Abstract: Yanchang Formation is an important exploring layer of Zhenyuan-Jingchuan Block in southwest Ordos basin. Research indicates that thick sandstones of Chang 8 Formation are composed of subaqueous distributary channel sandstones, mixtures of subaqueous distributary channel sandstones and mouth bar, sheet sandstones over mouth bar and slump sandstones, and thick sandstones of Chang 6 Formation are consisted of thin mouth bar sandstones and thick subaqueous distributary channel sandstones by analyzing sequence stratigraphy and sedimentology of two thick sandstones of Chang 8 and Chang 6 Formation based cores, well logs, slices and particle size analyzing. Only the sandstones called as sheet sandstones over mouth bar can enrich petroleum which formed under the background of high accomodation, high sediment supply and intense current in Chang 8 Formation. Relatively, petroleum enriches in subaqueous distributary channel sandstones which formed under the background of low accomodation, high sediment supply and intense current in Chang 6 Formation.

Key words: Zhenyuan-Jingchuan Block, Yanchang Formation, sequence stratigraphy, subaqueous distributary channel sandstones, sheet sandstones bar