Research on the Conductivity Mechanism and Saturation Evaluation Method of Altered Igneous Rock Reservoirs Based on Numerical Simulation

Altered igneous reservoirs are difficult to evaluate using conventional electrical models because of their complex mineral composition and mixed conduction mechanisms. To improve the understanding of electrical features in these reservoirs, this study combines laboratory experiments with digital rock simulation to analyze the conductivity mechanism of altered igneous rocks and to establish applicable conductivity and saturation models. X-ray diffraction, resistivity measurements, and CT scanning were used to characterize the mineral composition and pore structure of the samples and to reconstruct 3D digital cores. On this basis, digital rock models with different clay contents, wettability conditions, and water saturations were generated. Considering the alteration characteristics of igneous rocks, mathematical morphology and the lattice Boltzmann method were used to simulate pore-scale fluid occurrence, and the model reliability was verified by experimental results and fractal exponent analysis. Finite element simulations and laboratory data were then integrated to investigate the effects of clay content, wettability, and salinity on electrical properties. The results show that clay content increases the nonlinearity of the I-Sw relationship, whereas salinity weakens it. Clay minerals also affect the saturation exponent mainly by altering wettability. Based on these results, new conductivity and saturation models were developed, and a Bayesian-optimized BiLSTM network was introduced to predict clay-related parameters. Compared with conventional shaly sandstone models, the proposed method gives better predictions for altered igneous rocks. The study provides a practical basis for logging evaluation and petrophysical interpretation of complex altered igneous reservoirs.