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Impurities such as N 2 and O 2 can be co-injected with CO 2 to cut the overall cost of carbon capture and geological storage by lowering the CO 2 capture cost. However, only few field-scale studies have focused on the effect of these impurities, such as N 2 and O 2 , with low solubility and a low reaction rate in a CO 2 stream. A pilot-scale experiment on impure CO 2 aquifer storage containing N 2 and O 2 was conducted in Tongliao, Inner Mongolia, China, where 200 tonnes of CO 2 and 30 tonnes of air were injected into the upper Yaojia Formation. The geochemical characteristics of flow-back formation fluid in three monitoring wells were investigated by using a U-tube-based sampling system. The following conclusions can be drawn from the preliminary geochemistry analysis: (1) The underground chemical reactions in the CO 2 and air co-injection process are clearly pH and redox sensitive, which is in contrast with pure CO 2 aquifer storage. Oxygen enhances the remobilization of certain minerals in the aquifer formation and provides new reactive tracers such as uranium and SO 4 2− . (2) The migration pattern of the formation fluid and the geochemical components in the fluid are very complex, particularly when the reservoir exhibits high heterogeneities of geology, mineralogy, and groundwater chemistry. (3) Chromatographic partitioning processes of chemical components occur in both aqueous and gaseous phases. In the aqueous phase, the dissolved O 2 and CO 2 arrive at the monitoring wells earlier than they do in the gaseous plume. The CO 2 -induced ions arrive at the monitoring wells earlier than the O 2 -induced ions; however, the dissolved O 2 arrives earlier than the dissolved CO 2 . The N 2 and O 2 in the gas mixture arrive at the monitoring wells earlier than CO 2 and form the migration front of the underground gas stream. This study provides a new dataset for evaluating possible scenarios of underground migration behavior and metal remobilization in response to the co-injection of CO 2 with O 2 , and N 2 . [ABSTRACT FROM AUTHOR]