Abstract: The igneous rock reservoirs of buried hills in the eastern South China Sea exhibit complex lithology and strong heterogeneity in pore structures. Traditional destructive mechanical testing on core samples results in significant sample loss, high testing costs, and limited evaluation accuracy, which make it difficult to accurately characterize the mechanical response of reservoir rocks under overburden pressure. The microscopic pore structures and spatial distributions of mineral components in granite and diabase were nondestructively reconstructed based on micro-computed tomography (micro-CT) scanning and X-ray diffraction (XRD) testing to achieve efficient and accurate quantitative evaluation of igneous rock mechanical parameters under an overburden pressure condition. Based on the digital core, a high-fidelity numerical simulation model using the finite-discrete element method (FDEM) was established. Combined with results from laboratory triaxial compression tests, parameter sensitivity analysis and iterative inversion calibration were conducted to determine key input parameters, including elastic parameters, strength parameters, and fracture energy for both mineral phases and interphase contact surfaces. The refined simulation of the entire process of fracture initiation, propagation, coalescence, and ultimate failure of igneous rocks under compression was achieved. The results indicated that granite, characterized by micro-fractures as the primary storage space and low porosity, exhibited predominantly brittle failure. In contrast, diabase, with well-developed dissolution pores and higher clay mineral content, displayed predominantly plastic failure and relatively inferior macroscopic mechanical properties. After calibration and optimization, the simulation results from the FDEM model showed good agreement with laboratory test data, with calculation errors for key mechanical parameters such as peak compressive strength and elastic modulus controlled within 15%. The study confirms that microscopic pore structure and mineral composition are the dominant factors controlling the differences in mechanical properties of igneous rocks. The established digital core-FDEM coupled calculation method can provide a reliable theoretical basis and technical support for engineering practices such as reservoir evaluation and hydraulic fracturing stimulation in igneous rock reservoirs of buried hills in the eastern South China Sea.