Research On Sequence Stratigraphic Framework And Depositional System Of The Upper Forth Member Of The Eocene Shahejie Formation In The Boxing Sag, Dongying Depression

The study area is located in the Boxing Sag and the study interval is the upper 4th member of the Eocene Shahejie Formation(Es4U).The Es4~U is well documented in early studies about its bearing beach-and-bar reservoir in the lower to middle part of the interval.In this paper,using sequence stratigraphy and sedimentology theory as guidance,and based on core observation data,litholgic data,logging data and seismic data,the strata of the upper fourth member of the Shahejie Formation in the Boxing Sag is seen as a third-order sequence including the lowstand systems tract(LST),the transgressive systems tract(TST)and the highland systems tract(HST),and further subdivided into seven parasequence sets.The identification criteria of third-order sequence boundaries and systems tract boundaries have been established in this study.Based on the detailed observation and description of the cores from various wells located in different areas in the Boxing Sag,lots of soft-sediment deformation structures(SSDS)have been observed.SSDSs,which are commonly encountered in muddy deposits or massive sandy deposits in the LST or TST in the study area,have been systematically ignored.SSDS have been divided into two main categories:ductile deformation structures and brittle deformation structures mainly based on their morphological characteristics.Ductile deformation structures include load casts and flame structures,pillow structures,water-escape structures,contorted structures,and slump structures and brittle deformation structures include sedimentary dykes,microfaults,and autoclastic breccias.In this study deformation mechanism,driving forces and trigger mechanism of the SSDS have been studied.Seismic activities and subaqueous sediment gravity flow are the mainly triggers for the formation of the SSDS,and the deformation mechanism includes liquefaction,fluidization and thixotropy.Furtherly,sedimentary environments and the types of sediments bearing various types of SSDS have been summarized.The deformed intervals bearing seismically induced SSDS have been interpreted as seismites.Based on the founded criterion of identification for seismically induced SSDS,cores from15 wells have been studied and seismites were identified in the cores.Lateral comparison of seimites among wells with the basis of the high-resolution sequence stratigraphic framework have been carried out to figure out the lateral variation law of seismites.Guided by the theory of sedimentology,combined with the field survey data of Qinghai Lake,the types of sedimentary facies and microfacies were finely divided,and the corresponding logging facies signs were established.The Es4~U is mainly consisted of lacustrine sediments,furtherly,beaches and bars(sandy bars and pebbly bars)deposited in littoral environments,storm deposits sedimented in sub-littoral environments and gravity-flow deposits sedimented(slide-slump deposits and muddy debrites)in shallow profundal environments have been identified.Through the comprehensive analysis of core facies in single wells,profiles of facies among wells and distribution characteristics of facies in the study area,a series of conclusions can be drawn that:(i)littoral beaches and bars mainly occurred in the LST and sub-littoral storm deposits occurred locally in the LST in the study area.(ii)shallow profundal gravity flow deposits mainly occurred in the TST when the lake level rose quickly with minor beaches occurring in the southern part of the study area.(iii)littoral beaches and bars mainly occurred in the HST,while the area of them were far smaller than that of the beaches and bars in the LST.Different types of deposits changed regularly with the variation of the lake levels,and the variation of the lake levels also led to the change of the systems tracts(the shallow water of the lake in the LST,the quick rise and the most deep of the water in the TST and the relatively stable lake level in the HST).Control factors of the littoral sedimentary system in the LST have been studied,and we found that geomorphology,ancient water depth,ancient provenances and ancient wind fields are the main control factors of the beach-and-bar deposition system.The very gentle sedimentary slope,the shallow water depth,the strong paleo-wind force as well as the windward location of the study area,close proximity to the source area,the Luxi uplift,leading to the continuous wave force on the large-scale sandy deposits in the littoral area.Finally,the sedimentary model of the Es4~U in the study area was established.