A Three-Level Graded Characterization Framework for the Mesoscopic Mechanical Behavior of Ultra-Deep Heterogeneous Dolomite Based on the Combined Finite–Discrete Element Method

Ultra-deep dolomite formations exhibit pronounced mechanical heterogeneity governed by mineral composition, inter-particle cementation, and observation scale, complicating their mechanical characterization. Because intact downhole cores are difficult to obtain, a mesoscopic numerical framework that reproduces and classifies such behavior is needed. A two-dimensional mesoscopic model of Cambrian ultra-deep dolomite from the Tarim Basin was developed using the combined finite–discrete element method, incorporating mineral composition from X-ray diffraction and scanning electron microscopy with energy-dispersive spectroscopy. A Weibull-distribution discretization scheme was introduced to represent matrix and cementation heterogeneity separately. Sensitivity analyses examined three factor groups: mineral composition and spatial distribution, matrix and cementation shape parameters, and mesh size as the observation scale. Accordingly, a three-level graded characterization framework was proposed, in which the compositional level governs strength magnitude; the structural level governs the elastic modulus, pre-peak nonlinearity and failure mode; and the scale level governs response representativeness and convergence. Quantitative grading indicators and threshold ranges were extracted for each level. Triaxial validation against laboratory experiments at confining pressures of 0 to 220 MPa confirmed strength and stress–strain response consistent with the proposed grading. The model and framework offer a transferable methodological reference for the mesoscopic mechanical evaluation of ultra-deep heterogeneous dolomite.