A core was taken approximately 300 ft from a production well in the maturest area of the Prudhoe Bay Miscible Gas Project (PBMGP). Data from conventional and compositional core analysis, and from a reservoir-simulation history match showed that approximately 70% of the cumulative oil recovery from the project was the result of reducing the residual oil saturation near the injection well. Oil swelling enabled the remaining 30% recovery. The swelling mechanism was especially important near the production well where the core was taken. Both simulation and field data show that additional miscible-gas injection into this area would yield very Little incremental oil recovery.