Abstract: Inter - well communication ratio and control degree of waterflooding in Chaheji oilfield, a complex fault block reservoir of fluvial facies, are still low after primary well pattern thickening. A secondary infill development experiment on a single well group aiming at increasing inter -well communication was carried out in 1999, so as to determine whether the well pattern could be thickened further at 180 to 200 well spacing and to verify the economic benefits of incremental oil. Experiment results showed that interconnecting rate of oil layers went up by 17. 1% and waterflooding recovery rate increased by a big margin.