Currently, multistage hydraulic fracturing is the primary method for shale oil development. However, it has significant environmental drawbacks including large volumes of wastewater, greenhouse gas emissions, and toxic air pollutants, primarily due to high energy and water consumption as well as the use of chemical additives. In addition, the primary depletion method after multistage hydraulic fracturing can yield oil recovery factors of only a few percent. Therefore, there is an urgent need to develop cleaner and more sustainable methods for shale oil recovery. Currently, mainly some huff-and-puff enhanced oil recovery (EOR) methods without fracturing shale oil samples are being investigated. In this study, we managed to successfully investigate the feasibility of CO2 and hydrocarbon gas (HCG) injection as a cleaner and greener method for ultra-tight shale oil development without fracturing through core flooding experiments combined with X-ray scanning in a continuous injection mode. The results show that continuous CO2 and HCG gas injection yielded a maximum of 75% oil recovery without significant asphaltene precipitation or permeability reduction. A piston-like displacement front was observed during miscible HCG flooding. A two-stage displacement process was observed: the first stage is controlled by miscible gas displacement where pore filling is dominated by the occurrence of snap-off. The second stage is a diffusion-driven displacement where molecular diffusion plays a more important role with subsequent oil swelling and viscosity reduction after diffusive mixing. The findings not only prove the feasibility of gas injection (CO2 and HCG) for developing ultra-tight shale oil without fracturing but also contribute to enriching the theory of gas flooding mechanisms.