对相关煤系烃源岩的研究仍然有限。同一或相似沉积相带内各种煤系岩性（包括煤、伴生暗色泥岩和碳质泥岩）的沉积古环境与烃源潜力之间的耦合关系仍不清楚。在本研究中，利用主微量元素地球化学分析古环境，以评估其对烃源岩发育的影响。随后，采用有机地球化学方法评估烃源岩的生烃潜力。结果表明：（1）主微量元素数据显示，煤系烃源岩富含CaO、Sr和U，Sr含量顺序为暗色泥岩>煤>碳质泥岩。（2）中侏罗世湖相盆地经历了周期性的水深波动（浅-深-浅），古盐度以咸水至微咸水为主，古气候温暖湿润，古氧化还原条件以还原为主。（3）有机地球化学分析表明，暗色和碳质泥岩的有机质丰度高于煤。有机质主要为Ⅱ型干酪根，Ⅰ型和Ⅲ型贡献较小。镜质体反射率（Ro）值在0.54%至0.84%之间，平均值为437℃，表明处于低成熟至早成熟热演化阶段。这些发现表明，在同一煤系烃源岩体系中，与煤互层的湖沼相暗色和碳质泥岩是优质烃源岩。它们增强的生烃潜力归因于古盐度升高，这促进了富氢显微组分如基质镜质体的保存。目前处于热演化的低成熟至早成熟阶段（Ro：0.54-0.84%），这些烃源岩具有生成低成熟油气的巨大潜力。值得注意的是，它们的生烃潜力超过了煤层，为北山地区中侏罗统煤系烃源岩勘探提供了关键的理论支持。
