准噶尔盆地沙湾凹陷东部原油地球化学特征和来源

沙湾凹陷是准噶尔盆地腹部重要的勘探领域之一，为了更好地厘清凹陷内二叠系、侏罗系和白垩系储层中原油的来源，明确油气成藏过程和富集规律，对该凹陷东部典型井原油样品开展了碳同位素和生物标志化合物分析，并基于对原油地球化学特征的分析，开展了原油分类和油源对比。沙湾凹陷东部不同层系原油可以分为3类：第一类原油赋存于二叠系及侏罗系储层中，其全油δ¹³C值和Pr/Ph比值分别介于-31.0‰~-29.0‰和1.0~2.0，αααR规则甾烷相对含量具有C₂₇＜C₂₈＜C₂₉的特征，萜烷类多数表现出C₂₀＜C₂₁＞C₂₃TT、C₂₄TeT/C₂₆TT＜1的特征，且伽马蜡烷指数均小于0.30，这类原油主体来自中二叠统下乌尔禾组烃源岩，其中上二叠统上乌尔禾组油样中未检出Ts，β-胡萝卜烷/C₃₀藿烷比值大于1，三环萜烷分布表现出C₂₀＞C₂₁＞C₂₃TT的特征，且甲基菲分布分数反映了原油处于高成熟演化阶段，这表明其中混入了少量下二叠统烃源岩生成的原油；第二类原油赋存于中侏罗统储层中，其典型特征为全油δ¹³C值及正构烷烃单体δ¹³C值主体大于-29.0‰，Pr/Ph比值相对较高，介于2.0~2.5，且C₂₄TeT/C₂₆TT比值大于1，表明这类原油主体为侏罗系烃源岩生成；第三类原油赋存于下白垩统储层中，其全油δ¹³C值和Pr/Ph比值分别小于-31.0‰和1.0，αααR规则甾烷含量呈C₂₇≈C₂₈＜C₂₉特征，伽马蜡烷指数均高于0.50，正构烷烃单体δ¹³C值随碳数升高而逐渐降低，且主体小于-31‰，表明这类原油主要来自下白垩统烃源岩。

Geochemical characteristics and source of crude oil from the eastern Shawan Sag,Junggar Basin

The Shawan Sag is one of the important exploration areas in the hinterland of the Junggar Basin. In order to better clarify the source of crude oil from the Permian, Jurassic and Cretaceous reservoirs in the sag and identify hydrocarbon accumulation processes and enrichment pattern, the authors conducted a carbon isotope and biomarker analysis on the typical crude oil samples from the eastern Shawan Sag in this study, and further carried out crude oil classification and oil-source correlation based on the analysis of geochemical characteristics of crude oil. The results indicate that the crude oil from different strata in the eastern Shawan Sag can be divided into three types. The first type mainly occurs in the Permian and Jurassic reservoirs, and its whole oil δ13C value and Pr/Ph ratio range from -31.0‰ to -29.0‰ and from 1.0 to 2.0, respectively. The relative contents of αααR regular sterane display the characteristics of C27 < C28 < C29, and the terpanes are mainly characterized by C20 < C21>C23 TT and C24TeT/C26TT < 1, with the gammacerane index lower than 0.30. This type of crude oil is mainly derived from the source rocks in the Middle Permian Lower Wuerhe Formation. In the oil samples from the Upper Permian Upper Wuerhe Formation, Ts content is below the detection limit, the β-carotane/C30 hopane ratio is greater than 1, and the tricyclic terpane distribution displays the characteristics of C20>C21>C23TT. The distribution fraction of methyl phenanthrene reflects that the crude oil is in the stage of high-maturity evolution. All these indicate the mixing of crude oil generated in the Lower Permian source rocks. The second type occurs in the Middle Jurassic reservoirs. The whole oil δ13C value and individual n-alkanes δ13C value are higher than -29.0‰. The Pr/Ph ratio is relatively higher, ranging from 2.0 to 2.5, with C24TeT/C26TT>1, which indicate that this type of crude oil is mainly derived from the Jurassic source rocks. The third type occurs in the Lower Cretaceous reservoirs, and its whole oil δ13C value and Pr/Ph ratio are lower than -31.0‰ and 1.0, respectively. The αααR regular sterane contents display the characteristics of C27≈C28 29, with the gammacerane index higher than 0.50. The individual n-alkanes δ13C value gradually decreases with the increase of carbon number and is commonly lower than -31.0‰. This type of oil is demonstrated to be mainly derived from the Lower Cretaceous source rocks.