Abstract: Reservoir type of the Ordovician Yingshan Formation in the Bachu-Maigaiti region and the main controlling factors for favorable reservoir generation are the keys for further petroleum explorations in the region.Based on core observation,thin-section analysis and well-logging interpretation,the Ordovician reservoir is fracture-pore type.According to trace element and carbon or oxygen isotope analyses,the reservoirs include fractures and corrosion pores formed under closed burial diagenetic conditions.Fracture generation belt controlled by the Caledonian and Hercynian movements is favorable for the Yingshan Formation reservoir distribution.Karst plateau and slope formed by long-time denudation of the Hetian ancient uplift are qualified for epidiagenetic karst reservoir generation,and are important for further exploration.