Abstract Silica cementation exerts a key control on the compaction and geotechnical properties of mudstones, and by extension, the style of hydrocarbon and/or mineral systems present in a given sedimentary basin. Integrated microscopic and bulk geochemical observations demonstrate that siliceous mudstones in the Bowland Shale Formation, a target for UK shale gas extraction, exhibit abundant dispersed, discrete, μm-scale quartz cements, and exhibit silica enrichment (‘excess’) above a local detrital Si/Al threshold of 2.5. Dissolution of siliceous radiolarian tests during early diagenesis is identified as the main source of silica (opal A) required for quartz precipitation, either via opal CT or directly to quartz, and potentially generated as a product of anoxic marine ‘weathering’ (dissolution) of reactive silicates during early diagenesis. Excess silica correlates with free hydrocarbons (S1) normalised to total organic carbon (oil saturation index; OSI); we propose early diagenetic quartz precipitation suppressed pore collapse (‘buttress effect’), retaining the pore space capacity to host oil. Quartz precipitation was likely catalysed, for example via low porewater pH, elevated Al and/or Fe oxide content, and/or abundant labile organic matter. Juxtaposition of siliceous mudstones and mudstones lacking quartz cement indicates silica was immobile beyond the bed scale. Thus metre-scale siliceous packages likely represent more prospective units within the Bowland Shale (in terms of unconventional hydrocarbons), on the basis of early diagenetic biogenic-derived quartz cementation leading to improved hydrocarbon storage capacity coupled to enhanced brittleness. These findings are relevant for shale oil and shale gas systems, specifically where oil retained in pores subsequently cracks to generate gas. These findings also suggest the Bowland Shale is a sub-class of black shale, defined by the potential to host a relatively large volume of early diagenetic fluids, derived from anoxic bottom waters, which were potentially S- and/or metal-bearing. This is potentially relevant for understanding the genesis of adjacent and related Pb-Zn mineral deposits.

中文翻译：

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英国克雷文盆地密西西比鲍兰页岩地层中早期成岩石英的成因和意义

摘要 二氧化硅胶结作用对泥岩的压实和岩土特性起到关键控制作用，进而影响给定沉积盆地中存在的烃和/或矿物系统的类型。综合微观和大块地球化学观测表明，英国页岩气开采目标 Bowland 页岩组中的硅质泥岩表现出大量分散的、离散的、微米级的石英胶结物，并且在局部碎屑硅上方表现出二氧化硅富集（“过量”） /Al 阈值为 2.5。早期成岩作用期间硅质放射虫测试的溶解被确定为石英沉淀所需的二氧化硅（蛋白石 A）的主要来源，通过蛋白石 CT 或直接到石英，并可能作为缺氧海洋“风化”（溶解）的产物产生早期成岩作用中的活性硅酸盐。过量的二氧化硅与归一化为总有机碳（油饱和指数；OSI）的游离碳氢化合物 (S1) 相关；我们提出早期成岩石英沉淀抑制了孔隙坍塌（“支撑效应”），保留了容纳石油的孔隙空间能力。石英沉淀可能是催化的，例如通过低孔隙水 pH 值、升高的 Al 和/或 Fe 氧化物含量和/或丰富的不稳定有机物。硅质泥岩和缺乏石英胶结物的泥岩的并置表明二氧化硅在床层之外是不可移动的。因此，米级硅质包裹可能代表鲍兰页岩内更多的潜在单元（就非常规碳氢化合物而言），基于早期成岩生物成因石英胶结导致碳氢化合物储存能力提高，同时脆性增强。这些发现与页岩油和页岩气系统相关，特别是保留在孔隙中的石油随后裂化产生气体的系统。这些发现还表明，鲍兰页岩是黑色页岩的一个亚类，其定义是具有相对大量早期成岩流体的潜力，这些流体来自缺氧底层水，这些流体可能含有 S 和/或金属。这可能与了解相邻和相关铅锌矿床的成因有关。