Abstract: The "thick top" structure is typical and extensive occurrence in Chaidam basin. It can be divided into several types such as 1)tip roof back; 2)two-layer texture; 3)round top tent; 4)fault coincidence; 5)low mound, and 6)straw hat. In vertical direction, these types of struc-tures are mainly distributed in Oligocene (E₃), Miocene (N₁) and Pliocene (N₂¹) of the basin; in lateral direction, they are mostly in the north-ern, western and central areas of it. The mechanism of this structural formation is due to plastic argillaceous strata under the environment of intense compaction and wrench deformation. For petroleum exploration in it, the "thick top" structure is in favorable source-cap conditions, and the reservoirs formed are dominated bu fractured ones.

Key words: Chaidam basin, structure, type, distribution, mechanism