Abstract: Based on gravity, magnetic, electric, drilling and geologic data of peripheral basin, basement of Yanqi basin in Mesozoic and Cenozoic were classified into four structural types: imbricate, ramp, back thrust and basement arch.The lithology includes Proterozoic catametamorphic rock in south margin, granite in Hercynian period in northern margin, Mid-Upper Carboniferous epimetamorphic rock in Sishilicheng sub-sag and Mid Devonian epimetamorphic rock to the north of Yannan fault.The lithofacies belt extends from east to west, and is zoned axially.The basement is of Tarim platform to the south of Zhongmachang fault and of Hercynian fold to the north.The latter probably form anticlinal trap in the caprock due to its higher compressibility and unstability.The caprock has higher organic maturity resulting from deeper geothermal flow, so oil and gas are enriched in northern Yanqi basin, which is evidenced by the fact that the discoveries so far took place in northern Yanqi basin.

Key words: Yanqi basin, Basement structure, Basement rock, Structure