Abstract: To study the dynamic changes during CO2 flooding in reservoir formations and to evaluate the progress of the miscible front in the oil reservoir in Block Fan 142 which possesses extremely low permeability,and to clarify phases and flooding performances in such reservoirs, dynamic changes in pressures were reviewed. Comprehensively using geologic data, production data, downhole pressure monitoring and reservoir numerical simulation techniques allowed the analysis of patterns in pressure changes and pressure build-up after CO2 injection in the Well ClusterF142-7-X4. Research results showed that formation pressures decreased quickly in stages with development by using natural energy. During pressure build-up with closed producers and injected CO2, the formation pressures in Well F142-7-3 and Well F142-8-3 of F142-7-X4 Cluster restored slowly, whereas pressures in the other the 4 wells was quickly restored. Considering analyses related to minimum miscible pressure front and CO2 concentration front both vertically and horizontally, the progres of miscible zones may be determined to establish the method to clarify the CO2 miscible zones. Research results demonstrate that the dynamic tracking of pressures might provide a reliable foundation to determine the volume and timing of CO2 injection. In addition, these results may provide effective ways to determine phases in CO2 flooding and to clarify boundaries of miscible regions.