Abstract:

The even-odd predominance shown in n-alkanes has been recognized in Tertiary source rocks and crude oils in North China.The characteristics ofthe distribution of the n-alkanes with such predominance are that the rangeof even-odd predominance is between n-C₂₂ and n-C₃₄,in which the mostabundant constituents are the alkanes n-C₂₆,n-C₂₈ and n-C₃₀,with the ma-ximum value at n-C₂₈（the OEP value is 0.48-0.51）.It is worthy to be noted that the curve of the n-alkanes with even-oddpredominance of some samples is quite coincident to that with odd-evenpredominance of other samples in this region and that the even-odd peak C₂₈ coincides well with the odd-even peak C₂₇.This indicates that both ofthem belong to low-maturity organic matter.Microscopic examinations of thin sections and infrared spectra of aroma-tic hydrocarbons also demonstrate that both the n-alkanes with even-oddand odd-even predominance belong to the same organic matter,i.e.theyboth originate from terrigenous organic matter of lipid-rich higher planttype. Samples with even-odd predominance occur in a geological environment where evaporates,gypsum and carbonate rocks are associated.Like odd-even predominance,the even-odd predominance can also serveas a geochemical indicator in studying the origin,migration and matura-tion of petroleum as well as for oil-to-oil and oil-to-source rock correla-tion.The maturation of the organic matter with even-odd prdominancemay probably proceeds somewhat more slowly than that with odd-even predominance.