Abstract: The area 2 of Tahe oilfield of the Ordovician Yijianfang Formation reservoir is carbonate reef flat facies strata.This paper studies the characteristics of tectonic and sedimentary by analyzing reservoir cores,rock thin section,logging data and geochemical characteristics.This paper also reconstructs the top surface of Early Hercynian karst palaeogeomorphology of the study region and surrounding area by the method of combination of well and seismic,and the method of refilling and polishing.The surface is divided into 4 sub-geomorphic unit.We analyze the reservoir characteristics and diagenesis of the Ordovician reservoir,and evaluate the karst stages.The results show that the Early Hercynian karst and karst tectonic fracture is the main mechanism of reservoir reconstruction.Through the current drilling fracture cave unit distribution,combined with Early Hercynian karst palaeogeomorphology,ancient water system and fracture control,the favorable area for reservoir development zone is identified.The ring T443,TK220,T207 well near discharge Wadang northern region are the most favorable areas of karst reservoir.