1. Shenzhen Branch of CNOOC Ltd.( China) ，Shenzhen 518067，Guangdong，China;
2. School of Geosciences，China University of Petroleum ( East China) ，Qingdao 266580，Shandong，China

 The characteristics and main controlling factors of the low permeability sandstone reservoirs in the Zhujiang Formation of Yangjiang sag X oilfield have been studied by core observation，rock sample test，rock microscopic observation，scanning electron microscope and mercury injection testing． The results show that the low permeability reservoirs in the Zhujiang Formation of X oilfield are mainly feldspar lithic quartz sandstone，of which the lithic types consist mainly of acidic eruptive rocks and mica quartz schists，with medium to poor separation． The compaction is relatively weak and the cementation of carbonate and argillaceous is strong． A large number of argillaceous carbonate cements and clay mineral cements produced in the delta front sedimentary environment fill intergranular pores，block pore throats，and lead to low permeability of sandstone． The dissolution of feldspar is widespread，but the porosity and permeability increasing are not obvious． Pore type is mainly primary intergranular  pore，and pore throat connectivity is poor． The delta front sedimentation controls the rock composition，particle structure，argillaceous matrix，carbonate source and content of the reservoir． Carbonate and argillaceous cementation are the key factors leading to low permeability of the reservoirs． Low permeability argillaceous sandstone is vertically distributed among the thick mudstones of the prodelta facies． Low permeability calcareous sandstone is mainly distributed at the junction of large mudstone and sandstone． Underwater interdistributary bay mudstone provides carbonate material source for the delta front sedimentary sandstone．