Geochemical Characteristics of Crude Oil from B Gas Field in Xihu Sag,East China Sea Basin
Based on crude oil samples from 7 wells in B gas field,oil and gas geochemical analysis and research were conducted using gas chroma-tography and isotope mass spectrometry.The research shows that the light hydrocarbon composition of the crude oil in B gas field is low in normal alkanes and rich in isoalkanes and aromatics,indicating that the crude oil in B gas field mainly comes from terrestrial organic matter;According to the composition triangle diagram of C7 compounds(methylcyclohexane,n-heptane,dimethylcyclopentane)in crude oil,it is shown that crude oil mainly comes from humic parent material and is the mixed source oil of Huxiang oil and coal-formed oil;The carbon isotope value of gas field B is between-27‰ and-24‰,and the w(Pr)/w(Ph)is between 5.68 and 10.11,indicating that the crude oil of gas field B is formed in a strong oxidation environment and is formed by humic organic matter of coal measures;The maturity of crude oil is divided by heptane number and isohep-tane number,which belongs to the stage from low maturity to maturity.Considering that the crude oil in West Lake Sag undergoes relatively obvious evaporation and fractionation,and the light hydrocarbon composition of crude oil changes,combined with the Mango crude oil formation temperature formula,the formation temperature of crude oil in Field B is calculated to be 129.6℃～132.0℃,and the crude oil maturity is further calculated to be 0.92%～0.95%,which can be used to judge that the crude oil in West Lake Sag is in the mature stage.
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