应用TIMA分析技术研究Alum页岩有机质和黄铁矿粒度分布及沉积环境特征

Grain Size Distribution of Organic Matter and Pyrite in Alum Shales Characterized by TIMA and Its Paleo-environmental Significance

Alum页岩(中寒武—早奥陶世)是北欧一套重要的海相烃源岩,其成熟度跨度从为成熟-过成熟度阶段。由于我国下古生界海相烃源岩均已过成熟,未成熟-低成熟度的Alum页岩是研究下古海相的烃源岩生烃潜力特征的重要参照样品。因此,对这套成熟度较低的Alum页岩的生物组成特征、矿物组成及其沉积环境的分析,可为后续国内外下古生界海相烃源岩的对比研究奠定基础。本文以欧洲上寒武统富含有机质Alum页岩为主要研究对象,在有机碳含量(TOC)和有机岩石学观察的基础上,应用综合矿物分析技术(TIMA)进行扫描,通过细化样品扫描参数,获得了页岩矿物组成、含量及粒度分布。Alum页岩有机质成熟度较低(固体沥青反射率为0.30),TOC含量在11.16%~12.24%之间。有机质主要为浮游藻类降解形成的层状藻类体、底栖藻类来源的海相镜状体和裂缝中充填的固体沥青。TIMA扫描获得的有机质相对质量百分含量为9.79%~10.64%,略低于碳硫分析仪测定的TOC含量;黄铁矿含量为4.17%~4.49%。TIMA扫描获得的有机质与黄铁矿比值与化学法的C/S比值相近,均分布在2.18~2.55范围。粒径分布特征上,有机质粒径主要分布在0.9~27.0μm之间(80%以上颗粒分布在1.2~5.5μm);草莓状黄铁矿粒径分布在0.9~17.0μm之间(小于0.5μm的颗粒占78%以上),反映了缺氧甚至硫化的环境。综合C/S比、有机岩石学与TIMA黄铁矿粒度分布特征,认为该页岩形成于闭塞封闭甚至硫化的沉积水体体系。该研究为油气地质领域的烃源岩(包括页岩)的研究提供了一种新的技术支持。

BACKGROUND:Alum shale(Middle Cambrian to Lower Ordovician)is a set of significant marine source rock in Northern Europe,and the maturity is from immature to over mature.Because of high maturities of the Lower-Paleozoic marine source rocks in China,the immature-early mature Alum shales are important samples for the comparative studies of hydrocarbon generation potential.Thus,the analyses of organic matter,mineral composition and sedimentary characteristics of the Alum shale with low maturity,are the fundamental for the comparative studies on the Lower Paleozoic marine source rocks at home and abroad.OBJECTIVES:To uncover the mineral and organic matter compositions,and the sedimentary characteristics of Alum shales.METHODS:Based on the organic carbon content(TOC)and organic petrological observations,the integrated mineral analysis(TIMA)technology was used to scan the sample.Scan parameters were refined to obtain the mineral composition,content and particle size distribution of shale.RESULTS:The TOC and TRS contents of studied Alum shale(%Rb=0.3)were 11.16%-12.24%and 4.30%-5.31%,respectively.The maceral compositions included the lamalginite from planktonic algae degradation and vitrinite-like macerals from benthic algae.Solid bitumen filled in the fractures and pores.The organic matter(OM)and pyrite(Py)contents acquired from TIMA scanning analyses were 9.79%-10.64%and 4.17%-4.49%,respectively.The OM/Py ratios ranged from 2.18 to 2.55,similar to the C/S ratios from Rock-eval pyrolysis.Grain sizes of OM were 0.9-27.0μm,and 80%grains had sizes of 1.2-5.5μm.Grain size of Py distributed from 0.9 to 17.0μm,and the proportion of grains with the size of lower than 0.5μm was higher than 78%,indicating an oxygen-lacking or sulfurized environment.CONCLUSIONS:A combination of C/S ratio,and characteristics of organic petrology and Py grain size distribution indicates the Alum shale was deposited in a closed or sulfurized water system.A new technical support for the research of source rocks(including shale)in the field of petroleum geology is provided through the methods used and discussed in this study.