Middle-Upper Permian Siliceous Rocks In Lower Yangtze Area Origin And Shale Gas Potential

The Middle-Upper Permian in the Lower Yangtze area develops siliceous rocks and organic-rich black rock series,which are important target intervals for shale gas exploration.The main siliceous rock intervals include the Middle Permian Gufeng Formation and the Upper Permian In the Dalong Formation,there are still big differences in the understanding of the siliceous source,genesis and tectonic environment of siliceous rocks,which makes it difficult to accurately evaluate the potential of Middle-Upper Permian siliceous shale gas.In this paper,the Gufeng Formation in Well Gangdi 1 of Yangtze District and Dalong Formation in Niushan Section of Xuancheng District are the main research objects.Through field survey,core observation,rock thin section identification,main trace elements,XRD,organic geochemistry and other analysis,The source of siliceous material,the genesis of siliceous rock and the tectonic environment when the siliceous rock was formed have been studied in detail.On this basis,the deposition and distribution characteristics of the Gufeng Formation and the Dalong Formation in the study area are analyzed,and then the potential of shale gas in the target interval is evaluated,and the favorable shale gas zone is clarified,with the hope that it will be the best shale gas in the Lower Yangtze area.The main conclusions are as follows:(1)According to the characteristics of lithology,biofossils and element geochemistry,the Gufeng Formation and Dalong Formation were classified in detail,and the source,genesis and tectonic environment of siliceous rocks were analyzed.Research shows that the Gufeng Formation can be divided into 3 sections from bottom to top.The Gu-1 section is a calcareous mudstone-mudstone section,containing biological fossils such as foraminifera,intercalated with thin siliceous rocks,and has the characteristics of hydrothermal and biological integrated genesis.The tectonic environment formed on the continental margin;The Gu 2 Member is characterized by the interbedded black siliceous rock and siliceous mudstone,and a large number of radiolarian fossils are seen.The siliceous rock is mainly of biological origin and mainly formed at the transition from the continental margin to the ocean basin The Gu 3Member is mainly mudstone with a small amount of siliceous rock,formed in the environment of the continental margin.The Dalong Formation can be further divided into 3 members.From The Long 1member is mainly gray-black siliceous mudstone and shale.The siliceous and siliceous mudstone are formed in the environment of the continental margin due to the integrated biological and hydrothermal origin;The Long 2 member is mainly siliceous rock And siliceous mudstone,containing a lot of radiolarian fossils,siliceous rock is of biological origin,mainly formed in the transitional environment of continental margins to ocean basins.The Long 3 member is dominated by calcareous mudstone,and siliceous rock is mainly of hydrothermal origin,formed in the tectonic environment of the continental margin.(2)Through stratigraphic comparison and sedimentary facies research,clarified the lithofacies paleogeographic characteristics of the Gufeng Formation and Dalong Formation and the distribution characteristics of siliceous rocks.Siliceous rocks of the Gufeng Formation and Dalong Formation in the Lower Yangtze District are mainly formed in shelf-basin facies deposits.After the Qixia Formation was deposited in the Middle Permian(Gufeng Period),a large transgression occurred in the Lower Yangtze area,and the seawater became deeper.The original carbonate open platform of the Qixia Period turned into offshore siliceous rock deposition,but the ancient topography remained It inherited the trend of high southeast and low northwest in the Qixia period.The sedimentary facies of the whole area are simple and stable,most of the areas are basin facies and shelf facies deposits.From the east to the south of southern Anhui,the carbonate platform environment is formed in the northern Jiangxi area.The sedimentary facies are distributed in a north-east direction as a whole,showing a basin-shelf facies paleogeographic pattern,accompanied by many small depressions.As a whole,the thickness of the Gufeng Formation deposits gradually becomes thinner from the southwest to the northeast.During the Changxing period of the Late Permian,a large-scale transgression was experienced,which invaded from south and south west through the Yangtze Sea,and most of the study area was submerged by the Yangtze Sea.The Changxing Period presented a platform-platform edge-shelf facies sedimentary pattern as a whole.The lithology and lithofacies change boundaries were clear,and the change position of the platform and shelf was roughly on the Jiangyin-Guangde line.To the west of this line are the Dalong Formation siliceous rock and siliceous mudstone deposits in the semi-occluded sea stagnant shelf environment,intercalated with siltstone and thin limestone.The east of this line is mainly limestone of the Changxing Formation in the platform environment.(3)The distribution of organic-rich shale in the Gufeng and Dalong formations in the Lower Yangtze area and the geochemical characteristics of the shale gas were investigated.The shale gas potential of the Gufeng and Dalong formations was comprehensively analyzed,and the shale was selected.Favorable zones for gas exploration.The Middle-Upper Permian dark mudstone is developed in the Lower Yangtze area,among which the black mudstone of the Gufeng Formation is generally distributed between 20?80 m,and the sedimentary thickness of the Dalong Formation is mostly20?40 m.The distribution of organic carbon in source rocks has changed significantly.The Gufeng Formation and Dalong Formation have high organic carbon content.The TOC of the shale of the Gufeng Formation is generally higher than 5%,and the organic carbon content of the Dalong Formation is generally 3?5%.The organic matter types of Gufeng Formation and Dalong Formation are mainly type II and III,with a small amount of type I.In general,there are many sets of organic-rich shale developed in the Middle-Upper Permian,with large single layer thickness and cumulative thickness,high organic carbon content,good type,and maturity is mainly in the early stage of high maturity-over-mature,which is in favorable gas generation.Within the scope of this period,shale gas has great potential.The comprehensive analysis method was used to analyze the potential of shale gas in the study area,and the favorable areas for shale gas were selected.From the perspective of geological conditions,the Gufeng Formation and Dalong Formation shale gas have great potential,especially the Gufeng Formation and Dalong Formation2 are the most enriched siliceous intervals,with favorable fracturing conditions and moderate organic carbon content.Between 10.35% and 5.67%,they are favorable intervals for shale gas extraction,and are the most potential target intervals in the study area.Among them,the Wuwei Basin along the Yangtze River and the Permian mud shale shale gas in the Xuanguang Basin have obvious advantages.,Is the first choice for Permian shale gas exploration in the study area.