C

Ⅲ

oil gas-bearing group is the principal pay bed in Central Tarim Basin.It is composed of Donghe sandstone as reservoir rock and shale of Bachu Formation as sealing bed.The regoinal unconformity originated in ealier Hercynian tectonic movement did not only destroy the hydrocarbon pools formed earlier

but also stopped the main source rocks from hydrocarbon generating of Middle-Upper Ordovician.Since the Late Hercynian

the main source rocks started secondary hydrocarbon generating and expelling

and trap forming stage was the main pool-forming stage of the C

Ⅲ

group.Multi-tectonic deformation resulted in a series of Carboniferous third-order structural belts of east-west strike and a lot of traps.The spatial arrangement of the third-order structural belts formed hydrocarbon confluent ridges.The traps can develop into hydrocarbon pools or not are determined by the location whether the traps are on the ridges or not.