Sequence stratigraphy of Wufeng-Longmaxi Formation in the southeastern Chongqing area and its geological significance

Based on the measurement and field observation of outcrop, the sedimentary and sequence characteristics of Wufeng-Longmaxi Formation in the southeastern part of Chongqing were studied deeply by combining drilling, well logging and measured data with the theories and methods of sedimentology and sequence stratigraphy, the geological significance for oil and gas was explored. The Wufeng-Longmaxi shale has obvious sedimentary features in the field, high degree of recognition, thickness from 60-130 m, siliceous shale in the lower part, gradually increasing sandy content and lithologic transition to muddy siltstone. Through logging curve characteristics, lithology, sedimentary response and sea level rise and fall, Wufeng-Longmaxi Formation can be divided into four third-order sequences. The transgressive system tract and the high stand system tract are developed symmetrically, with good continuity in the horizontal direction and obvious features in the vertical direction. The Sq1 transgressive system tract developed extensively in the study area, depositing a set of siliceous shale. The upper Sq2 began to develop high-level system tract deposition, corresponding to the deposition of argillaceous siltstone. In the lower part of Sq3, the mudstones of the transgressive system tract were developed, and Sq4 at high level system tract dominated the development of clay shale facies carbonaceous shale deposition. Wufeng-Longmaxi Formation was controlled by tectonic movement and sea level rise and fall during the whole sequence filling. The source of Wufeng-Longmaxi Formation mainly came from Xuefeng paleo-uplift on the southern and eastern fronts. The early volcanic thermal events provided siliceous rocks for a wide range of silicification in shale. At the same time, it provided the living conditions for plankton in this period and promoted the development of organic matter in the shale of Wufeng-Longmaxi Formation, which provided sufficient gas source for enrichment of shale gas.