The Carboniferous-Permian System in Ordos Basin was formed in marginal barrier sea

→

delta

→

river

→

limnetic environment

and constitute a sedimentary sequence of silicarenite

→

detrital silicarenite

→

clastic sandstone in ascending order.The Benxi

Taiyuan

Shanxi Formations of the System are composed respectively of three subsequences.River channel sandbodies and deltaic front sandbodies are favourable reservoirs for natural gas;the reservoir spaces of the reservoirs could be classfied into four types such as big pore coarse throat type(I)

big pore fine throat type(Ⅱ)

small pore fine throat type(Ⅲ)and fine proe minimal throat ytpe(Ⅳ).Among them

type I occupies 11.19%

type Ⅱ occupies 50.53%

type Ⅲ occupies 28.06% and type Ⅳ occupies 9.5%.Commercial gas flows are usually preserved in the first three types.