Diagenesis exerts a key control on the evolution of mineralogical systems in shales, and by extension, on the development of rock properties amenable to unconventional reservoir prospectivity. To develop an understanding of the control of primary grain assemblage on the diagenetic pathways and products in shales, integrated high-resolution petrographic and multiple geochemical studies were conducted on samples from a vertical transect of oil-gas mature (vitrinite reflectance ranging from 0.98 to 2.72%) Whitehill Formation, a shale unit identified as the main potential gas reservoir in South Africa. Results show that the formation comprises three intervals with contrasting primary grain assemblages, including organic matter content, maceral type, and detrital mineral grains, as well as the submillimeter-scale arrangement of these constituents. The lower interval occurs as parallel-laminated black shale dominated by components derived from primary production, such as Tasmanites cysts, colonial algae, radiolarian tests, and amorphous organic matter with up to 5.78% TOC, and depleted in siliciclastic detritus. The mid and upper intervals contain biologically produced components (up to 16.57% TOC) and high quantities of siliciclastic mud dominated by clay minerals and micas, and are organized into thick discontinuous gray laminae. Petrographic data suggest that the high organic content supported a variety of bacterial metabolic pathways, which caused the precipitation of mineral cements in the sediment pore spaces prior to compaction. However, the early diagenetic reactions and products in these intervals display systematic variations that reflect heterogeneity in their primary grain assemblages. The presence of framboidal pyrite throughout the lower interval suggests sulfate reduction as the predominant metabolic pathway. In absence of significant volumes of siliciclastics, the acidity associated with this process was poorly buffered, and the pore waters were reducing. These conditions led to the dissolution of opaline radiolarian tests, which recrystallized within Tasmanites cysts and interstitial pores to quartz. On the contrary, carbon isotopes indicate that organic oxidation in the mid and upper sections was mainly promoted by methanogenic archaea and that the associated acidity was effectively buffered by the detrital aluminosilicates. The resulting bicarbonate alkalinities allowed for the formation of interstitial carbonate cements (dolomite, calcite). Aluminum released during the acid-consumption reactions recrystallized within sheltered and interstitial pores to chlorite. Considering that many other shale successions exhibit similar stratigraphic variability of primary grain assemblages as the Whitehill Formation, this type of study will improve the characterization of shale mineralogical systems and bulk properties amenable to sustainable unconventional exploration.

成岩作用对页岩矿物系统的演化起到关键控制作用，进而对适合非常规储层勘探的岩石性质的发展起到关键控制作用。为了了解原生颗粒组合对页岩成岩路径和产物的控制，对油气成熟垂直断面的样品进行了综合高分辨率岩相学和多种地球化学研究（镜质体反射率范围为 0.98 至 2.72 %) Whitehill Formation，一个被确定为南非主要潜在气藏的页岩单元。结果表明，该地层包括有机质含量、微晶类型和碎屑矿物颗粒以及这些成分的亚毫米级排列等三个具有对比的原生颗粒组合的层段。塔斯马尼亚人包囊、群落藻类、放射虫测试和无定形有机物，TOC 高达 5.78%，并在硅质碎屑中耗尽。中上层段含有生物产生的组分（TOC 高达 16.57%）和大量以粘土矿物和云母为主的硅质碎屑泥，并组织成厚厚的不连续的灰色薄层。岩相学数据表明，高有机物含量支持多种细菌代谢途径，这导致矿物胶结物在压实前沉积物孔隙空间中沉淀。然而，这些层段的早期成岩反应和产物显示出系统变化，反映了其原始颗粒组合的异质性。整个较低层段都存在草莓状黄铁矿，这表明硫酸盐还原是主要的代谢途径。在没有大量硅质碎屑的情况下，与该过程相关的酸度缓冲不良，孔隙水减少。这些条件导致乳白色放射虫试验的溶解，这些试验在Tasmanites包囊和间隙孔到石英。相反，碳同位素表明中上部的有机氧化主要由产甲烷古菌促进，相关的酸度被碎屑铝硅酸盐有效缓冲。由此产生的碳酸氢盐碱度允许形成间隙碳酸盐胶结物（白云石、方解石）。在酸消耗反应过程中释放的铝在隐蔽的间隙孔中重结晶为亚氯酸盐。考虑到许多其他页岩层序表现出与 Whitehill 组相似的原生颗粒组合的地层变异性，这种类型的研究将改进页岩矿物系统的特征和适合可持续非常规勘探的整体特性。