Paleoenvironment Analysis Of The Second Member Of Bayingebi Formation Of The Lower Cretaceous In The Bagemaode Region,Yin'e Basin

In this paper,the second member of Bayingebi Formation of the Lower Cretaceous in Bagemaode Region of Yin'e Basin is taken as the research target layer,in which oil shale and organic poor mudstone are mainly developed,which is the most favorable carrier for the restoration of paleoenvironment.Based on the drilling core,this paper analyzes the Paleoenvironment of the study area by means of biomarker,sporopollen and organic carbon isotope,and further discusses its main impact on the enrichment of organic matter.The lithology of the second member of Bayingebi Formation of the Lower Cretaceous in Bagemaode Region of Yin'e Basin is mainly oil shale,mudstone,silty mudstone,marl and dolomite.There are abundant animal and plant fossils in oil shale,including mesophylls,bivalves,ostracods and plant stems.The mineral composition of oil shale is mainly quartz and clay minerals,followed by plagioclase and calcite;the mineral composition of mudstone and silty mudstone with low organic content is similar to that of oil shale,but does not contain potassium feldspar;in addition to dolomite,there are a small amount of quartz and plagioclase in dolomite.The TOC value of the oil shale in the second member of Bayingebi Formation of the Lower Cretaceous in Bagemaode Region,Yin'e Basin is between 4.86 wt%and 13.6wt%,with an average value of 7.58wt%.The organic matter types are mainly type I and type II₁,which are in the immature low mature stage;the TOC value of the poor organic matter mudstone is between 0.71 wt%and 4.77wt%,with an average value of 3.51wt%.The organic matter types are mainly type II₁ and type II₂,which are also in the immature low mature stage mature stage.According to the enrichment degree of organic matter,the target layer is divided into five sedimentary units from the bottom to the top,among which sedimentary unit II?TOC average value is 7.33 wt%?and sedimentary unit V?TOC average value is 8.13wt%?have the highest organic matter abundance,and high-quality oil shale is the most developed;sedimentary unit IV has the worst organic matter enrichment degree?TOC average value is 3.18wt%?.The carbon number distribution of n-alkanes in the oil shale and organic poor mudstone of the second member of Bayingebi Formation of the Lower Cretaceous in bagemaode Region of Yin'e Basin is mainly nC₁₅ⁿC₃₅,with long and medium chains predominating,while the content of short chain n-alkanes is less;isoprene like hydrocarbons are mainly pristane?Pr?and phytane?Ph?;hopane is mainly composed of?18?,21??and?17?,21??The main and molane are mainly of?17?,21??configuration;the steranes are mainly of?5?,14?,17??H??and?5?,14?,17??H??,among which?5?,14?,17??H??steranes are dominant,and C_27-29 regular steranes are dominant.The results of comprehensive analysis of rock pyrolysis parameters and biomarkers show that the organic matter in oil shale and organic poor mudstone is the mixed source of aquatic organisms and terrestrial higher plants such as bacteria and algae.However,the oil shale is dominated by aquatic organisms,while the poor organic mudstone is dominated by terrestrial plants;from different sedimentary units,the organic matter of sedimentary units II,III and V mainly comes from aquatic organisms such as microorganisms and plankton,with little impact on the terrestrial organic matter;the organic matter of sedimentary units I and IV is greatly affected by the terrestrial organic matter,with poor organic matter types The source of organic matter is a mixture of plankton,microorganisms and terrestrial higher plants.The climate of the second member of Bayingoebi fFormation is developed from warm and humid to semi-hot and semi-arid;the ancient lakes are alternately developed with salinity fresh water,semi salt water and salt water;although the redox property of the water body fluctuates,it is in a strong reducing environment as a whole.The change of the water quality of the upper ancient lakes also shows that the frequent fluctuation of the global paleoclimate in the early Cretaceous Bbaremian-Aptian period also has similar response characteristics in the continental paleoenvironment of the study area.Rich sources of organic matter and semi-warm and semi-humid climate are favorable conditions for the enrichment of organic matter in the study area.On the one hand,in warm and humid climate,terrestrial plants and aquatic organisms flourish together,and with the increase of humidity,aquatic organisms contribute more and more,which can provide material basis for the enrichment of organic matter;on the other hand,the rise of lake water level can produce water stratification,make the bottom water body in a reducing environment,which is conducive to the preservation of organic matter.In the semi-hot and semi-arid climate,although a stronger reduction environment can be formed,the abundance of organic matter is relatively low due to the limited number of sources of organic matter;at the same time,in the warm and humid climate,if the precipitation is rich,the supply of terrigenous debris will increase,to a certain extent,the organic matter will be diluted,resulting in the decrease of the abundance of organic matter.In conclusion,for this study area,the organic matter in the second member of Bayingebi Formation is relatively rich,and the overall preservation condition of organic matter is the paleolake environment of reduction and strong reduction.The difference of organic matter accumulation mainly comes from the change of paleoclimate,among which the most favorable condition for organic matter accumulation is the reduced fresh water condition in the semi warm and semi humid climate.