Abstract: Under the guidance of petroleum accumulation system (PAS) theory and combined with the data of geology, geophysics and geochemistry, the basic geological conditions and characteristics of the PAS in C sag within the Weixinan depression have been analyzed systematically, and its model for hydrocarbon accumulation has also been set up in this paper. The results show that the mid-deep lacustrine mudstone of the second member of the Liushagang formation is the primary source rock in the PAS. Faults, unconformitis and permeable sandstone layers constitute the hydrocarbon conduit system, in which vertical connectivities of faults and lateral permeabilities of unconformities are more important. Many kinds of structural traps, which match well with generation,expulsion,migration and accumulation of hydrocarbon are developed in this PAS. The favorable exploration objects in this PAS include traps (near C sag) accompanying with faults that connect source rocks with reservoirs and traps located in the Weixinan lower uplift with unconformities and connective permeable sandstone layers as effective conduit systems.