Abstract: Igneous-rock reservoirs in buried hills in the eastern South China Sea are characterized by complex lithology and pore structures as well as strong heterogeneity. Conventional destructive core testing is constrained by substantial sample consumption, high cost, and limited characterization accuracy, making it difficult to accurately determine the mechanical properties of the reservoir rocks. To efficiently and accurately obtain the mechanical parameters of igneous rocks under overburden pressure, the microscopic pore structures of granite and diabase were nondestructively reconstructed using micron-scale computed tomography (micro-CT), while their mineralogical compositions were characterized by X-ray diffraction (XRD). High-fidelity numerical models based on the combined finite-discrete element method (FDEM) were subsequently constructed using digital rock technology. In conjunction with laboratory triaxial compression test results, parameter sensitivity analysis and iterative inversion were performed to calibrate key model inputs, including the elastic and strength parameters and fracture energies of the mineral phases and interfaces between dissimilar phases. This approach enabled detailed simulation of the entire compression-induced failure process, including crack initiation, propagation, coalescence, and eventual macroscopic failure. The results indicate that the granite is characterized by abundant microfractures and low porosity and exhibits predominantly brittle failure, with markedly higher strength and elastic modulus. In contrast, the diabase contains more dissolution pores and a higher proportion of clay minerals, exhibits more pronounced plastic failure behavior, and has comparatively lower mechanical properties. The results obtained from the optimized FDEM models agree well with the laboratory measurements, with errors in key mechanical parameters, including peak strength and elastic modulus, controlled within 15%. The findings demonstrate that microscopic pore structure and mineralogical composition are the primary factors governing the differences in the mechanical properties of the igneous rocks. The proposed digital rock–FDEM coupled calculation method provides a reliable technical basis and theoretical support for hydraulic-fracturing design, reservoir evaluation, and related engineering applications in igneous buried-hill reservoirs in the eastern South China Sea.