Although shale heterogeneity has recently emerged as a key topic in unconventional petroleum systems, hydrocarbon (HC) retention and pore evolution in organic-rich shales still remain poorly constrained. In this study, an organic-rich Eagle Ford Shale sample (EqVR_o = 0.92%) from a shale-oil prospecting well in South Texas was selected for gold-tube pyrolysis experiments to investigate organic matter (OM) -hosted pore development versus confining pressure during thermal maturation. Before heating, the OM-type heterogeneity and associated pore characteristics in the sample were examined by thin section and scanning electron microscopy imaging, combined with elemental mapping by X-ray energy-dispersive spectroscopy. Four forms of OM occurrence that are closely associated with different mineral matrix domains are distinguished, including the ductile oil-bearing primary OM and rigid primary OM in the siliceous-argillaceous seams, as well as the secondary OM in coccolith-rich lenses and in foraminifera chambers. From heating experiments, the pressure effect on pore evolution was found to be pronounced for the secondary OM in the foraminifera chambers, in which the coalescence of small pores to larger pores upon OM maturation was accelerated by increasing fluid pressure, accompanied by arrangement and densification of OM structures. However, the expulsion effect as a function of confining pressure seems to be evident only for the secondary OM in the coccolith-rich lenses. In contrast, the two types of primary OM in the siliceous-argillaceous seams do not show any apparent changes in pore development over the investigated pressure range of 10 to 120 MPa, probably due to stiffer mechanical properties of primary OM as compared to secondary OM. The discrepancies in pore evolution of different OM types as a response to the variation in confining pressure upon maturation primarily depend on the physical-chemical properties of these various OM materials.

Of further importance, observed differences in OM evolution and associated pore characteristics in response to the variation in confining pressure imply that the microscale heterogeneity of shale fabric controls the extent of oil retention and pore evolution. In contrast to the intragranular pores in the foraminifera chambers, the intergranular pores in the coccolith-rich lenses from the Eagle Ford Shale sample are relatively smaller, yet more likely interconnected. Thus, these coccolith-associated pores may provide not only significant storage space but also a major flow pathway for HC production. Our investigation at the microscopic scale provides concepts indicating that HC retention and OM-hosted pore evolution occur differently in two contrasting types of mineral matrix pores, which is critical to successful unconventional shale oil and gas exploration in sedimentary basins.

尽管页岩非均质性最近已成为非常规石油系统中的一个关键话题，但富含有机质页岩的碳氢化合物 (HC) 保留和孔隙演化仍然受到很差的约束。在本研究中，富含有机物的 Eagle Ford 页岩样品 (EqVR _o = 0.92%) 来自德克萨斯州南部的一口页岩油探井，被选择用于金管热解实验，以研究热成熟过程中以有机质 (OM) 为主体的孔隙发育与围压的关系。在加热之前，通过薄片和扫描电子显微镜成像检查样品中的OM型非均质性和相关的孔隙特征，并结合X射线能量色散光谱的元素映射。区分出四种与不同矿物基质域密切相关的 OM 赋存形式，包括硅质-泥质煤层中的韧性含油原生 OM 和刚性原生 OM，以及富含球石的晶状体和有孔虫中的次生 OM房间。从加热实验中，发现压力对孔演化的影响在有孔虫室中的次生OM中很明显，其中随着流体压力的增加，OM成熟时小孔向大孔的合并加速，伴随着OM结构的排列和致密化。然而，作为围压函数的排出效应似乎仅在富含球石的晶状体中的次级 OM 中是明显的。相比之下，硅质-泥质煤层中的两种原生 OM 在所研究的 10 至 120 MPa 的压力范围内没有显示出任何明显的孔隙发育变化，这可能是由于与次生 OM 相比，原生 OM 的力学性质更硬。

更重要的是，观察到的 OM 演化和相关孔隙特征响应于围压变化的差异意味着页岩织物的微观非均质性控制着油滞留和孔隙演化的程度。与有孔虫室中的粒间孔相比，Eagle Ford 页岩样品中富含球粒石的透镜体中的粒间孔相对较小，但更可能相互连接。因此，这些与球粒石相关的孔隙不仅可以提供重要的储存空间，还可以提供 HC 生产的主要流动途径。我们在微观尺度上的研究提供的概念表明，HC 保留和 OM 承载的孔隙演化在两种不同类型的矿物基质孔隙中发生不同，