华北地台中元古代微生物白云岩: 超微组构、有机矿化过程和成因机制研究

Enriched in organic matter and micro-porosity, and together with potential of forming high quality reservoirs, microbial dolomites have often been taken as one of the promising targets for oil-gas exploration. Microbial activity is the only mechanism proved so far for dolomite precipitates in low temperature. This model, however, has rarely been confirmed through and used in ancient dolomites, especially for the Proterozoic during which massive dolomites were widely formed. Targeting some potential microbial dolomite intervals recognized from the Mesoproterozoic Gaoyuzhuang, Wumishan and Tieling formations on the North China platform, the project will study meter-scaled cycles, micro-facies, mineral associations, and analyze the isotopes (e.g., C,S,Cr,Zn) and RSEs (e.g.,Mo,U,V,Re,Cr) of the intervals to determine the depositional environment and sea-water chemistry under which microbial dolomites have been deposited. Using La-ICP-MS, SEM, TEM and EDS techniques, the study will focus on the ultra-microfabrics, bio-mineralization, configuration of organo-minerals, and chemical composition of multi-level microfabrics in the dolomites, so that the microbial dolomite types, identifying indicators, major controls, and potential bio-chemical processes potentially involved in the dolomite formation can be properly revealed. With aid of XRF, NanoSIMS, EDS techniques, combined with crystallography, the composition and texture of transitional series between proto-dolomite and ordered dolomite will be analyzed and compared, so that the process and mechanism for this transition can be determined. This study will further complete through supplement with microbial dolomite mechanism in early Earth environments, and also provide important information of life-environment interaction in deep Earth history, and help to narrow the targets oil-gas exploration in ancient carbonates.

微生物白云岩富有机质和微孔,常形成优质烃源储集和油气勘探目标层.尽管微生物活动被证明是地表条件下原生白云石沉淀的唯一机制,但在白云岩极发育的中元古代并未证实.立项拟选取华北Pt2高于庄、雾迷山和铁岭组产微生物白云岩层段,开展米级旋回、沉积微相、矿物组合及同位素(C,S,Cr,Zn)和氧化还原敏感元素(Mo,U,V,Re,Cr等)分析,确定微生物白云岩发育的沉积环境和海洋化学条件.运用La-ICP-MS,SEM,TEM,EDS等技术揭示微生物白云岩的超微组构、矿化过程、有机矿物配置和多级微组构的化学组成，查明微生物白云岩类型、标识、主控因素和可能的生物化学过程。运用XRF, NanoSIMS技术分析过渡系列矿物并结合晶体光学研究,揭示原白云石向有序白云石转化过程和机制.补充地球早期环境下白云岩的生物成因模式；为深刻认识早期生命与环境相互作用和和古代白云岩区优选油气勘探目标提供信息和指导。

为揭示白云岩的成因、生物矿化机制及其可能的海洋化学条件, 本项目对华北地台中元古代碳酸盐岩开展了多学科的综合研究。发现华北微生物成因白云岩相当普遍，主要发育于贫氧、富有机质的潮下带浅水环境，常与微生物席相关; 表明海水氧含量、硫酸盐浓度和微生物发育程度对原生白云岩的形成有明显的控制作用。.应用FESEM、EDS、XRD、微区元素填图以及同位素和元素丰度分析等技术方法，在层状细粒白云岩中发现了保存良好的有机组分(EPS, 丝状, 球状细菌体)和多种有机矿化超微组构(纳米球, 微球和多种小球状体)；它们与培养实验和现代环境中识别的有机矿物微组构及其与白云石微粒的配置关系十分相似, 表明属微生物成因。样品中大量的纳米球(20–200 nm)主要附着于EPS或集中在菌体周边。纳米球通过粘结形成多形晶(60 nm–2 μm), 后者与有机质进一步联合形成哑铃状或球状集合体(10–30 μm; 微球粒)。由微球粒与有机质形成的核心和贫有机质文石环边一起形成小球粒(30–120 μm); 后者则往往是微生物白云岩的基本建构单元, 并在随后的成形过程中逐步转化为变化不等的近菱形白云石晶体。. 矿物和化学组分分析表明小球粒核心的微粒及其环边的微晶均为镁方解石组分, 但微球粒具有较高Ca2+ 和Si2+ 成分, 环边则富Mg2+和Cl2- 离子。微球粒集中区的原位碳同位素(δ13C)值较富胶结物区部位低约0.3– 0.8‰。这可能表明, 小球粒中Mg2+的加入较晚, 主要源自水柱。由EPS厌氧降解形成的纳米球粒可能扮演了白云石沉淀及随后多形晶生长的晶核角色；而多形晶和微球粒通过与周边海水相互作用产生的有利微环境则是促进小球粒环边微晶胶结物沉淀的重要机制.. 本研究从微组构、矿物组分和元素配置关系等方面证明, 华北中元古代存在大量的微生物成因白云岩，其主要发育在贫氧、富有机质的浅潮下带环境，可用做指示古海洋环境条件的沉积学标识。其中存在的残余菌体, EPS以及与之交织的有机矿物不仅可作为地球早期生命的证据, 同时也是深时微生物与环境相互作用过程的重要记录。