This study aims at identifying some of the potential contributors to increase oil recovery through field gas huff-n-puff in a thick, heterogeneously layered, and low-permeability shale-sand-carbonate reservoir. It has been observed that the composition of the produced volume is important. At least in some cases, more oil volume can be obtained in standard conditions by optimizing the composition of the produced volume. The general trend is that the more heavy components are produced, the more oil volume will be obtained in standard conditions. Furthermore, the optimum pressure for a given reservoir composition, which may vary during the enhanced oil recovery time frame, can be determined by conducting a sensitivity study. The optimum pressure is the pressure that is able to facilitate the production of heavy components as much as possible. This optimum pressure can then be translated to controllable operation parameters. The production of the heavy components can be increased with huff-n-puff gas injection by producing more oil or gas volume in reservoir conditions relative to that in primary production since both oil and gas phases bear some heavy components even though at different degrees. Despite bearing less heavy components, the gas phase can potentially compensate for such a shortcoming by having a higher mobility than the oil phase in reservoir conditions. All of these are intended to be achieved within the scope of numerical simulation.