Pore and fracture characteristics and diagenetic evolution mechanisms of low-maturity lacustrine shales from the Paleogene Shahejie Formation in the Jiyang Depression, Bohai Bay Basin, Eastern China

Abstract

Pore and fracture characteristics and diagenetic evolution pathways are crucial for understanding the storage and flow capacity of shale reservoirs. Compared with marine shales in North America and South China, lacustrine shales typically exhibit low maturity, high clay content, and abundant laminae. The pore and fracture characteristics and diagenetic evolution pathways of low-maturity lacustrine shales remain poorly understood. This study integrates thin-section observations, fluorescence analysis, organic geochemical analysis, X-ray diffraction (XRD) mineralogy, low-temperature nitrogen adsorption (LTNA), and high-resolution scanning electron microscopy-energy dispersive X-ray spectroscopy-cathodoluminescence (SEM-EDS-CL) image analysis to systematically investigate pore and fracture types, pore size distribution, diagenetic reactions, and major factors controlling pore development in the Es3x–Es4s lacustrine shales from the Jiyang Depression, Bohai Bay Basin. The low-maturity lacustrine shales primarily consist of interparticle pores between mineral particles, interparticle pores between clay platelets, intercrystalline pores between carbonate minerals, and dissolution pores, whereas organic-matter pores are undeveloped. The observed fractures are primarily horizontal bedding fractures, followed by structural fractures and abnormal pressure fractures. Calcareous mixed shales and siliceous mixed shales have a higher proportion of large-sized mesopores (>30 nm in pore size) and macropores compared with argillaceous mixed shales. Pore development is mainly influenced by the type, content, and existing forms of inorganic minerals, but less affected by TOC content. Clay mineral content and siliceous mineral content are weakly positively correlated with total pore volume. The pore decline by pore-filled carbonate cements may exceed the pore increase by carbonate recrystallization and dissolution, leading to the weakly negative correlation between carbonate mineral content and total pore volume. Among the four main laminae, sparry calcite laminae (SCAL) and sparry dolomite laminae (SDL) exhibit higher reservoir quality compared with argillaceous laminae (AL) and micrite calcite laminae (MCAL). The reservoir quality of the corresponding three laminae combinations is ranked as follows: AL + SCAL > AL + MCAL + SDL > AL + MCAL. Laminae combinations are beneficial to the development of horizontal bedding fractures. The early stage of middle diagenesis is a key stage for the formation of secondary pores. When the R_o values exceed approximately 0.6%, the total pore volume increases obviously because of dominant diagenetic reactions including organic acid dissolution, carbonate recrystallization, and rapid smectite illitization.