Abstract: WANG Qun-chao et al Arnan oifield,located100km to the north of Silin-haote in the Inner Mongolia Autonomous Region,is a lay-ered fault block oil reservoir with edge water and con-trolled by structural zones.The low permeability sandstone reservoir has highly developed faults,fractured structures,thick oil-bearing intervals in the major fault blocks,poor physical properties and serious inhomogeneity.Subdivision of layer series of development was carried out in major blocks with thick oil layers.Evaluation showed that devel-opment efficiency was optimal after the oil-bearing layers were subdivided into4to5layer series each of which control 7to8substratum and12to15m thick oil layer.Implementation of the development program enlarged pro-ducing reserves and improved waterflooding efficiency.