Evolution Law And Significance Of Biomarker During Thermal Maturation Of Lacustrine Type Ⅰ Organic Rich Shale

Biomarker compounds widely distributed in lacustrine organic-rich shale record rich information on paleontology,paleoenvironment,and organic matter thermal maturity.The use of biomarker compounds to characterize organic matter input and water environment in ancient lakes has always been a hot spot in organic geochemistry research.However,most biomarker compounds are unstable and vulnerable to the late thermal evolution,leading in polysolution of biomarker compounds in their applications.As a result,the application of maturity criteria of different biomarker chemicals in determining the maturity of lacustrine source rocks has been a point of contention.Clarifying the evolution law of biomarker compound parameters of lacustrine source rocks as maturity changes is critical for assessing the application and usefulness of biomarker compounds at various phases.To this end,oil shale samples from the Eocene Huadian Formation in the Huadian Basin（Vitrinite reflectance R_o=0.27%,TOC=26.27%）and mud shale samples from the First Member of the Cretaceous Qingshankou Formation in the Songliao Basin（R_o=0.47%,TOC=2.28%）were selected for the pyrolysis experiment.The artificial maturation sequences of lacustrine source rocks in these two sets of distinct basins were carefully explored to investigate the evolution law of biomarker compounds in typical lacustrine source rocks.Mainly obtained the following knowledge:（1）The pristane/n-heptadecane content ratio(Pr/n C₁₇),phytane/n-octadecane content ratio(Ph/n C₁₈)and pritane/phytane ratio（Pr/Ph）of the two sets of shale samples declined and then increased with the increase of maturity.It demonstrates that the indentification effect of isoprenoid alkane paleoenvironmental indicator is influenced by maturity.The gammacerane index(GI=G/C₃₀H)of both samples is positively correlated with R_oin their respective experimental thermal evolution intervals,while C₂₉/C₂₇（regular sterane）is negatively correlated with R_owhen R_o≤1.09%.However,the Huadian samples have better correlations than the Songliao samples,which is due to the differences in the hydrocarbon-generating components and parent materials of the samples.The Huadian sample has a higher concentration of terrigenous organic matter,with vitrinite dominating,whereas the Songliao sample has a higher concentration of exinite.This results in a discrepancy in the content and composition of the biomarker molecules created by pyrolysis of the two,resulting in a difference in the correlation between them.To sum up,the above parameters can be uesful in identifying the macroscopic paleo-sedimentary environment or paleontological origin,but maturity should be addressed in research and practical applications,and multi-parameter thorough corroborative analysis is the key.（2）The evolution law of saturated hydrocarbon parameters with maturity of Huadian sample is better than that of Songliao sample.It may be caused by the differences in the sedimentary water environment and components of the samples’hydrocarbon-generating parent materials.The C₃₁hopane 22S/（22S+22R）ratio of Huadian samples is a good maturity parameter for indicating the immature-mature stage（R_o≤0.91%）,while the C₃₂22S/（22S+22R）ratio for indicating the immature-high maturity stage（R_o≤1.34%）,but both ratios lose their indicative significance after reaching equilibrium.The ratios of total tricyclic terpenes/total hopanes and C₂₃tricyclic terpenes/C₃₀hopanes can indicate the maturity of source rocks or crude oil when R_o≤2.19%,and are more reliable maturity indicators in high evolution stages.Overall,the C₃₀βα-morane/αβ-hopane maturity parameter index is the most optimal maturity parameter index that may give an effective indicator for the maturity of organic matter in this experiment’s thermal evolution range（R_ois about 0.27%-3.56%）.（3）Among the series of aromatics maturity parameters studied,the tetramethylnaphthalene parameters（Te MNR）and dimethyl-phenanthrene series parameters（DPR2-DPR4）of the two sets of pyrolytic samples increased with the maturity,and have a strong positive correlation with R_o（R_oabout 0.5%to 2.0%）（R²was 0.8810 to 0.9781）.Therefore,for lacustrine type I organic rich shale,Te MNR,DPR2-DPR4 are more reliable aromatic hydrocarbon series parameters for evaluating the maturity of source rocks or crude oil.Factors such as microbial action and depositional environment have great effect on the maturity parameters of trimethylnaphthalene,so caution should be used when using it.