Multi-scale characterization and control factors of bedding-parallel fractures in continental shale reservoirs: Insights from the Qingshankou Formation, Songliao Basin, China

Abstract

Bedding-parallel fractures play a critical role in enhancing storage capacity and horizontal permeability in tight shale reservoirs, significantly influencing the enrichment and productivity of shale oil. This study focuses on the continental shale of the Qingshankou Formation in the Songliao Basin, China, and systematically investigates the development characteristics and controlling factors of bedding-parallel fractures using a multi-scale dataset including core observations, image logs, scanning electron microscopy (SEM), and optical microscopy. Results show that bedding-parallel fractures are extensively developed across nano-to macro-scales, with nano-scale fractures reaching densities of up to 10⁵ fractures/m. Fracture density is negatively correlated with both fracture scale and aperture. These fractures are generally sub-parallel to bedding or exhibit low-angle orientations (mostly <10°), and are typically discontinuous with limited lateral connectivity. Quantitative analyses indicate that fracture density is positively correlated with the contents of clay minerals, pyrite, and carbonate minerals, while showing weak or negative correlations with brittle minerals such as quartz and feldspar. Higher fracture densities are also observed in shales with elevated TOC content and higher thermal maturity. Furthermore, the type, thickness, and density of lamination significantly influence fracture development. At micro- and nano-scales, bedding-parallel fractures primarily act as hydrocarbon storage spaces and micro-flow conduits, whereas at the macro-scale, they mainly serve as horizontal flow pathways. These findings elucidate the multi-scale development mechanisms of bedding-parallel fractures in continental shale and underscore their dominant role in improving reservoir performance.