The reservoirs of the sha一3 member in the Machang Oil Field is composed mainly of meditim—to fine—grained arkose．0n the basis。f a study of the CO!‘es，thin sections，grainsize，SEM，physical properties，X—ray diffractions，capillary pressures and diagram analy— sis of the pore structure，the paper discusses the lithological composition，diagenetic lea— tures，pore types，evolution and structures and potential sensitivity to reservoir conditions of sandstones．On that basis，an evaluation of classification of the sandstone reservoirs has been made．