Molecular diffusion plays an important role in shale oil recovery. CO2 huff-n-puff is a feasible way to recover shale oil in multistage fractured horizontal wells, and natural fracture is important during this process. In this work, the CMG-GEM model is built based on the Bakken formation geological settings and the Bakken live oil PVT data, in which natural fracture effective permeability is populated with a series of Dykstra-Parsons (DP) coefficients and spatial correlation lengths. DP coefficient varies from 0.396 to 0.867, and correlation length varies from 50 ft to 3,000 ft. CO2 huff-n-puff starts when primary recovery factor reaches 3%, in either single-cycle or multiple-cycle schemes: single-cycle is set with a fixed injected CO2 reservoir pore volume, and multiple-cycle is set with fixed huff/soak/puff days. Findings in this work highlight the complex molecular diffusion behavior in the comprehensive field-scale simulations. The findings imply the importance of accurate in-lab diffusion coefficient measurement between CO2 and light oil components. Besides, it is demonstrated that CO2 injectivity in shale reservoirs is a function of both natural fracture heterogeneity and distribution patterns.