低熟泥页岩芳烃地球化学特征及其成熟度演化——以沙四上段与长7段为例

Differential geochemical characterization and thermal evolution of aromatic hydrocarbons in low-maturity shales: a case from the upper part of 4th member of Shahejie Formation and Chang 7 Member

以东营凹陷沙河街组四段上亚段(沙四上段)与鄂尔多斯盆地延长组7段(长7段)低熟泥页岩为例,利用气相色谱—质谱技术,分析芳烃地球化学特征,揭示芳烃化合物在生物来源、环境、成熟度上的差异性与演化特征。结果表明:沙四上段泥页岩3~6环芳烃相对丰度较高,长7段泥页岩4环以上芳烃丰度相对较低;沙四上段的芳烃参数荧蒽/芘大于长7段的,指示前者芳烃演化程度较高;沙四上段与长7段泥页岩芳烃是成岩作用与异常热作用混合成因的结果,其中沙四上段芳烃分布特征受热作用影响更大。沙四上段比长7段具有更高的陆生高等植物有机质输入。4-环芳烃参数三亚苯/[艹屈]与甲基二苯并噻吩具有良好的正相关关系,指示三亚苯/[艹屈]是芳烃成熟度评价的有效参数。该结果为低熟泥页岩评价提供参考。

The low-maturity shales from the upper part of 4th member of Shahejie Fomation in the Dongying Depression and the Chang 7 Member in the Ordos Basin were sampled and analyzed for aromatic geochemical characteristics using gas chromatography-mass spectrometry to reveal the difference and evolution of aromatic compounds with respect to organic sources,environment and maturity.The results show that the shale in the upper part of 4th member of Shahejie Fomation contains higher relative abundance of 36 ring aromatic hydrocarbons,and the shale in the Chang 7 Member has lower relative abundances of aromatic hydrocarbons above 4 rings.The fluoranthene/pyrene ratio of the aromatic parameters in the upper part of 4th member of Shahejie Fomation is greater than that in the Chang 7 Member,indicating that the aromatic evolution of the former is higher.The aromatic hydrocarbons in all shales are the result of diagenesis and pyrolysis,but the aromatic hydrocarbon distributions of the upper part of 4th member of Shahejie Fomation are more influenced by thermal effect.The higher plant index shows that the sample from upper part of 4th member of Shahejie Fomation has higher organic matter input from higher plants than the Chang 7 Member sample.The 4-ring aromatic parameter tryphenylene/chrysene ratio has a good positive correlation with methyldibenzothiophene ratio,indicating that tryphenylene/chrysene ratio is an effective aromatic hydrocarbon parameter for assessing maturity.The results provide a reference for the evaluation of low-maturity shales.