Sequence Stratigraphic Characteristics From The Late Triassic Through The Middle Jurassic In The Sichuan Basin

Sichuan Basin is the largest continental basin in the south of China,and it is a continental sedimentary basin developed after many tectonic movements beased on the Triassic Craton Basin.This thesis investigated the sedimentary facies characteristics,sequence stratigraphy,sedimentary center migration of Late Triassic to Middle Jurassic in Sichuan Basin,and the sequence stratigraphy of key oil and gas exploration interval(Shaximiao Formation),based on the logging,seismic,cores,field outcrops and other related data,with the guided by the theories of sequence stratigraphy,sedimentology,tectonic geology,seismic stratigraphy and logging geology.Four different sequence stratigraphic boundaries and their characteristics are identified,which includs(1)unconformity interface formed by regional tectonic uplift movement;(2)large scouring discontinuities;(3)erosion surface and related conformability surface(4)lithology,lithofacies and related conformability surface.The main unconformity interfaces since the Late Triassic are identified as follows:(1)unconformity between Xujiahe Formation and Leikoupo Formation(T3x/T2l);(2)unconformity between the second member and third member of the Xujiahe Formation(T3x3/T3x4);(3)unconformity between the Jurassic and Triassic(J1b/T3x6).According to the sequence stratigraphic boundaries,the strata from the Late Triassic to the Middle Jurassic are mainly divided into 4 tectonic sequences and 9 thirdorder sequences.The outcrop and core data of the Shaximiao Formation are deeply analyzed.Combined with regional survey data and seismic data,the sequence boundaries characteristics of the Shaximiao Formation,the Lianggaoshan Formation and the Suining Formation and the internal sequence division scheme of the Shaximiao Formation are re-combed,and the Shaximiao Formation is divided into two third-order sequences,corresponding to five fourthorder sequences.The regional stratigraphic division and correlation of the Shaximiao Formation are carried out,and the macroscopic distribution characteristics of each layer of Shaximiao Formation are analyzed and compared.The thickness of the Shaximiao Formation is generally characterized by thick in the north and thin in the south.The thickness of the first member of the Shaximiao Formation has a trend of thickening from southwest to northeast.The second member of the Shaximiao Formation shows a gradual thickening trend from the Ya’an and Leshan in the south and west to the Suining and Nanchong in the middle then to the Tongjiang in the north and east.It is determined that the first member of Xujiahe Formation in the western Sichuan Foreland Basin has experienced a transgression and regression process.The paleogeographic pattern is characterized by early open shelf shallow sea and late open shallow bay,with some rivers flowing into the north,east and south.The western part of the basin is connected with the Tethys Sea;the second member of Xujiahe Formation is mainly composed of fluvial and delta deposits.The third and fifth members of Xujiahe Formation are mainly lacustrine sediments.The fourth and sixth members of Xujiahe Formation are mainly developed with fluvial and delta facies deposits.There are four main sedimentary facies in Jurassic: alluvial fan facies,fluvial facies,delta facies and lacustrine facies.Alluvial fan are mainly developed in Zhenzhuchong Member at the bottom of Jurassic,and lacustrine facies are mainly developed in Dongyuemiao Member,Ma’anshan member and Da’anzhai member in the upper part of Ziliujing Formation.The Middle Jurassic strata are mainly delta and fluvial sediments with local lacustrine sediments.Based on the planar thickness characteristics of each sequence in Sichuan Basin,the the sedimentary center migrated from west and south to east and north during the Xujiahe Formation to the Shaximiao Formation,which indicated the Sichuan Basin underwent multiphase tectonic transformation.