Abstract Previous characterization of a low permeability (10−16 ≤ Kh ≤ 10−12 m s−1) and high salinity (> 5 M of Cl−, > 10× seawater salinity) aquiclude, in Upper Ordovician-aged sediments situated on the eastern flank of the Michigan Basin, where a deep geological repository for low and intermediate level nuclear waste is proposed, suggested a microbial origin for CH4 and CO2 based on their stable isotope values. CH4 is believed to have been produced and trapped during the Paleozoic in a relatively discrete and high organic matter (OM) horizon at the shale/carbonate transition. To further investigate the efficiency of confinement of this aquiclude, here we present detailed isotopic and geochemical stratigraphic profiles of OM and biomarkers, with the objective of understanding the origin of CH4 and its relative timing of confinement. Classical diagnostic ratios of polycyclic aromatic hydrocarbons, relative abundances of hopanes and degraded hopanes coupled to dual compound-specific isotopic analysis of n-alkanes, as well as δ13C analysis on different fractions of OM were measured. A partitioning of the Ordovician sedimentary succession into two systems is observed with an upper system that is self-sourced and confined, hosting the microbially derived CH4 with evidence of late diagenetic OM biodegradation and a lower system that reveals later secondary oil migration which has overprinted the organic geochemical record. We propose a conceptual model that would explain the generation and preservation of this paleo-bioreactor over time using the maturation history reconstruction derived from this dataset. This work not only provides essential field empirical evidence of a relationship between methanogenesis and late diagenetic biodegradation, but also shows the power of dual-compound-specific isotope analyses in deciphering between different maturation processes affecting OM in natural subsurface settings.

中文翻译：

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奥陶系含水层（密歇根盆地）有机质成熟历史的分子和同位素评估：晚期成岩生物降解的证据

摘要 先前描述的低渗透率 (10−16 ≤ Kh ≤ 10−12 ms−1) 和高盐度 (> 5 M of Cl−, > 10x 海水盐度) 含水层，位于东部上奥陶世时代沉积物中密歇根盆地的侧翼提出了一个用于中低放核废料的深层地质处置库，根据 CH4 和 CO2 的稳定同位素值提出了它们的微生物来源。CH4 被认为是在古生代期间在页岩/碳酸盐转变的相对离散和高有机质 (OM) 层位中产生和捕获的。为了进一步研究该隔水层的限制效率，我们在此展示了 OM 和生物标志物的详细同位素和地球化学地层剖面图，目的是了解 CH4 的起源及其限制的相对时间。测量了多环芳烃的经典诊断比率、与正构烷烃的双化合物特定同位素分析相结合的藿烷和降解藿烷的相对丰度，以及对 OM 不同馏分的 δ13C 分析。观察到奥陶纪沉积层序划分为两个系统，上层系统是自源和封闭的，含有微生物来源的 CH4，具有晚期成岩 OM 生物降解的证据，而下层系统则显示出后来的次生石油迁移，该系统覆盖了有机地球化学记录。我们提出了一个概念模型，该模型将使用从该数据集导出的成熟历史重建来解释该古生物反应器随时间的产生和保存。