Hydrocarbons in the Oulituozi oil-gas field, Liaohe Tertiary basin, Northeast China, are accumulated at the top of a subvolcanic trachyte porphyry. This porphyry was intruded in the Eocene Shahejie Formation, which is the principal hydrocarbon source of the region. Reservoir space is dominated by crypto-explosive breccias, crack networks, vertical open cracks, tensional microfissures and structural fractures. Crypto-explosive breccias and crack networks were produced by explosion of water and volatiles below a crystalline rind. Vertical open cracks were formed by the upwelling forces of magma from the depth. Tensional microfissures were produced by contraction of the crystallized magma on cooling. Structural fractures were created as a result of post-intrusion regional stress. In the surrounding rocks of the intrusion, there are numerous faults and fractures that connect the hydrocarbon sources to the fracture system of the trachyte porphyry. Hydrocarbons migrated to the present spaces after cooling of the intrusion.