四川盆地震旦系灯影组葡萄状白云岩成因讨论

A discussion about origin of botryoidal dolostone of the Sinian Dengying Formation in Sichuan Basin

四川盆地震旦系灯影组葡萄状白云岩是天然气藏的重要储集层和铅锌矿床的重要容矿围岩,但关于其特征和成因研究却充满争议。基于传统沉积学研究方法和手段,结合地球化学分析,并与前人研究成果进行对比,认为四川盆地灯影组葡萄状白云岩主要由顺地层、(层间和穿层)溶蚀孔缝中产出的拟晶白云石化的葡萄状团块和葡萄状、栉壳状胶结物或充填物组成。葡萄状团块主要顺地层产出,包括核心和包壳两部分:核心为葡萄石,主要成分为泥、微晶高镁方解石,由沉积阶段的海水沉积作用和微生物捕获、粘结作用形成;包壳主要成分为等厚纤维状环边文石,主要形成于沉积—同生阶段,有厚、薄之分,其中薄包壳形成于相对动荡的海底,而厚包壳形成于相对安静的海底。葡萄状、栉壳状充填物主要在溶蚀孔缝中产出,主要成分为等厚纤维状文石和高镁方解石,为准同生阶段大气淡水作用和海水沉积作用的产物;也存在顺地层葡萄状团块、内碎屑等颗粒粒间或顶部产出的葡萄状、栉壳状胶结物或层状胶结壳,主要为同生—准同生阶段海水潜流带沉积。在葡萄状或栉壳状团块、充填物、胶结物的形成过程中,文石和高镁方解石结晶析出后迅速发生拟晶白云石化作用,并保留其原生的结构和构造。文中关于葡萄状团块和拟晶白云石化作用的研究,为葡萄状白云岩成因研究提供了新的思路和认识。

The botryoidal dolostone of the Sinian Dengying Formation in Sichuan Basin is an important type of gas reservoirs and host rocks of lead-zinc deposit. However,there are still some contentious issues on its characteristics and origin. In this paper,based on the traditional research methods of sedimentology and analyses of geochemistry,the origin of botryoidal dolostone is studied. Compared to the previous researches,the results show that the botryoidal dolostone of the Sinian Dengying Formation in Sichuan Basin is mainly composed of mimetic dolomitized botryoidal lumps,and botryoidal or ctenoid fillings and cements,which mainly develop along bed,interbed,or in the dissolved pores and fractures crossing bed. The botryoidal lumps develop mainly along bed,and are comprised of cores and coatings. The cores of botryoidal lumps are grapestones whose major components are cryptocrystalline and microcrystalline high-Mg calcites formed by marine deposition as well as microbial capturing and binding effects during the depositional stage. The major components of coatings are isopachous fibrous zoned aragonites formed during the depositional-contemporaneous stages. There are two kinds of coatings,in which,the thin coatings were formed in a relatively-turbulent seabed,while the thick ones in a relatively-quiet seabed. The botryoidal and ctenoid fillings develop in dissolved pores and fractures. Their major components are isopachous fibrous aragonites and high-Mg calcites formed by meteoric leaching and marine deposition during the penecontemporaneous stage. Meanwhile,there are also botryoidal and ctenoid cements or lamellar cement layers in the intergranular pore or top of grains （e.g. botryoidal lumps and intraclasts） along bed,which deposited in a marine phreatic zone during the contemporaneous-penecontemporaneous stages. During the forming process of botryoidal or ctenoid lumps,fillings and cements,aragonites and high-Mg calcites were immediately mimetic dolomitized after crystallization and precipitation,and preserved primary textures and structures. The research of botryoidal lumps and mimetic dolomitization can provide a new thought and understanding about the origin of botryoidal dolostone.