Abstract: An immature oil sample from Tang-6 Well and a mature oil sample from Sha-7 Well in Subei basin are selected and mixedeach other with different ratios so as to investigate changes between biomarker indices.The saturated hydrocarbon separated from themixed samples is analyzed by GC and GC-MS, respectively. The study shows that different changes occur in mixed sample indices andmixed ratio, e.g. n-alkanes and terpenes parameters assume linear changes approximately, but others (e.g. steranes) assume nonlinearones. The reasons caused are that the concentration of steranes, terpenes and n-alkanes in the immature oil differs from that in the matureoil. It is indicated from estimation that when the difference of the concentration among them are big, they appear in nonlinear change andtend toward increasing concentration, and this trend doesn't disappear until compounds of low concentration account for absolute predom-ination. The phenomenon is called the allocation principle of concentration-feature contribution ratio in the mixed oils. The results showthat the mixed ratio is not only dependent upon biomarker indices, but also, it is more important to make clear the original concentrationsrelated to these compounds. The "immature oil" pre-recognized from steranes maturity parameters could be the mixed oil dominated by themature oil.

Key words: immature oil, biomarker, experiment, maturity, Subei basin