Sequence stratigraphic features of the Middle Permian Maokou Formation in the Sichuan Basin and their controls on source rocks and reservoirs

Well Shuangyushi 1 and Well Nanchong l deployed in the NW and central Sichuan Basin have obtained a high-yield industrial gas flow in the dolomite and karst reservoirs of the Middle Permian Maokou Formation, showing good exploration prospects of the Maokou Formation. In order to identify the sequence stratigraphic features of the Maokou Formation, its sequence stratigraphy was divided and a unified sequence stratigraphic framework applicable for the entire basin was established to analyze the stratigraphic denudation features within the sequence framework by using the spectral curve trend attribute analysis, together with drilling and outcrop data. On this basis, the controls of sequence on source rocks and reservoirs were analyzed. In particular, the Maokou Formation was divided into two third-order sequences – SQ1 and SQ2. SQ1 was composed of members Mao 1 Member and Mao 3, while SQ2 was composed of Mao 4 Member. Sequence stratigraphic correlation indicated that the Maokou Formation within the basin had experienced erosion to varying extent, forming “three intense and two weak” denuded regions, among which, the upper part of SQ2 was slightly denuded in the two weak denuded regions (SW Sichuan Basin and locally Eastern Sichuan Basin), while SQ2 was denuded out in the three intense denuded regions (Southern Sichuan Basin–Central Sichuan Basin, NE and NW Sichuan Basin). The development of source rocks and reservoirs within sequence stratigraphic framework was significantly affected by sequence boundary; the grain banks that can form effective reservoir were predominately distributed in SQ1 highstand systems tract (HST), while effective source rocks were predominately distributed in SQ1 transgressive system tract (TST). It is concluded that the sequence division method is objective and reasonable, which can effectively guide oil and gas exploration in this region.