Authigenic minerals and their control over micro-pore development for saline lacustrine shales: A case study of the Shahejie Formation, Jiyang Depression, Bohai Bay Basin

This study focuses on the saline lacustrine shales of the Shahejie Formation in the Jiyang Depression, Bohai Bay Basin. Using a range of techniques, including scanning electron microscopy (SEM), cathodoluminescence (CL), and energy-dispersive spectroscopy (EDS), we systematically investigate the characteristics of authigenic minerals in the shales and their influence on micro-pore evolution. The results indicate that coarse-grained calcites or calcites with epitaxial overgrowth precipitated substantially in an alkaline diagenetic environment, leading to significantly reduced porosity. The diagenetic dolomites, in spite of low content and scattered distribution, also indirectly hindered pore preservation due to the weakened reworking by acidic fluids during cementation in an alkaline environment. Ankerites are mostly formed in a Fe²⁺-rich acidic environment, frequently associated with illitization and albitization. Albites and ankerites exhibit pronounced positive correlations with thin-section porosity, although they are not direct pore carriers themselves. This relationship reflects the synergistic effects of acidic fluid-induced reworking and mineral transformation. Illites act as important pore carriers, and their platy or fibrous aggregates facilitate the formation and interconnectivity of intergranular pores, bedding-plane pores, and microfractures in an acidic environment. Authigenic quartz, except for some tiny quartz grains that fill pores, can occur as distinctive vein-like aggregates that provide rigid support for surrounding pore structures, thereby mitigating the damage to pores caused by subsequent compaction and cementation. Overall, the pore evolution in the shales of the Shahejie Formation is dominated by alternating dissolution-induced porosity enhancement in an acidic environment and cementation-induced porosity reduction in an alkaline environment. Furthermore, various authigenic mineral assemblages and their spatial distributions can serve as important indicators for assessing the potential for pore preservation in shale reservoirs.