Lithofacies And Paleogeography Of Middle Permian In Sichuan Basin

The Middle Permian in Sichuan Basin has great potential for oil and gas exploration and it has attracted the attention of many geologists.However,there are many different views on the development types and distribution characteristics of sedimentary systems in this period.This paper is based on the system in the Sichuan Basin in Permian Qixia-Maokou Formation as the research object which is taking sedimentology and lithofacies paleogeography discipline theory as the instruction.On the basis of field section geological survey,combined with a large number of newly added drilling and logging data in recent years,the division and correlation of the Middle Permian in Sichuan Basin are carried out,and the stratigraphic framework is constructed,and the characteristics of the plane distribution of the strata are determined.Then the sedimentary facies types and characteristics of Qixia-Maokou Formation in the Middle Permian in Sichuan Basin were studied and the sedimentary model was established.Finally,the lithofacies paleogeographic map of Qixia-Maokou Formation of Middle Permian in Sichuan Basin is compiled with the member as the mapping unit,which systematically reveals the lithofacies paleogeographic characteristics of Middle Permian.Qixia Formation of Middle Permian in Sichuan Basin consists of two parts and Maokou Formation of four parts.The bottom of Qixia Formation is dominated by carbonate rocks,and the local area is distinguished by siliceous rocks and aluminaceous clay rocks of the underlying Liangshan Formation.At the top of the Maokou Formation is carbonate rock,and the siliceous rocks in some areas are distinguished from the coal-bearing mudstone and siltstone of the overlying Longtan Formation.At the bottom of Maokou Formation,the eye globular limestone is distinguished from the massive limestone,dolomitic dolomite and gray matter of Qixia Formation.The curve characteristics of GR and RT are significantly different between different layers.The First Member of Qixia Formation gradually thickens from north to south and west to east.There is little change in the thickness of the Second Member of Qixia Formation.The members of Maokou Formation except the last one are thicker in the south than in the north and are thicker in the east than in the west.Affected by the massive eruption of Emeishan Basalt,the Forth Member of Maokou Formation is partially missing and completely denuded in Zitong-Zhongxian-Bishan-Guren-Nejiang area.There are mainly open platform facies,limited platform facies,platform marginal facies and continental shelf facies in the Sichuan Basin.Open platform facies is developed in the main part of the basin and shoal and open sea subfacies are developed in the platform.The restricted platform is developed in the southwest of the basin and the platform is dominated by shoal and lagoon deposits.The continental shelf facies are developed in Guangyuan-Dazhou in the north of the basin and southeast of the basin.The platform margin facies consists of platform margin shoal and inter shoal subfacies which are mainly around the shelf facies area.Early to late sedimentation of the Middle Permian Qixia Formation in the Sichuan Basin,the two-stage sedimentation shows an inherited development and the sea level has changed from rapid rise to slow decline.Influenced by the West Sichuan Central Yunnan oldland on the southwest edge of the basin,the sedimentary period of the first parts of Qixia Formation is generally stable from Emei area to the Northeast basin,with the development of restricted platform open platform sedimentation,which is the main part of the basin sedimentary pattern,in which there are many different scales of platform beach sedimentation;there are shelf sedimentation and platform margin facies on the Huayingshan Yunyang line.In the second member of Qixia Formation,the paleogeographic pattern changes little,the water body of the basin becomes shallower gradually,and the bioclastic beach dominated by the wengda algal assemblage is largely developed in the platform facies.In the early stage of Maokou Formation of Middle Permian in Sichuan Basin,sea level rose to the highest point.Later,with the occurrence of the Dong Wu movement,the dome shaped uplift of the crust formed by the rise of the Emei mantle plume made the Maokou Formation under the Emeishan basalt in the southwest of the Upper Yangtze Plate partially missing.During the period from the first to the third member,the main part of the basin is open platform.Influenced by the rise of Emei mantle plume and sea level,Kaijiang Liang platform basin and Deyang wusheng shelf,which are deep into the basin,are developed,and their scales are shrinking.Around the shelf,there are platform margin facies deposits.Mao-2 and Mao-3 are important beach-forming periods.During the sedimentary period of the Fourth Member of Maoxi formation,influenced by the large-scale eruption of Emeishan basalts,the sedimentary rocks remained little and were largely absent in the central part of the basin.In other areas,it is still represented by limestone deposition in open platform,and there are several platform shoals.The Kaijiang Liangping shelf in the North developed again,which laid the foundation for the development of karst reservoir and the development of reef beach reservoir in the later Changxing formation.