Based on the analysis of lithology, rock structure, sedimentary structure, profile texture, and paleontological combination data from Block-6 of Tahe Oil Field, it is concluded that the Ordovician reservoirs consist of continuously developing limestone that deposited in the environment of carbonate platform facies. The large scale of karst pipelines that formed by dissolution along the joints or fissures are the favorable reservoir space. Porphyritic dolomitic calcarenite develops in reservoirs and generally contains oil. The dolomization in the reservoirs is determined by the specific tidal flat-lagoon environment in the platform margin in this located area. The dolomization improves the reservoir property and produces large amounts of secondary pores due to decreasing rock volume during the transformation from calcite to dolomite. Fractures and stylolites mainly develop in homogeneous micritic and microcrystalline limestones which are generally formed in the low-energetic low-laying lands of carbonate open platform. Thick-bedded homogeneous micritic and microcrystalline limestones with fractures is favorable to karstification. Existence of high angular faults and fractures promoted developing a lot of vertical karst caves while existence of low angular fractures and changing phreatic surfaces affected the migration of underground rivers which made these limestones corrode along the beds and shapes several sets of large-scaled bedding caves. These bedding caves became the most important reservoir space in Tahe Oil Field.