上扬子区龙马溪组页岩中黄铁矿成因

Study on the genesis of pyrite in the Longmaxi Formation shale in the Upper Yangtze area

黄铁矿作为页岩的特征矿物,其结构特征具有重要意义,构成草莓状黄铁矿集合体的微晶在生长过程中受限于周围环境,可将其作为评判沉积环境水体环境的指标。因此,草莓状黄铁矿结构对还原古海洋沉积水体相有着重要意义。以扬子地区龙马溪组页岩中黄铁矿样品为基础,基于氩离子抛光-扫描电镜成像技术,对图像中草莓状黄铁矿集合体粒径,微晶粒径进行测量。结合龙马溪组页岩中有机质含量、硫含量数据,对黄铁矿成因和形成环境方面指示意义进行综合分析,得出以下结论:①研究区草莓状黄铁矿平均粒径、最大粒径(MFD)和集合体平均粒径与微晶平均粒径比值三者均偏小,分析沉积环境为闭塞硫化还原环境,草莓状黄铁矿微晶粒径对沉积环境的氧化还原状态具有指征意义。②草莓状黄铁矿形成于同生-准同生阶段;简单成因自行晶状黄铁矿形成于同沉积时期,复杂成因黄铁矿形成于成岩时期。③龙马溪组页岩中孔隙类型发育,黄铁矿周围伴有大量晶间有机质孔和铸模孔,黄铁矿有助于改善储层孔隙。深水陆棚相页岩中黄铁矿与有机质关系密切,对页岩气成藏开发有积极影响。

As a characteristic mineral of shale,pyrite is of great significance. The microcrystalline of strawberry pyrite aggregates is limited to the surrounding environment during the growth process,so it can be used as an indicator to evaluate the sedimentary environment. Therefore,strawberry pyrite is important to restitute palaeomarine sedimentary water facies. Based on argon ion polishing-scanning electron microscopy imaging of pyrite samples in the shale of Longmaxi Formation in the Upper Yangtze region,the particle size and micrograin size of the strawberry pyrite in the image are measured. Combined with the organic matter content and sulfur content in the Longmaxi Formation shale,a comprehensive analysis of the genesis and formation environment of pyrite is carried out,and the following conclusions are drawn:(1)The average particle size,maximum particle size(MFD)of the strawberry-like pyrite and the ratio of the average particle size of the aggregate to the average particle size of the microcrystals were both small in the study area,and the sedimentary environment is occluded and reduced. The crystallite size of the strawberry pyrite has an indication of the redox state of the sedimentary environment significance.(2)Strawberry-like pyrite is formed in the syngenetic-quasi-probiotic stage;the simple cause of self-crystallized pyrite is formed in the same sedimentary period,and the complex origin pyrite is formed in the diagenetic period.(3)The pore type in the Longmaxi Formation shale is developed. There are a large number of intercrystalline organic pores and mold pores around the pyrite. The pyrite content helps to improve the reservoir pores. Pyrite in deep-water continental shelf shale is closely related to organic matter and has a positive impact on shale gas accumulation and development.