Reservoir space types, controlling factors, and evolution models of interbedded shale in the Jurassic Lianggaoshan Formation, Sichuan Basin

Abstract

The type, structure, and origin of shale pores, which serve as both storage spaces for hydrocarbon accumulation and percolation pathways, fundamentally determine reservoir quality and further influence sweet spot identification and production performance in shale oil reservoirs. The heterogeneous interlayers such as siltstone and silty mudstone developed in continental shale have greatly altered the pore structure, fluid storage state and percolation capacity of the reservoir, resulting in significant differences in its reservoir properties compared with shale. This study investigates lacustrine interbedded shale reservoirs through a comprehensive approach incorporating X-ray diffraction (XRD), thin-section petrography, field emission scanning electron microscopy (FE-SEM), low temperature nitrogen adsorption, and nuclear magnetic resonance (NMR) techniques. The results demonstrate that the reservoir properties of different shale lithofacies are generally better than those of fine siltstone lithofacies, especially the organic-rich laminated argillaceous shale (O-rLAS) lithofacies, which has the best reservoir properties. Organic matter (OM) - related pores (particularly organo-clay composite pores) and the interlayer fractures/intercrystalline pores of clay mineral constitute the primary contributors to the reservoir space. Thermal cracking of organic matter and clay mineral transformation enhance the storage capacity of shale lithofacies, whereas cementation is identified as the main factor responsible for the poor reservoir quality of fine siltstone lithofacies. The development of OM-related pores is jointly controlled by thermal maturity, total organic carbon (TOC), and OM type, as well as clay mineral content. The occurrence state of clay minerals with OM and clay mineral type predominantly influence the development of clay mineral interlayer pores. Dissolution pores are mainly affected by TOC and felsic mineral content, while felsic mineral content significantly impacts the development of intergranular pores and microfractures. This study enriches the theoretical understanding of diagenetic and reservoir-forming mechanisms in continental shale reservoirs. It provides critical geological basis and theoretical support for accurately identifying and evaluating the high-quality "sweet spots" of continental interbedded shale oil. These findings offer practical guidance for efficient exploration and development of such complex reservoirs.