Early Cretaceous Sedimentary Records And Provenance Analysis Of The Upper Member Of Shahezi Formation In Songliao Basin

The large thick Cretaceous strata have been deposited in the Songliao Basin,of which the Shahezi formation was developed in the syn-rift stage of tectonic evolution during the volcanic intermission period.Because of its special sedimentary environment,it has became an important reservoir in the deep part of the basin.The target layer studied in this thesis is the upper member of the Shahezi formation.The analysis of the sedimentary environment and the source area has great significance for restoration of the paleoenvironment and the exploration of oil and gas.Through the detailed description of the core,by using the structural unit division method,11 types of lithofacies have been divided,including Gm,Gs,Sm,Ss,Sr,Sh1,Sh2,Fsc,Fl,Fm,C.Fan delta facies and lacustrine facies can be identified according to the combination law of the lithofacies,which are divided into fan delta plain subfacies,fandelta front subfacies,shore subfacies,shallow facies,braided distributary channel microfacies,sand sheet microfacies,subaqueous channel microfacies,longitudinal gravel bar microfacies,flood plain microfacies,abandoned channel microfacies and swamp microfacies.The upper member of Shahezi formation shows the characteristics of fan delta facies intersecting shore-shallow lakes,reflecting the multistage cyclicity changes under the sufficient source supply during the fault period.Through the detrital zircon U-Pb dating of five samples of the upper member of Shahezi formation,the maximum deposition age is about 111~115Ma,which belongs to Aptian to early Albian.It is a good constraint on the age of the upper part of Shahezi formation.The detrital zircons showed three major peaks,which were 105~140Ma,155~200Ma and 220~260Ma.The peak contrast of the five samples was almost identical,indicating that the source area did not change in the depositional period of the upper member of the Shahezi formation.The triangular diagram of the sandstone clastic compositions shows that it belongs to the recycling orogenic belt which is lithic recycling.The sandstone source rock of the upper member of the Shahezi formation is I-type granite according to the detrital composition,the morphological characteristics of zircon and the analysis of trace elements shows that belongs to the active continental margin and may be product of the collision between Songnen block and the Xing'an massif.Based on the comparison with Central Great Xing'an Range in zircon age spectrum,we assume that the provenance mainly cames from the Central Great Xing'an Range.The detailed description of the core and detrital zircon U-Pb data from this thesis can provide a good basic geological material for further study of Shahezi formation.