焦石坝五峰组-龙马溪组页岩硅质生物成因的证据及其地质意义

Evidences of biogenic silica of Wufeng-Longmaxi Formation shale in Jiaoshiba area and its geological significance

以页岩露头样品和钻井岩屑样品元素测量结果为依据,结合岩心和薄片观察,分析焦石坝五峰组-龙马溪组页岩的硅质生物成因的证据,并探讨其地质意义。结果表明:焦石坝五峰组—龙马溪组页岩中存在大量的笔石、角石、珊瑚等古生物化石和放射虫微体生物化石;SiO_2含量介于40%~80%,平均达63%;过量硅含量为2.6%~55.31%,其中五峰组和龙马溪组页岩下段过量硅含量较高,平均分别为41.3%和25.5%;Al/(Al+Fe+Mn)值介于0.62~0.87,与纯生物成因硅质岩相近,Al-Fe-Mn三角图上,研究区页岩样品绝大部分落在生物成因区,表明页岩硅质成分属于生物成因;深水陆棚贫氧、缺氧沉积环境有利于页岩有机质的富集和有机孔隙的发育,对于页岩天然裂缝的形成和后期人工压裂都具有重要意义。

Based on the elemental content of the ground outcrop samples and the drilling cutting samples,combined with the observation of core and thin section,the evidences of biogenic silica of Wufeng-Longmaxi Formation shale in Jiaoshiba area were analyzed,and the geological significance was discussed as well.The results show that there are a lot of paleontological fossils,such as graptolite,oncoceras,coral,and a lot of microfossils,such as radiolarian,in Wufeng-Longmaxi Formation shale.The content of SiO2 is between 40% and 80%,about 63% on average.The content of the assess SiO2 is between 2.6% and 55.31%,and the contents in Wufeng Formation and the lower member of Longmaxi Formation shale are higher,which are up to 41.3% and 25.5% on average respectively.The value of Al/（Al＋Fe＋Mn） is between 0.62 and 0.87,which is close to the value of pure biogenic siliceous rocks.In the triangular Al-Fe-Mn diagram,most of the samples locating in the biogenic silica area indicate the silica of Wufeng-Longmaxi Formation shale is biogenic.High content of biogenic silica indicates deep-water shelf,hypoxic or anaerobic sedimentary environments is beneficial of enrichment of organic material and formation of organic pores,which shows important significance to the formation of natural fracture and the later artificial fracturing.