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NMR T2 relaxation times for fluids in a porous medium are, in principle, proportional to the relevant occupied pore size. Here, we exploit this relationship to monitor the pore size distribution occupied by methane (CH4) at 100 bar and room temperature in both sandstone and carbonate rock cores, while it is displaced by a range of miscible fluids: carbon dioxide (CO2), nitrogen (N2), nitrous dioxide (N2O), and helium (He). The process is then reversed, with methane being used to displace these injected fluids. Time-resolved T2 distributions are thus able to probe the preference of the methane for smaller or larger pores during these core-flooding processes. In the cases of CO2 and N2O, methane was found to preferentially occupy larger pores during both injection and displacement for both rock types. In the case of N2, no significant preferential pore size occupation was evident, whereas in the case of He, methane preferentially occupied smaller pores. For the fluids used in this work, preferential occupation of comparatively smaller pores during these displacement experiments correlated with a comparatively greater surface adsorption capacity.