Abstract: The reservoirs of the Lower-Middle Ordovician Yingshan Formation in Ka1 region of themiddle Tarim Basin are mainly fissure and fissure-pore types.Reservoir generation and distribution areclosely related to fissures.Fissure identification,origin classification and orientation analyses were car-ried out with FMI data from 10 drilled wells.High-resistant fractures account for the first place in theregion,usually filled with calcite and mud.High-conductive and drilling-induced fractures account forthe second place.Drilling-induced fractures and borehole breakouts are controlled by present stressfield,while high-conductive and high-resistant fractures caused by tectonic genesis are closely related topalaeo-stress field.The present stress field in Ka1 region is mainly NE and NEE oriented.The palaeo-stress field during the Middle and Late Ordovician was mainly NNE and NEE oriented,and the NNEoriented fractures were formed before the NEE oriented ones.