Since the Late Paleozoic

the four large-sized basins(i.e.Ordos

Sichuan

Junggar and Tarim)in central-western China have experienced a similar evolution process from cratonic depression to intra-continental depression and finally to foreland(pseudo-foreland)basin

and their deposition features in thick clastic rocks.Hydrocarbon source rocks were widely developed in the intra-continental depressions

while thick high-quality source rocks were deposited in the pseudo-foreland basins

providing favorable basis for hydrocarbon accumulation in the clastic strata in the four basins.There are three types(inner-sourcing

out-sourcing and hybrid-sourcing)and six subtypes of petroleum accumulation systems.The inner-sourcing petroleum accumulation system is the major target for large- and medium-sized oil/gas reservoirs.For the inner-sourcing systems in the four basins

their oil and gas distribution has features of oil in upper part and gas in lower part as well as more gas to the south

their hydrocarbon accumulation were developed early in the north and late in the south.The temporal and spatial combination of source rock and reservoir is the major factor controlling hydrocarbon accumulation and enrichment.Slope zone and piedmont thrust belt are play fairways for these inner-sourcing systems.The distribution of the out-sourcing and inner-sourcing system are significantly complimentary

so greatly broadening the area and horizons for petroleum exploration in the clastic strata.The out-sourcing systems are distributed mainly in the south parts in the four basins.Hydrocarbon resource condition of marine strata and vertical migration pathways are major factors controlling hydrocarbon accumulation and enrichment in the out-sourcing systems.The paleo-uplift zones of marine strata and the Meso-Cenozoic faults zones are the play fairways for petroleum exploration in the out-sourcing system.