Abstract: In recent years, great success for petroleum exploration of the Ordovician Yingshan formation in north slope in Tazhong area of Tarim basin has been achieved, and the quasi?layered and large?scale hydrocarbon accumulation province has begun to take shape. The Yingshan karst reservoir is characterized by multiple phases, complicated oil, gas and water distribution and no unique oil?water contact.This paper presents fine studies of two subsidiary structural zones of Tazhong?I and Tazhong?10, the fluid distribution of Yingshan formation and the most complicated Zhonggu?51 Wellblock. The biomarker correlation of the crude oil and the comprehensive analyses of the deposition, structure, drilling and formation testing of Zhonggu?51 Wellblock show that the hydrocarbon accumulation zone is found in the karst reservoir developed in 30~180 m apart from the top of Yingshan formation, especially at the lower Ying?1 member and the Ying?2 member, dominated by mixed hydrocarbons in origin; the early charged hydrocarbon and the late high mature gas charging occur along twomajor strike?slip faults, making the early oil pool turn into condensate pool, and far apart from the faults appear major oil pools with some gas pools; the Zhonggu?51 Wellblock is surrounded by four strike?slip faults and two rows of overthrust faults (Tazhong?10 and Tazhong?I faults) and divided into three fault blocks in which water is located in the lower part and oil?gas in the higher part, and relatively sealing of oil and gas among them. Therefore, it is suggested that the next drilling or exploration in this area should focus on the higher parts in the fault blocks