四川广元-陕西镇巴地区上奥陶统五峰组地球化学特征及沉积环境意义

摘要

The black shales with a great potential in the Upper Ordovician Wufeng Formation from the Guangyuan region, northern Sichuan to the Zhenba region, southern Shaanxi is interpreted as the key horizons for the marine oil and gas exploration in southern China. Referenced to the previous results, the present paper focuses on the analysis of palaeoclimate, palaeodepth and palaeoredox conditions in the study areas. The analytical results including 0 . 64 for the average V/( V+Ni ) ratio;1 . 87 for the average U/Th ratio;2 . 98 for the average V/Cr ratio, 15. 07 for the average Ni/Co ratio and 0. 23 for the average MnO/TiO2 ratios suggest the highly reduced conditions in the marginal shallow sea during the deposition of the Wufeng Formation. The high Ba contents, and low Sr contents and Sr/Ba ratios for the black shale samples from the Wufeng Formation indicate that the Wufeng Formation was once laid down in the fresh water environments. Geochemically, the samples display LREE enrichment. The δCe values vary between 0. 77 and 1. 57 with an average of 1. 11, indicating positively Ce anomalies or slightly negative Ce anomalies. The average Ce/La ratio (2. 22) and average δEu (0. 59) indicate the dysaerobic to anaerobic shallow marine environments in the study areas.%上奥陶统五峰组黑色页岩有机质含量高,生烃潜力巨大,为南方海相油气勘探的重点层位.对于上奥陶统五峰组的沉积环境,历来争议较大,主要为浅水成因论和深海成因论.本文从前人的研究成果出发,运用元素地球化学方法,对研究区五峰组的古氧相、古气候、古水深等沉积环境进行分析.分析结果显示,研究区V/(V+Ni)平均值为0.64;U/Th为1.87;V/Cr为2.98;Ni/Co为15.07,V/(V+Ni)、V/Cr、U/Th等值均指示研究区五峰组沉积环境为强还原环境.MnO/TiO2比值在0.02~1.89之间,其平均值为0.23,指示离陆地较近的边缘浅海环境;样品总体上表现为高Ba低Sr的趋势,Sr/Ba比值普遍较低,平均值为0.02,远远小于1,指示五峰组沉积环境应该为淡水沉积.稀土元素总体表现为轻稀土富集,δCe值位于0.77~1.57之间,平均值为1.11,为Ce正异常或不明显的Ce负异常,Ce/La平均值为2.22,δEu平均值为0.59,表明研究区五峰组形成于贫氧-厌氧的浅海沉积环境.