Abstract: Field cyclic CO 2 injection operation has been implemented in 13 wells/times in a complex small fault-block reservoir in Qintong depression, Jiangsu oilfield. Totally 5,755t CO 2 have been injected leading to an incremental oil production of 3,390t (excluding production in 2002). Production enhancement effects, however, vary greatly in different wells. Mathematical statistics and analysis are carried out in three different types of reservoirs including high water-cut reservoirs, low permeability low efficiency reservoirs and heavy oil reservoirs. The relationship between incremental oil production and experimental parameters such as rate of injection, shut-in time and flowback is established and the factors that influence the effects of cyclic CO 2 injection are analyzed. Cyclic CO 2 injection has the advantages of small investment needed and taking effects quickly and is suitable for use in small fault-block reservoirs.