Abstract: To address the insufficient understanding of the spatial mass transfer, diffusion, and migration mechanisms between high-carbon gas and formation gas during high-carbon gas injection in gas reservoirs, a macroscopic 2D physical experimental device for CO₂-CH₄ detection under formation conditions was designed and constructed. This device was used to systematically investigate the planar and vertical migration patterns and diffusion mass transfer characteristics of high-carbon gas within the reservoir. Experimental results show that under well shut-in conditions with an injected gas carbon content of 80%, high-carbon gas injection can increase reservoir recovery by approximately 30%. Similar to water-flooded oil reservoirs, there exists gas channeling between injection and production wells, with the injected gas primarily migrating along the direction of the maximum pressure gradient. Significant diffusion phenomena occur during high-carbon gas injection, and the diffusion rate of high-carbon gas in high-permeability layers is much greater than that in low-permeability layers. Residual gas at the reservoir edge-bottom and gas trapped in the reservoir top attic outside the swept zone are difficult to mobilize. Numerical simulations further indicate that reasonable control of injection rate and reduction of permeability contrast can effectively enhance sweep efficiency and gas recovery. Increasing well spacing to reduce inter-well channeling, mitigating diffusion retention caused by permeability heterogeneity, and optimizing injection-production rates can improve gas injection displacement efficiency. This study provides important theoretical foundation and technical support for the development of high-carbon gas reinjection in offshore gas reservoirs.