The Provenance Of The Xujiahe Formation In The Sichuan Basin And Remganetization Of The Hydrocarbon Bearing Strata In Yangtze Block

This thesis is composed of two parts of study.The first part is related to the provenance of detrital zircons in the Late Triassic Sichuan foreland basin.The other part of study is related to the remagnitization of hydrocarbon bearing strata.During the Late Triassic,the closure of the Paleo-Tethys Ocean involved several blocks(e.g.South China,North China,Qaidam and Qiangtang)that converged to form the main part of the present central China(Roger et al.,2010;Ding et al.,2013).During that period,the Late Triassic Sichuan foreland basin formed and filled,and it was flanked to the west by the Longmen Shan thrust-fold belt and Songpan-Ganzi turbidite complex,to the north by the Qinling-Dabie Orogen and North China Block,and to the east by other regions of Yangtze Block and Western Jiangnan Orogen.Given this geologic context,the sediments in the foreland basin may hold answers to the sources of the sedimentary basin,the evolution of the surrounding mountain belts and the crustal growth history of central China.In order to reveal these information,we conduct the in situ U-Pb and Lu-Hf isotope data for Upper Triassic detritus.U-Pb age data for the Late Triassic detrital zircons generally show populations at 2.4-2.6 Ga,1.7-1.9 Ga,710-860 Ma,410-460 Ma,and 210-300 Ma.By fitting the zircon data into the tectonic,sedimentologic,and paleographic framework,we propose that the north Yangtze Block and South Qinling-Dabie Orogen were the important source areas of sediments in the northern part of the foreland basin,whereas the Longmen Shan thrust-fold belt was the main source region for detritus in other parts of the foreland basin.The South Qinling-Dabie Orogen could also have served as a physical barrier to block most detritus shed from the southern North China Block into the foreland basin during the sedimentation of the Xujiahe Formation.Our results also reveal that part of the flysch from the eastern margin of Songpan-Ganzi region had been displaced into the Longmen Shan thrust-fold belt before the deposition of the foreland basin sediments.In addition,the Lu-Hf data indicate that Phanerozoic igneous rocks in central China show insignificant formation of juvenile crust.Extraordinary pervasive remagnetization has occurred in the South China,and the maturation of organic matter and migrated hydrocarbon might be two of the mechanisms.This study examines the lower Silurian shale of the Longmaxi Formation,which is also the most important target for shale-gas exploration in China.To test a hypothesized connection between hydrocarbon generation and re-magnetization,and contribute to shale-gas exploration in the region,we undertook an integrated magnetic,geochemical,and petrographic study of 160 specimens from the shale.The results suggest that the shale contains a reliable remanent magnetization(Dec = 41.4°,Inc = 40.8°,?95 = 6.8°).The magnetization predates tilting,and the paleopole plots close to the Late Triassic segment of the South China apparent polar wander path.The rock magnetic data and scanning electron microscope(SEM)observations confirm that framboidal magnetites carry the bulk of the magnetization,which suggest a Late Triassic chemical remanent magnetization in the shale.87Sr/86Sr and magnetic analyses indicate that the amount of magnetite was unaffected by fluid alterations around the veins,but is strongly covariant with the amount of total organic matter.Moreover,SEM observations reveal possible evidence of the replacement of pyrite framboids by magnetite,probably in the presence of organic acids.These analyses,therefore,suggest that the remagnetization was caused by organic maturation rather than orogenic fluids,and that the maturation occurred in the Late Triassic.This timing of organic maturation has been validated by independent modeling studies,and provides important constraints on the complex thermal history of the Longmaxi Shale as well as contributing to shale-gas exploration efforts.Another remagnetization is related to the migrated hydrocarbon,and we examine the dolomite in the Dengying Formation in the Yuhang region.The oil contaminated dolomite contains a reliable remanent magnetization which is close to the one of the Early Palaeozoic,and the magnetic carrier is the greigite.However,the un-contaminated dolomite samples show the unstable remanent magnetization,and the hematite is the main ferromagnetic minerals.The strong covariance of NRM intensities with TOC contents suggest that the migrated hydrocarbon was responsible for the remagnetization.In other words,the path of migrated oil always shows abnormally high NRM intensities,which can benefit the oil exploration efforts.