Energy storage huff-n-puff is an effective way to improve the low recovery rate of small fault-block oil reservoirs, in which the effective well-pattern is hard to establish. At present, there is a lack of research on media optimization and mechanisms of energy storage huff-n-puff in low-permeability small fault-block reservoir. Based on online nuclear magnetic resonance (NMR) technology, the effect of 6 different injection medium on huff-n-puff in low-permeability reservoir were systematically compared, including formation water, surfactant AH0603, CO₂, N₂, and surfactant assisted gas. More importantly, the microscopic oil utilization degree of different media was characterized. The results show that the uptake and extraction degree of the surfactant system AH0603 is 39.54%, which is 20.9% higher than that of the formation water. The AH0603-assisted CO₂ uptake and recovery rate is the highest, reaching 42.13%, which is 2.34% higher than the AH0603-assisted N₂ uptake, and 6.13% and 9.01% higher than the CO₂ and N₂ uptake respectively. It can be seen that AH0603 auxiliary CO₂ uptake and release can improve the utilization effect of crude oil in pores ranges from 0.2 μm to 2 μm, significantly increase the lower limit of pore utilization, and the on-site effective utilization distance is 2.99 m. This study reveals the microscopic seepage laws of multiphase fluid flow in low-permeability and small-block oil reservoirs, providing a basis for the energy storage and uptake development technology of complex oil reservoirs and technical support for the CO₂ flooding and co-storage mechanism.