Abstract: This paper utilizes theory and method for dynamic system of hydrocarbon migration and accumulation to classify Baiyinchagan sag in Erlian basin into the upper and the lower systems. The lower one is high pressure-self source-half closed system which is oil-rich in the south ramp belt and north steep slop belt, and the upper one is low pressure-mixed source-half closed system which is oil-rich in the south ramp belt. In resent years, the breakthrough of petroleum exploration with several tens of million tons of incremental reserves in this area proves this theory and method can be successfully applied to the risk oil exploration of old sags.

Key words: Baiyinchagan sag, hydrocarbon accumulation, dynamic system, abnormal fluid pressure, reservoir-forming pattern