Research On The Permian Sedimentary System In The Eastern Sichuan Basin

The Sichuan Basin is one of the main oil and gas resource producing areas in China,and the Permian as an important oil and gas producing layer in the basin has great exploration and development potential.However,due to the imbalanced exploration and development of various blocks,the lack of in-depth research on sedimentary strata in local areas,and the disunity of the early Permian stratigraphic division standards,it is difficult to further carry out the oil and gas exploration of the Permian in the area.This paper takes the Permian strata in the eastern Sichuan Basin as the research object,based on previous studies,guided by stratigraphy and sedimentology theory,using outcrop profile analysis,stratigraphic division and comparison,sedimentary contrast analysis,and lithofacies palaeogeography map compilation and other methods and means have systematically carried out the study of the Permian sedimentary system in the area.This paper aims at the target stratum in the study area,on the basis of the preliminary exploration work,selects the continuous strata with good exposed profiles,observed and described in detail in terms of color,thickness,lithology,sedimentary structure,paleontology,etc.,the characteristics of the field outcrop are summarized,and the typical geological boundary characteristics in each layer of the Permian system are clarified.At the same time,the grinding pieces are taken layer by layer,and the field sections are improved through thin section identification and correction,the comprehensive histogram of typical section is established,and then the lithology,thickness characteristics and vertical and horizontal distribution of Permian group of the study area are studied.Combined with regional sedimentary background data,the types of Permian sedimentary facies in the eastern Sichuan Basin are analyzed and summarized.The types and characteristics of sedimentary facies and sedimentary subfacies in the study area are analyzed and described in detail.It is considered that the Permian in the east of Sichuan is a set of marine carbonate deposits as a whole.The types of sedimentary facies mainly include open platform,restricted platform,platform margin reef beach,carbonate gentle slope,platform margin slope,platform basin,etc.Among them,open platform facies can further divided into shoal,open sea and intertank sea subfacies;restricted platform facies can be divided into tidal flat and lagoon subfacies;platform margin reef beach facies can be divided into shoal and reef subfacies;gentle slope facies can be divided into inner gentle slope,middle gentle slope and outer gentle slope subfacies;platform margin slope facies can be divided into upper slope and lower slope subfacies.The sedimentary facies of the section in the area are divided and compared,and it is believed that the Middle and Upper Permian are dominated by open platform facies and gentle slope facies.In the Late Permian,influenced by the Dongwu movement,the paleogeographic pattern of the study area has changed.Local restricted platform and platform basin sedimentary facies are developed in Wujiaping Formation,and siliceous rocks are developed in Changxing Formation/Dalong formation in large area,from west to east,the water body gradually deepened,from open platform/gentle slope to platform basin sedimentary environment.In order to further reshape the sedimentary environment,the Permian lithofacies paleogeography map of East Sichuan Basin is compiled with the formation as the unit.At the same time,the Permian carbonate sedimentary model maps were established,and the lithofacies palaeogeography characteristics and evolution rules of the middle and Upper Permian are analyzed.Although the study area has experienced many transgressions and regressions,the paleogeographic pattern has not changed much,and it is still dominated by open platform and gentle slope sedimentary environment.The Permian as a whole has a paleogeographic pattern of high in the west and low in the east.The water body gradually deepens from the west to the east,and the hydrodynamic conditions gradually weaken.On this basis,the basic conditions of oil and gas geology are preliminarily discussed.It is considered that the source rocks are mainly located in Qixia Formation,Wujiaping formation,Dalong Formation and rarely exposed Gufeng Formation,which are divided into two types:muddy and carbonate rocks;the reservoirs are mainly located in Maokou Formation,Wujiaping Formation and Changxing Formation,which are mainly carbonate rocks;the cap rock is developed in each group,of which the Qixia Formation,Wujiaping Formation and Dalong Formation is the main cap rock of Permian in the basin.The Permian in the study area is mainly composed of the underlying and overlying continuous source reservoir cap assemblage and the discontinuous source reservoir cap assemblage.