渤海湾盆地沧东凹陷古近系孔二段页岩层系原油地球化学特征

利用原油物性、族组分分离与定量、饱和烃色谱—质谱、芳烃色谱—质谱、稳定碳同位素等分析测试资料,系统分析了渤海湾盆地沧东凹陷古近系孔二段页岩层系原油地球化学特征。研究表明,原油属于中质、中凝稠油,流动性较差;原油中烃类含量偏低,饱芳比和非沥比均较低。不同构造单元上的原油生物标志物及稳定碳同位素分布特征相似,反映其母质来源及生烃演化过程基本一致。饱和烃色谱以正态分布的单峰型为主,轻碳优势不明显,C27、C28、C29规则甾烷呈"上升型"分布,三环萜烷含量低,反映原油有机母质来源具有湖相水生生物和陆源高等植物的双重贡献,且以陆源高等植物为主。伽马蜡烷指数、重排霍烷含量及分布、Pr/Ph、三芴系列化合物、原油稳定碳同位素分布等指标反映了原油形成于弱氧化—还原条件下的淡水—微咸水沉积环境。正构烷烃分布特征和奇偶优势比、αββ/(ααα+αββ)C29与20S/(20S+20R)C29甾烷异构化指数以及霍烷异构化指数均指示原油成熟度偏低。

Geochemical characteristics of crude oils in the second member of Kongdian Formation shale system,Cangdong Sag,Bohai Bay Basin

Crude oils in the shale system of the second member of Kongdian Formation (Ek₂) in the Cangdong Sag of the Bohai Bay Basin were analyzed for physical properties, group composition, quantitative gas chromatography-mass spectrometry of saturated hydrocarbon and aromatic hydrocarbon fractions and stable carbon isotopes. The crude oils belong to medium quality, medium freezing point heavy oil with poor fluidity. The hydrocarbon content in the crude oils is low, and both ratios of saturated hydrocarbon to aromatic and nonhydrocarbon to asphaltene are low. The characteristics of crude oil biomarkers and stable carbon isotope distributions in different structural units are similar, indicating that these crude oils have roughly similar sources and hydrocarbon evolution processes. Almost all the samples have saturated hydrocarbons with a slight dominance of light carbons and are normally distributed with a single peak. The distribution of C₂₇,C₂₈ and C₂₉ regular steranes shows a rising trend, while the tricyclic terpane content is very low, demonstrating that the original organic matter had contributions of lacustrine aquatic organisms and terrigenous higher plants, with the latter being the major contributor. The gammacerane index, the content and distribution of rearranged hopanes, Pr/Ph, dibenzothiophene, dibenzofuran and fluorine series, and stable carbon isotope distribution and other indicators suggest that the crude oil source rocks formed in fresh-brackish and weakly oxidizing to reducing lacustrine environment. The odd/even predominance, the αββ/(ααα+αββ)C₂₉ and 20S/(20S+20R) C₂₉ isomerization index and the hopane isomerization index indicate that the crude oils have low thermal maturity.