The reservoir rocks of A'nan sandstone oilfield are dominated by feldspar lithic sandstone

and are proximal subaqueous fan deposits with prominent influences of volcanic activity.They feature in high content of pyroclastic debris and tufaceous cements

low composition maturity and structure maturity.Although their burial depth is shallow

epidiagenesis such as compaction and cementation are intense

leading to the poor development of primary porosity.As the major secondary pores

the dissolution pores feature in micro-pore throat and poor sorting of pore throat

resulting in poor storage capacity and permeability.Therefore

A'nan oil reservoir is typical low permeable pyroclastic sandstone reservoir.The pore structure deteriorates during water flooding due to various factors such as water sensitivity and velocity sensitivity.As a result

the displacement-pressure increases

while water-free oil recovery and ultimate oil displacement efficiency decrease.During waterflooding

water injection pressure increases continuously

and pressure buildup is significant

but well deliverability is commonly low.Several measures such as layer-subdivision and infill drilling are carried out to improve oilfield development.