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.
