Study On Sequence Stratigraphy And Immature Oil Accumulation Condition Of The Paleogene Shahejie Formation,Cangdong Depression

There are abundant immature oil in the clastic rock and carbonate rock of the Shahejie Formation in the Cangdong depression,Bohai Bay Basin,which shows a good prospect of immature oil exploration.The study of control function of stratigraphy and immature oil accumulation condition is not only helpful to better understand the sequence-sedimentary characteristics of continental rift basins and enrich the theory of immature oil accumulation,but also provides an important basis for exploration of immature oil in the Shahejie Formation,Cangdong depression.In this paper,core observation,thin section observation,logging data analysis,seismic data analysis and indoor tests and analysis were used,sequence stratigraphy and sedimentary system of the Shahejie Formation in the Cangdong depression,immature oil accumulation conditions were mainly studied and discussed.Following achievements and understandings have been obtained:(1)The sequence stratigraphic model of the Shahejie Formation has been established.The Shahejie Formation can be divided into five third-order sequences,SQ1-SQ3 correspond to Sha3 Member,SQ4 corresponds to Sha2 Member,and SQ5corresponds to Sha1 Member.SQ1-SQ4 are sequences of clastic rock type,and SQ5is a sequence of carbonate rock type.The sequence stratigraphic framework of the study area from north to south can be summarized into three types:single-fault asymmetrical two depressions sandwich one uplift,double-fault middle uplift and double-fault asymmetrical graben.The stratigraphic thickness center of the third-order sequences in the Cangdong depression is located in the downwall of Xuxi fault and Cangdong fault,and they are in oblique distribution.Sandbodies of clastic sequence are mainly distributed on the east and west sides of the Cangdong Depression and the periphery of the Kongdian bulge.Longitudinally,sandstones are mainly developed in the lowstand system tract,transgressive system tract and lower part of the highstand system tract mainly deposit mudstone.In the carbonate sequence,bioclastic limestone is mainly distributed in the east side of Cangdong fault,Kongdian bulge and its surrounding,which are palaeogeomorphologic highlands or gentle slope zones.Longitudinally,bioclastic limestone deposit in the lowstand system tract,while the dark mudstone with huge thickness deposit in the transgression and high stand system tract.(2)The depositional model of the Shahejie Formation has been established.The Cangxian uplift,Xuhei bulge,Dongguang bulge and Kongdian bulge all provided provenance to the study area.There are four sedimentary systems in the Shahejie Formation:fan delta,braided river delta,meandering river delta and lake.The temporal and spatial distribution of sedimentary system is controlled by tectonic activity,sediment supply and maine transgression.The clastic rock sedimentary system developed in the SQ1-SQ4 sedimentary period in the Cangdong depression.Palaeogeomorphologic features resulted from different fault activity intensity or fault interaction,inherited paleogeomorphology,and the different sediment supply in time controlled the sedimentary system type and its spatial and temporal distribution characteristics.During the SQ5 sedimentary period,the three factors of weakened fault activity,weakened sediment supply and marine transgression caused the simultaneous development of clastic rock and carbonate rock sedimentary systems in this sequence.(3)The Shahejie Formation has good immature oil accumulation conditions.On the basis of this understanding,the immature oil accumulation model of the Shahejie Formation has been established.(1)The dark mudstone of the Shahejie Formation was distributed in SQ1-SQ3 and SQ5,and the organic matter is of high abundance.The main types of organic matter are?-?₁,there is a small amount of?type organic matter in the SQ1-SQ3 source rock.The maturity of organic matter is between immature to low mature.During the sedimentary periods of SQ1-SQ3 and SQ5,the salinization of lake water caused by maine transgression promoted the enrichment of organic matter in source rocks and the generation of immature oil.(2)Clastic reservoirs were developed in SQ1-SQ4 of the Shahejie Formation,and both clastic and carbonate reservoirs were developed in SQ5 at the same time.Plain distributary channels,front subaqueous distributary channels,mouth bar,sheet sand of different types of deltas as well as sandy beach bar are important clastic reservoir types.The porosity types of clastic reservoir are mainly medium-high porosity,and the permeability types are mainly ultra-low to medium permeability.The carbonate reservoir is bioclastic limestone,with medium porosity and medium permeability.(3)Alternate deposition of sandstone and mudstone in different system tracts of SQ1-SQ3 can form small range of reservoir-cap assemblages laterally.SQ4 is dominated by sandstone,and it is difficult to form effective cap rocks and reservoir-cap assemblage in this sequence.The combination of biological limestone in the SQ5 low stand system tract and the dark mudstone in the lacustrine transgression and high stand system tract can form large range of reservoir-cap assemblages laterally.(4)There are four types of trap in the Shahejie Formation,including lithologic trap,stratigraphic trap,structural trap and structure-lithologic trap.Among them,sequence stratigraphy framework and sedimentary system controlled lithologic trap and stratigraphic trap to some extent.(5)The transport system of immature oil in the Shahejie Formation includes reservoir sandstone body,unconformity surface and fault.Sedimentary systems and sequence stratigraphy boundary controlled the formation of reservoir sandstone body and unconformity surface,while faults were associated with Paleogene extensional or strike-slip processes in the study area.(6)The immature oil accumulation models of clastic sequence stratigraphy include self generation and self storage type and lower generation and upper reservoir type,and the oil reservoir types include stratigraphic,lithologic and structural immature oil reservoirs.The immature oil accumulation model of carbonate sequence stratigraphy is mainly self-generation and self-storage type,and the oil reservoir type is lithologic immature oil reservoir.