Hydrocarbon gas flooding/Huff-n-Puff (HNP) can improve the oil recovery in the unconventional reservoirs. Here, the mechanisms accounting for fluid flow in the low-permeability and ultra-low permeability reservoirs were experimentally and theoretically investigated. Core plugs collected from a typical China oilfield were utilized for the experiments. Additionally, methane was used as the injection agent to conduct natural gas HNP/displacement experiments. The results indicated that the use of natural gas as an energy supplement agent and the HNP development method can effectively improve the recovery efficiency of the aforementioned two types of reservoirs. During the HNP process, the oil recovery is effectively enhanced mainly in the first round and second round. Meanwhile, during gas injection and HNP, natural gas can evidently weaken the extraction and reduce the precipitation of heavy components. However, the natural gas injection can establish an effective driving pressure system in low-permeability core plugs, and the interaction between natural gas and oil can change the mobility ratio. Furthermore, it aids in avoiding viscous fingering and premature breakthroughs. Moreover, the oil can be sandwiched between the interface of the gas and water phases to form a slip channel in a hydrophilic core sample, which can quickly produce oil. Finally, a numerical model was developed by considering the reservoir parameters of Changqing Oilfield, China. The oil recovery after eight rounds of CH4 HNP was 80% higher than that achieved via depletion development. Additionally, the oil recovery curves are especially similar in the previous three HNP rounds. These curves show obvious differences from the fourth round onwards, which indicates that the asphaltene deposition and CH4 diffusion slightly affect the oil recovery factor during the initial production period.