川东地区下侏罗统自流井组东岳庙段泥页岩沉积特征及物质来源

Sedimentary characteristics and sources of shale of Dongyuemiao member of Lower Jurassic Ziliujing Formation in eastern Sichuan Basin

川东地区下侏罗统自流井组东岳庙段发育一套深色泥页岩,具有较大的页岩气资源勘探开发潜力。将宏观大地构造背景与微观岩石学特征分析相结合,利用微量元素(包括稀土元素)数据,对川东地区下侏罗统自流井组东岳庙段泥页岩的沉积环境和物质来源进行了分析与讨论。研究结果表明:(1)川东地区下侏罗统东岳庙段自下而上依次沉积灰色介壳灰岩、灰色粉砂质泥岩、深灰色介壳泥岩、灰黑色页岩、灰—深灰色泥岩。(2)研究区东岳庙段泥页岩稀土元素总体具有轻稀土元素富集、重稀土元素亏损的特征,δCe为负异常,Ce_(anom)平均值为-0.015,V/(V+Ni)平均值为0.70,Sr/Cu平均值为2.43,Sr/Ba平均值为0.16,(La/Yb)N平均值为1.32,泥页岩沉积环境为相对贫氧的弱还原环境,整体为温暖湿润气候下的淡水沉积,且沉积速率较快,有利于有机质的富集和保存。(3)Zr/Sc-Th/Sc图和La/Sc-Co/Th图显示研究区沉积物母岩为花岗岩;Th/Sc值为0.78、Th/Co值为0.65、Th/Cr值为0.14、La/Sc值为2.14,均接近上地壳的相应元素比值,表明母岩主要为来自上地壳;La,Ce元素校正后平均质量分数分别为34.5×10^(-6)、70.71×10^(-6),La/Yb,LREE/HREE,(La/Yb)_(N)校正后的平均值分别为11.32,8.13,7.67,δEu校正后的平均值为0.82,表明其与大陆岛弧相近,且La-Th-Sc,Th-Sc-Zr/10和Th-Co-Zr/10等3个三角图显示其构造背景为大陆岛弧。因此,研究区东岳庙段物质来源于北东侧大巴山和东南侧江南古陆(雪峰古陆)。

A set of dark shale is developed in Dongyuemiao member of Lower Jurassic Ziliujing Formation in eastern Sichuan Basin,which has great potential for shale gas exploration and development.By combining the macro tectonic background with the analysis of micro petrological characteristics,and using trace element(including rare earth elements)data,the sedimentary environment and material sources of the shale of Dongyuemiao member of Lower Jurassic Ziliujing Formation in eastern Sichuan Basin were analyzed and discussed.The results show that:(1)Gray shell limestone,gray siltstone,dark gray shell mudstone,gray black shale,and gray-to-dark gray mudstone were deposited from bottom to top in Dongyuemiao member of Lower Jurassic in eastern Sichuan Basin.(2)Rare earth elements of shale of Dongyuemiao member in the study area are characterized by enrichment of light rare earth elements,heavy rare earth elements loss.δCe is a negative anomaly,with an average Ceanom value of-0.015,V/(V+Ni)average value of 0.70,Sr/Cu average value of 2.43,Sr/Ba average value of 0.16,and(La/Yb)N average value of 1.32.The sedimentary environment of shale is relatively oxygen-poor and weakly reductive,and it is generally in a warm and humid climate with light water deposition,with fast deposition rate,which is conducive to the enrichment of organic matters.(3)The Zr/Sc-Th/Sc and La/Sc-Co/Th plots show that the sedimentary parent rock in the study area is granite.The values of Th/Sc,Th/Co,Th/Cr and La/Sc are 0.78,0.65,0.14 and 2.14,respectively,all of which are close to the corresponding element ratios of the upper crust,indicating that the parent rock is mainly granite from the upper crust.The average mass fractions of corrected La and Ce elements are 34.5×10^(-6) and 70.71×10^(-6),respectively,and the average values of corrected La/Yb,LREE/HREE,(La/Yb)N are 11.32,8.13 and 7.67,respectively,and the average value of correctedδEu is 0.82,indicating that they are similar to those of continental island arc.The triangle diagram of La-Th-Sc,Th-Sc-Zr/10 and Th-Co-Zr/10 show that its tectonic background is a continental island arc.Therefore,the provenance of Dongyuemiao member in the study area comes from Daba Mountain in the northeast and Jiangnan ancient land(Xuefeng ancient land)in the southeast.