Geological Patterns and Conventional Well-log Prediction of Bedding Fractures in Lacustrine Shales: A Case Study from the Upper Submember of the 4th Member of the Shahejie Formation, Dongying Sag

The effective development of unconventional shale oil reservoirs depends on the accurate prediction of seepage pathways. In the lacustrine shales of the upper submember of the fourth member of the Shahejie Formation (Es4U) in the Dongying Sag, bedding fractures serve as both primary storage spaces and critical fluid conduits. However, intense reservoir heterogeneity makes it challenging to reliably identify fracture-prone intervals using well-log responses alone.

Based on systematic core descriptions and fracture statistics from research wells, this study integrates thin-section petrography, X-ray diffraction (XRD), total organic carbon (TOC) analysis, and logging data to perform a multi-scale correlation of bedding fracture occurrences.

The results demonstrate that: (1) bedding fractures are diagenetic products triggered by clay mineral dehydration/transformation and hydrocarbon-induced overpressure; (2) the development of these fractures is synergistically governed by carbonate and clay mineral contents, organic matter abundance, and lamina density, with laminated clay-rich lithofacies identified as the dominant zones for fracture occurrence.Building upon these geological insights, a vertical development model for bedding fractures was established to provide quantitative geological constraints for logging evaluation. By utilizing conventional logging suites (e.g., integrating AC, GR, and DEN curves), intervals characterized by carbonate content of 20%–60%, clay content of 20%–35%, TOC > 2%, and well-developed laminated structures are identified as potential fracture-bearing zones.

This model effectively bridges geological genesis with logging attributes, providing a direct geological foundation for the prediction of favorable seepage pathways in shale reservoirs using borehole geophysical data.