Abstract: Low saturated reservoirs of Hongtai area mainly existed in middle Jurassic Xishanyao formation,with characteristics of poor reservoir physical property,high salinity of formation water,big water type change,complex oilgas-water relation,high bound water saturation and big difficulty in logging identification.Based on the drilling,logging,well testing and analysis test data,through comprehensive analysis of sedimentary environment,structure characteristics and lithology,the genetic mechanism study of low saturated reservoirs in Xishanyao formation of Hongtai 2 and 3 structures was carried out both from macro and micro aspects.The results indicated the deposition hydrodynamic conditions and the hydrocarbon accumulation period played a controlling role in reservoir saturation level.From micro aspect,there existed difference between the low saturated reservoirs and the conventional reservoirs,the pore type was mainly of secondary pores,with developed micro pore throats and high content of clay minerals such as kaolinite,chlorite and illite.Vertically,it was characterized by heterogeneity between each other.Affected by multiple factors,the saturation was lower while the irreducible water saturation was higher in Xishanyao formation of Hongtai area.