Songliao basin has principally experienced extensional and reverse stages. A set of near NW tension fractures and two sets of oblique shear fractures were chiefly formed in the stage of extensional structure

whereas two sets of near NE and NW shear fractures and one set of EW spreading fractures were developed in the stage of reverse structure. Fault growth activities could not cease basically until the late Qingshankou Formation. Structural fractures were not well developed due to sediments in plasticity in the Third Forth Member of Quantou Formation and Qingshankou Formation in particular. Most of fractures formed in the stage of extensional structure are discovered to be filled with calcite and in inefficient due to prolonged process of diagenesis. During the reverse stage

structural fractures were extensively formed by compressional process because the reservoir in the basin was already in deep diagenesis. Therefore

fractures in Sanzhao area were mainly formed in the reverse stage.