Abstract: The Ordovician oil-gas field in Tazhong No.1 slope-break zone in Tarim basin is the first large-sized field where the reservoir is dominated by reef and shoal complex facies. This field is characterized by big differences of testing production and gas/oil ratio in different wells, complex oil and gas properties, having both normal oil and gas condensate, generally containing H2S in natural gas, and varied contents of non-hydrocarbon gas in different well blocks. The study shows that the gas is rich in the northern part and the oil is rich in the southern part; such distinctive oil and gas in sectional distribution mainly originated in the Cambrian. It is concluded that Himalayan movement is the main reason for forming this large-sized field of Ordovician due to cosmically gas invasion and better preserved conditions in this area.

Key words: Tarim basin, Ordovician, reef, oil-gas accumulation, condensate gas