Electrical Imaging–Based Statistical Indicators for Fracture Identification in Shale-Oil Reservoirs

Electrical imaging logs are widely used for the quantitative characterization of unconventional reservoirs. However, their applicability is severely limited in pervasively fractured continental shale-oil reservoirs due to strong heterogeneity and complex resistivity responses.To address this limitation, an electrical statistical framework is developed that integrates variogram-based attributes with smoothness functions to identify fracture occurrence and quantitatively characterize fracture development in shale reservoirs. Resistivity percentile curves derived from electrical imaging logs are analyzed using a moving-window scheme to extract multidimensional parameters, including variance, Shannon entropy, variogram metrics, and smoothness indices. These parameters are jointly used to construct a fracture development index. In addition, fracture-prone intervals are identified using an adaptive thresholding approach constrained by geological rules and resistivity separation characteristics. To suppress the influence of stratigraphic background trends, a local background-resistivity normalization is applied, enabling fracture classification based on resistivity ratios.The method is validated using shale-oil reservoirs of the Shahejie Formation in the Dongying Sag, eastern China. The results demonstrate that, within a tolerance range of 0.125 m, fracture identification derived from electrical image logs achieves an 85.4% agreement with core descriptions. The identification accuracies for high-conductivity and high-resistivity fractures reach 90.41% and 80.65%, respectively, while approximately 47% of the identified fractures exhibit high conductivity.The proposed approach provides new insights into fracture-filling properties and their vertical distribution, highlighting its applicability to shale-oil reservoir characterization, sweet-spot evaluation, and hydraulic fracturing design. This research was supported by the National Oil & Gas Major Project (No. 2024ZD1405102).Fig. 1. Electrical Imaging–Based Statistical Indicators for Fracture Identification in Well FY3