四川广元上寺剖面上二叠统大隆组优质烃源岩发育主控因素初探

Main control factors of high quality hydrocarbon source rocks of the Upper Permian Dalong Formation at Shangsi section of Guangyuan,Sichuan Province

对四川广元上寺上二叠统剖面进行岩石学和地球化学分析,并利用Al/Ti值评估古生产力,而利用Mo、U、V、Cr、V/Cr和V/(V+Ni)来评估沉积环境,进而探讨烃源岩中有机质的积累与海洋初级生产力、沉积环境(氧化还原条件)以及沉积速率之间的关系。研究发现,在大隆组中部,TOC、过剩铝、Cu、Ni和Cd出现高值(TOC平均值为5.82%),具备优质烃源岩特征。对该段地层的生物地球化学分析表明,氧化—还原敏感元素Mo、U、V、Cr丰度高,V/Cr>4.25和V/(V+Ni)>0.7指示长期的厌氧沉积环境;而此时Al/Ti则指示只有中等的古生产力。低的沉积速率可能是有机质浓缩的主要原因。与上地壳平均页岩相比,大隆组中部明显富集Cd、P、Mo、U、Cu、Ni、V和Zn,亏损Co和Mn,显示出与现代上升流发育区近似的微量元素地球化学特征,指示该烃源岩层发育于上升流盛行的地区。

Geochemical and petrological analyses of the Permian section in Guangyuan of northeastern Sichuan suggest that multiple factors influenced the accumulation of organic matter.In this study,we conduct a comprehensive investigation on the geochemistry of the middle part of Dalong Formation to explore the link of accumulation of organic matter to the marine primary productivity,sedimentary environment（oxidation-reduction conditions）and sedimentary rate respectively.We found that high values of Mo,U,V,Cr,as well as Mo/U,V/Cr,V/（V＋Ni）are accompanied with high TOC value,which suggests anoxic conditions prevailed during the deposition of the middle part of Dalong Formation.The medium degree of paleoproductivity and lower sedimentary rate which weakened the dilution of terrigenous matter to the organic matter,made high TOC present in sediments.Compared with the average shale of upper,the sediments from the middle part of the Dalong Formation are significantly enriched in Cd,P,Mo,U,Cu,Ni,V,Zn are depleted in Co,Mn,showing the similar geochemical characteristics as modern upwelling zones.The results indicate that high quality hydrocarbon source rocks are developed in an environment prevailing with the upwelling flow.