Fine characterization of micro-nano fractures and analysis of network connectivity: Mechanistic controls on hydrocarbon enrichment in shale reservoirs

Abstract

Micro-nano fractures serve as the bridge connecting nanopores and macro-fractures. The unclear understanding of their developmental characteristics and controlling factors significantly hinders the large-scale, efficient development of continental shale oil. To address this, this study employs the entropy weight method to establish an evaluation model for fracture development strength that comprehensively considers fracture number, average width, areal density, and areal porosity. Additionally, topology is introduced to evaluate fracture connectivity. The research clarifies the differences in micro-nano fracture developmental characteristics and primary controlling factors among different lithofacies and elucidates the impact of micro-nano fracture development on pore structure and hydrocarbon accumulation in Gulong shale. The results indicate that the HQS (high-organic laminated felsic shale) lithofacies exhibits high micro-nano fracture development strength and connectivity, yielding the highest comprehensive evaluation index. The HCS (high-organic laminated mixed shale) shows high development strength but low connectivity, resulting in a secondary comprehensive evaluation index. Higher organic matter content correlates with greater fracture development strength; clay mineral content controls the characteristics of nano-fracture development; felsic mineral content positively influences fracture connectivity. The development of micro-nano fractures not only enhances macropore content and average pore size but also effectively connects pores of various scales, increasing the effectiveness of the pore-fracture system. Lithofacies with low fracture connectivity (primarily HCS) exhibit more complex pore structures. Shale oil in such lithofacies mainly accumulates via a self-sealing model, making it difficult to form complex fracture networks during hydraulic fracturing and hindering efficient development. Conversely, the HQS lithofacies demonstrates optimal pore-fracture connectivity, favorable oil content, and represents the most favorable lithofacies for Gulong shale oil development. These findings contribute to the optimization of sweet-spot intervals for shale oil exploration in the study area.

Keywords

Gulong shale / Micro-nano fractures / Development strength / Connection degree / Favorable lithofacies

CRediT authorship contribution statement

Boyang Wang: Writing - original draft, Resources, Methodology, Funding acquisition. Dengfu Yuan: Methodology. Jingjing Li: Writing - review & editing. Shichao Li: Resources, Methodology. Fei Xiao: Resources, Funding acquisition. Shansi Tian: Methodology. Mengjing Yin: Methodology, Formal analysis. Jianguo Yang: Writing - review & editing, Funding acquisition.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This project funded by National Natural Science Foundation of China (Grant Nos. U2244207 and 42472196), Young Elite Scientists Sponsorship Program by CAST (No. YESS20240090), Young Scientists Subject of the National Science and Technology Major Project (NSTMP) for New-Type Oil and Gas Exploration and Development (No. 2025ZD1400807), Heilongjiang Leading Goose Team (TY24001).

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