The Mesozoic sedimentary-structural units within the area of 500000 km

2

to the westof the Helan Mountains

south of the boundary line

east of Xinjiang and north of the Qi-lian Mountains are dividid into Alxa depressed region (including 11 depressions) andBeishan uplift region according to recent data of geology and geophysical exploration.Thebasements are composed of metamorphic rocks belonging to Pre-Caledonian

Caledonianand Hercynian Epoches and the overlying beds are composed of Meso-Cenozoic sedimentswhich range from 2000 to 8000m in thickness.The structural evolution shows distinctdifferentiation in east and west parts.Middle-Lower Jurassic and Lower Cretaceous sediments are better developed in thewestern depressions which have the conditions for hydrocarbon generation

but the areasare usually not broad enough (in more than 80 depressions

only 40% have the areasgreater than 1000km

2

).The Middle-Lower Jurassic to the west of Ejin Qi-Alxa Youqi

and the Lower Cretaceous in the northwest part of the corridor

Zongnan and Zongbei De-pressions have better oil potential.