Development characteristics and controlling factors of natural fractures in lacustrine shale oil reservoirs: A case study of the second member of the Funing Formation (E1f2), Qintong Sag, Subei Basin, eastern China

China hosts substantial lacustrine shale-oil resources and represents a key strategic replacement for sustaining reserves growth and production. Natural fractures are critical for enhancing the flow capacity of low-porosity, low-permeability, strongly heterogeneous lacustrine shale reservoirs and also exert fundamental controls on shale-oil accumulation and preservation. In the second member of the Funing Formation (E₁f²) in the Qintong Sag, Subei Basin (eastern China), fractures are abundant and diverse, yet their development characteristics remain insufficiently constrained and a systematic evaluation of controlling factors has not been fully conducted. This study integrates core-based fracture description, thin-section petrography, and borehole image logs. Fractures are classified according to geological origin, mechanical mechanism, and geometric relationships with bedding, and their development characteristics are quantitatively documented. For multiple geological attributes—including distance to faults, mechanical layer thickness, TOC, and XRD-derived mineral contents—we employ Theil–Sen estimators to conduct a “score–confidence interval” ranking of effect strength and thereby delineate the hierarchy of controlling factors.Results indicate that bedding-parallel fractures, intra-layer shear fractures, and cross-layer shear fractures are dominant, whereas intra-layer tensile fractures and bedding-parallel shear fractures are subordinate. Fractures are predominantly high-angle, with apparent fracture height on core surfaces generally <15 cm. Fracture strikes comprise multiple sets, with a dominant NNE–SSW orientation. Fractures exhibit an overall moderate degree of infilling, and calcite is the principal cement. Distance to faults is negatively correlated with structural-fracture density and is identified as the primary control, whereas mechanical layer thickness and clay-mineral content are secondary factors and also show negative correlations with structural-fracture density. In contrast, higher TOC and greater lamination density promote the development of bedding-parallel fractures and constitute the primary controls, whereas higher clay-mineral content and greater mechanical layer thickness act as secondary factors that are unfavorable for bedding-parallel fracture development. These results clarify fracture distribution patterns in E1f2 and provide geological constraints for shale-oil exploration and development in eastern China, while also offering a transferable framework for the quantitative evaluation and ranking of fracture-controlling factors.