Study On The Formation Mechanism And Development Model Of Lacustrine High Quality Source Rocks In Bozhong Sub-basin

The Bozhong sub-basin is one of the most important petroliferous basin of the Bohai Bay Basin.At present,with the deepening of oil and gas exploration in Bozhong sub-basin,the research on the formation mechanism and development model of lacustrine high-quality source rocks in Bozhong sub-basin is becoming more and more important,which has become a difficult and important scientific problem restricting further oil and gas exploration in this area.In this study,on the basis of high-resolution sequence stratigraphic division,by means of organic geochemistry,inorganic geochemistry,scanning electron microscopy,whole rock X-ray diffraction and paleontology identification,the source rocks of the first and third member of Shahejie Formation(Esi and Es3)in Bozhong sub-basin are evaluated,the sedimentary environment of ancient lake is reconstructed,and the development factors of source rocks are discussed The formation mechanism and development model of the lacustrine high quality source rocks in Esi and Es3 are summarized.Based on the division plan of Paleogene sequence stratigraphy in Bohai Bay Basin,the high-precision sequence stratigraphy of Es1 and Es3 is analyzed according to the data of geochemistry,geophysics and paleontology.The third-order sequences of Es1 and Es3 are further divided into two fourth-order sequences,i.e.EST and RST.The interface between EST and RST is condensed section,which generally has abnormally high organic matter abundance,low felsic and Al2O3 content,high clay mineral content,and high pyrite content.In view of the deficiency of previous statistical methods to determine the lower limit of organic matter abundance of high-quality source rocks,in this study,the lower limit of residual organic matter abundance is obtained by calculating the original organic matter abundance and the amount of hydrocarbon generation and expulsion based on the pyrolysis data of rocks.The lower limit of TOC of high-quality source rocks in Es1 and Es3 is determined to be 2.00%.According to the evaluation of the source rocks Es1 and Es3,the organic matter abundance of the two sets of source rocks is good,TOC of Es1 is 0.48-11.61%,with an average of 2.85%;TOC of Es3 is 0.92-17.50%,with an average of 3.21%.The types of organic matter of Es1 and Es3 source rocks are mainly type ?,containing part of type ? kerogen,and relatively less type ? kerogen.The maturity of organic matter has entered the stage of low maturity to maturity.The parent material of Es1 and Es3 source rocks are the mixed source of lower aquatic organisms such as algae and higher land-based plants,and algae is more dominant.Vertically,the high-quality source rock of Es3 is more developed than that of Es1,and the condensed section is an important position for the development of the high-quality source rock of Es3.In plane,the high-quality source rock of Es1 in Bozhong area is mainly distributed in the south of Shanan area,and the high-quality source rock of Es3 is mainly distributed in the Bozhong and Huanghekou areas.The paleoclimate of source rocks in the study area is analyzed by using sporopollen and X-ray diffraction data.The sporopollen of Es3 is mainly composed of Pinuspollenites-Ulmipollenites,both of which contain more than 50.00%.The content of Pinuspollenites-Ulmipollenites in Es1 is obviously reduced,not more than 50.00%.The content of Ulmipollenites and carpiniptites is obviously increased,reaching 30.00%,and Ephedrine is also found,which is twice as much as that in Es3.The clay mineral data show that the content of kaolinite in Es3 is obviously higher than that in Esi,and the content of illite is lower than that inEs1.The results show that the paleoclimate of Es3 is relatively warm and humid,while that of Es1 is relatively hot and dry.We use geochemical and algal data to analyze the salinity of the ancient lake water in the study area.In Es1,the assemblages of Pediastrum boreanum and Sentidinium are dominant,while in Es3,the assemblages of Sentidinium are dominant,and the algae diversity is obviously reduced.Geochemical data show that Es1 has higher gammacerane index,Ba,CaO content and Ca/(Ca+Fe)ratio than Es3.Generally,the salinity of Es1 is brackish water-brackish water,while Es3 is fresh water-brackish water.We have recovered the deposition rate of two sets of source rocks in the study area.The average sedimentation rate of Es1 is only 3.25cm/ka,and that of Es3 is 22.64cm/ka.Through the analysis of deposition rate and organic matter abundance,it is considered that the lower deposition rate in Es1 has aggregation effect on organic matter,and the higher deposition rate in Es3 has dilution effect on organic matter.The minimum dilution effect of deposition rate on organic matter is 4.47cm/ka.We calculated the paleoproductivity of two sets of source rocks and discussed the main controlling factors.In the study,the author focuses on the analysis of the previous deficiencies in the evaluation of paleoproductivity,points out that the evaluation of paleoproductivity cannot ignore the time attribute of productivity,and derives the calculation formula of paleoproductivity suitable for the generated and discharged hydrocarbon source rock formation.The average paleoproductivity ofEs3 is 1055.84 g C/(m2·a),and that of Es1 is 103.55 g C/(m2·a).Paleoclimate and nutrient input are the main factors controlling paleoproductivity.Eutrophication of paleolakes in Es3 has resulted in higher paleoproductivity in Es3.We use geochemical and trace element data to study the redox characteristics of the ancient water body in the area.Pr/Ph data show that both Es1 and Es3 have good reduction environment,and some data points of Es1 reflect strong reduction environment.V/(V+Ni)shows that both Es1 and Es3 are anoxic.The abnormally high Ph content and Pr/nC18 in Es1 and Es3 show that there is water stratification.In this study,strawberry pyrite with a particle size of 5.7 ?m was found inEs3,which is the evidence to infer the existence of water temperature stratification in the middle of the lake.Based on the above research,we believe that the high-quality source rocks in Es3 are formed under the condition of high productivity,organic matter is diluted to a certain extent by sedimentation rate,and formed under the condition of water temperature stratification;the high-quality source rocks in Es1 are formed under the condition of low productivity,fixed organic carbon is accumulated by low sedimentation rate,and the oxygen content in water decreases under the condition of high water salinity.At the same time,the formation of water salinity stratification under good preservation conditions in Es1.The main controlling factor for the formation of high-quality source rocks in Es3 is high productivity,and the main controlling factor for the formation of high-quality source rocks in Es1 is the accumulation effect of low deposition rate on organic matter.