川东北地区典型海、陆相原油地球化学特征及来源

Geochemical characteristics and origins of typical marine and terrestrial oils in the northeastern Sichuan Basin

基于饱和烃和芳烃组分的GC/MS和GC/MS/MS分析,揭示了川东北天然气勘探区探井及露头剖面中发现的典型海、陆相原油(油苗)的分子地球化学特征,并结合烃源岩分析资料探讨其来源。研究结果表明,大普光构造带的新清溪1井和元坝构造带的元坝9井中侏罗统千佛崖组原油饱和烃呈姥鲛烷优势(Pr/Ph>1.5);三环萜烷很少,ETR值低于1.2;芳烃中芴系列含量高;经与相关烃源岩干酪根碳同位素组成的对比,确认两者油源均来自本层位湖相烃源岩。但这两井原油的重排藿烷类化合物丰度相差悬殊,表征两者油源岩性质有所不同。大普光构造带的川岳84井二叠系原油Pr/Ph值(0.82)较低,三环萜烷丰富,含有C26降胆甾烷,油-岩对比认为来源于上二叠统龙潭组海相烃源岩。江油二郎庙飞仙关组、长兴组油苗呈显著的C29甾烷优势,24-异丙基-/24-正丙基-胆甾烷比值较高(1.1),C30甲基甾烷中以3β-、2α-甲基甾烷为主,C26甾烷中27-降胆甾烷占优势,芳烃中缺少三芳甲藻甾烷,且全油碳同位素很轻(-34.1‰～-34.6‰),表明源于寒武系—震旦系烃源岩。

Saturated and aromatic fractions of typical marine and terrestrial oils and seepages collected from exploratory wells and outcrop in the northeastern Sichuan Basin,respectively,were characterized in molecular geochemistry using GC/MS and GC/MS/MS and their origins were also discussed in combination with analytical data of source rocks.The results indicated that oils from the Middle Jurassic Qianfoya Formation in Well Xinqingxi-1 in Puguang structural belt and Well Yuanba-9 in Yuanba structural belt show a pristane predominance over phytane（Pr/Ph1.5）,low amount of tricyclic terpanes with ETR values below 1.2 and abundant fluorene series in aromatic hydrocarbons.Based on the correlation of carbon isotopes between oils and kerogens from related source rocks,the oils were validated to be derived from lacustrine source rocks of the Qianfoya Formation.However,these two oils could have different organic sources and depositional settings due to a significant difference in diahopane abundance.The Permian oil in Well Chuanyue-84 of Puguang structural belt shows a relatively low Pr/Ph ratio（0.82）,abundant tricyclic terpanes and detectable C26 norcholestane.Carbon isotopes indicated a genetic correlation between the oil and marine source rocks of the Upper Permian Longtan Formation.The seepages from the Feixianguan and Changxing formations in Erlangmiao,Jiangyou area are characterized by dominant C29 sterane over C27 and C28 homologues,a high 24-isopropyl-/24-n-propyl-cholestane ratio（1.1）,predominant 3β-and 2α-methylsteranes in methylated steranes,a low 24-/（24-＋27-）norcholestane ratio（below 0.2） and the absence of triaromatic dinosteroids.These age-related biomarkers,in conjunction with the isotopic depletion for whole oils（-34.1‰ to-34.6‰）,suggested that the seepages could originate from Sinian-Cambrian source rocks.