Recognition and impact on reservoir quality of fractures in fine-grained sedimentary rocks using image and array acoustic logs

Fine-grained sedimentary rocks have been widely regarded as effective reservoirs and source rocks of tight oil and shale oil. Natural fractures play a critical role in promoting oil enrichment and enhancing production. Clarifying the contribution of fractures to reservoir quality is important but lacks systematic research. Limitations exist in both core outcrop descriptions and seismic predictions of fractures. New technological advancements in well logs offer new avenues for multi-scale fracture characterization. Image logs and array acoustic logs are used to systematically investigate natural fractures in the Member 7 of Upper Triassic Yanchang Formation (Chang 7) shale oil reservoirs in the Ordos Basin. Natural fractures include tectonic fractures and bedding-parallel fractures. An absolute match between core data and image log data is achieved through the precise core homing. The reliability of image logs is enhanced by establishing the fracture identification templates under the core scale. The Vp/Vs ratio, energy attenuation, waveform distortion, and S-wave slowness anisotropy of array acoustic logs are effective for detecting and predicting natural fractures around the borehole. The impact of natural fractures on reservoir quality is evaluated by considering fracture parameters, radial extension and their relationship with in-situ stress. The results show that combining the occurrence and type of fractures detected by image logs and the fracture extension state indicated by fast and slow S-wave radial variations, the effectiveness of fractures around the wellbore can be accurately evaluated. T₁-T₂ maps from 2D NMR logs also offer insights into the relationship between fractures and reservoir quality. Natural fractures that are nearly parallel to the direction of the in-situ SH_max have proven to be effective and significantly enhance hydrocarbon productivity. The research above may aid in optimizing sweet spot evaluation and stimulating shale oil production.