The Study Of Geochemical Characteristics Of Crude Oils In Bohai Sea Area

Bohai Sea area is abounding in oils and gases.Many scholars are always focous on the exploration of oils and gases,so the studies of generation and exploration on oils and gases have not only important theoretic sense,but also obvious applied prospect.In this paper,on the basis of experiment and geological background of Bohai Sea area,the following studies have been dealt with,including physical and chemical properties,group compositions and n-alkane carbon isotopic compositions, geochemical characteristics of light hydrocarbon,saturated hydrocarbon and aromatic hydrocarbon.Sedimentary environments,organic source,maturity and characteristics of biomarkers have been discussed.Physical and chemical properties studies show that,the crude oils density scope is in 0.75～1g/cm³,light-weight oils,middle-weight oils and heavy-weight oils are all existing;the crude oils viscidity scope is also very comprehensive;the wax content and sulphur content are both in great disparity in different areas.The difference of the sulphur content is mainly decided by sedimentary environments,while the wax content may be concerning with the degradation in some externt.Group compositions and n-alkane carbon isotopic studies show that,the content of saturated hydrocarbon and aromatic hydrocarbon are both wide,are separately in 10～80%and 13～40%;n-alkane carbon isotopic is distinct in different areas,it may relate with the water salty.Geochemical characteristics of light hydrocarbon,saturated hydrocarbon and aromatic hydrocarbon studies show that,the crude oils are mature oils,the content of Ro is in 0.73～0.97%;the Pr/Ph is in 1.0～1.5,which indicates a suboxic environment; hopane distribution appears characteristics of freshwater-lake;C₂₇,C₂₈,C₂₉ sterane distribution appears unsymmetrical "V",the content of C₂₇ and C₂₉ steranes of oils is in 24.5～36.3%and 31.9～49.7%,which indicates continental facies organic source.The oils in Bohai Sea area can be divided in three types.Dongying Formation oils,the first member of Shahejie Formation oils,the third member of Shahejie Formation oils,the first/the third member of Shahejie Formation mixed-source oils and Dongying/the first member of Shahejie Formation mixed-source oils,according the difference and discipline of the oils geochemical characteristics.Dongying Formation oils are in low amounts of gammacerance and 4-methyl sterane;the first member of Shahejie Formation oils are in high amounts of gammacerance and middle amounts of 4-methyl sterane;the third member of Shahejie Formation oils are in low amounts of gammacerance and high amounts of 4-methyl sterane;the first/the third member of Shahejie Formation mixed-source oils are in high amounts of gammacerance and 4-methyl sterane;Dongying/the first member of Shahejie Formation mixed-source oils are in high amounts of gammacerance and low amounts of 4-methyl sterane.In the end,the mathematical models of biodegraded mixed-source oils biomarkers change along with the mixed proportion,and the calculation method for the contribution rate of oils sources are established by mathematical demonstration and artificial oil-mixing experiment,in this way,this paper find that,the main source of Dongying/the first member of Shahejie Formation mixed-source oils are Dongying Formation crude oils,their contribution is in 65～90%;to the the first/the third member of Shahejie Formation mixed-source oils,the contribution of the first and the third member of Shahejie Formation crude oil is about 50%.