Unconformity Characteristics of Sichuan Basin and Its Geological Implication

Featured by an obvious multicycle,Sichuan Basin encountered four tectonic evolutions of pre-Sinian(AnZ) basement formation stage,Z_1-T_2 cratonic basin stage,T_(3x)~(1-6) foreland basin stage and Jurassic-Miocene depression basin stage.There are 21 sequence boundaries developed in the basin and many tectonic events brought about the formation of multistage unconformities. These unconformity interfaces include transgressive onlap sequence and exposure sequence;and the interfaces are composed of karst leached belt,dissolution residue,sedimentary sequence (shallower upward),various exposure markers of both crust and mud crack.Three unconformities(e/ Z,D / S and T_3 / T_2)of Tong-wan, Caledonian and Indosinian orogenies are significant to form a reservoir.Both trap and dissolution pore can form between the upper and lower of unconformity;and reservoir condition can be improved.Meanwhile,unconformity is a favorable migration pathway for oil and gas.