Abstract: According to origin mechanism and seismic reflectance on unconformity at Cretaceous bottom, unconformity in the studied area is divided into four types, each of which has different effects on the hydro- carbon migration and accumulation.Above and below the unconformity, there exist five types of lithology combinations: TypeⅠincludes glutenite above it, mudstone and sandstone below it; Type Ⅱ sandstone above it, mudstone and sandstone below it; Type Ⅲ glutenite above it, mudstone and glutenite below it; Type IV sand- stone above it, mudstone and glutenitebelow it; Type V grit above it, mudstone, sandstone and glutenitebelow it.Among them, Type Ⅰ and type Ⅱ are the most favorable to oil and gas migration and accumulation in this area, for this unconformity and fault composed of channels of good conductivity, the leached zone below it acts as a better passage for petroleum migration, oil and gas can be migrated laterally and effectively by the weathered argillic horizon as a barrier above it.While the basal conglomerate and overburden thick sandstones cut by normal faults formed in late period of Himalayan movement have enabled oil and gas to migrate upward rapidly.The hydrocarbon reservoirs at Cretaceous bottom unconformity could be shaped by three styles of barriers such as argillic horizon barrier, leached zone half-weathered mudstone barrier and thin layer barrier.

Key words: Junggar basin, Luliang uplift, unconformity, lithology combination, leached zone, argillic horizon, basal conglomerate