Sequence Stratigraphy And Depositional Models Of The Middle Permian In Sichuan Basin

This study is carried to better understand the favorable reservoir zones in Middle Permian, Sichuan Basin. The principles of Sequence Stratigraphy and Sedimentology are introduced to this study. Also sequence stratigraphic framework, sedimentary facies characteristics, lithofacies palaeogeography characteristics and sedimentary models are studied.Firstly, an unconformity and a lithofacies shift surface were identified within Middle Permian by outcrop measurement of the Yangtze River, and divided the Middle Permian into three 3-order sequence stratigraphy units. The PSQ1 is composed of Liangshan Formation and Qixia Formation. While The PSQ2 and PSQ3 form Maokou Formation.Secondly, the sedimentary facies characteristics of Middle Permian in Sichuan Basin is concluded by the reference to previous studies, analysis of rock components, tectonic setting, logging characteristics, seismic profiles and vertical sedimentary sequences. Therefore, three major sedimentary facies are identified in study area: middle ramp, external ramp and open internal ramp. And then the map of lithofacies palaeogeography can be drawn and sedimentary model of Middle Permian in Sichuan Basin can be built.Thirdly, under the restraint of the PSQ1 framework, natural gamma logging data and natural gamma spectrometry logging data are further analyzed by using different data processing for one-dimensional spectrums analysis. Comparing their different effects on identification of Milankovitch Cycles, a more effective logging method are proposed to divide high-frequency sequence more accurately.Fourthly, a reasonable program theory and technology for analysis of high-frequency sequence stratigraphy are concluded. The theory and technology are based on the analysis of the one-dimensional spectrums, digital filtering processing and Two-dimensional spectrum analysis. Exploration practice results indicate that the high-frequency sequence stratigraphy boundary might be subjected to short-term exposure and experienced dissolution and finally formed paleokarst among interlayer. This high-frequency sequence stratigraphy boundary can also exist in Transgressive Systems Tract.Finally, the favorable reservoir zone can be predicated by the analysis of the sedimentary facies evolution process and carbonate depositional system in gentle slope. And the reservoir can be divided into four levels. Ancient weathering crust formed on the top of Maokou Formation in Luzhou uplift is the most important favorable reservoir. And the paleokarst reservoir developed along the layer in the margin of Luzhou ancient uplift is the secondly favorable reservoir. Fractured reservoirs developed in eastern Sichuan are the third favorable reservoir. Finally, Karst reservoirs developed among interlayer which are formed under short-term exposure and dissolution are the fourth favorable reservoir.