Abstract: According to cores,logging and testing analysis,the sedimentary characteristics of Chang-9 reservoir in northwest region of Ordos basin were studied.The results indicated it wholly belonged to marsh environment and mainly developed braid-river delta plain subfacies.The sedimentation was controlled by two provenance systems in northwest and northeast,especially the northwest provenance.The petrographic study showed the sandstone in Chang-9 reservoir were mainly of arkose and lithicarkose with low compositional maturity and moderate structural maturity.Affected by the sedimentation,the sand body of Chang-9 reservoir was vertically characterized by uniform type without graded change and composite graded type.The main reservoir body was of distributary channel microfacies and it was good for oil accumulation.