渤海湾盆地东营凹陷沙河街组泥晶碳酸盐岩中的微生物沉积作用

MICROBIAL SEDIMENTATION OF SHAHEJIE FORMATION MICRITIC CARBONATE IN THE DONGYING DEPRESSION,BOHAI BAY BASIN

东营凹陷古近系沙河街组发育纹层状泥晶碳酸盐,其产出与相邻泥、页岩中有机质含量之间存在较好相关性,但对其成因认识尚不明确。为了正确认识陆相湖泊环境中纹层状泥晶碳酸盐的成因机制,通过岩芯(619.65m)、薄片(NY1井9片,FY1井3片)观察、荧光显微镜(2片)、扫描电镜(10片)观察和地球化学分析对上述纹层状泥晶碳酸盐进行研究。观察发现纹层中存在纳米级球状和丝状碳酸盐矿物组构,与微生物活动导致其细胞外微环境物理化学条件改变而形成的胞外聚合物和丝状细菌十分相似。泥晶碳酸盐岩纹层有机-无机碳同位素对分析结果显示,无机碳同位素δ13 Ccarb值为2.1‰~4.8‰,有机碳同位素值为-27.9‰^-22.6‰,两者呈明显负相关。基于该结果提出单一碳库有机-无机碳同位素演化模式佐证泥晶碳酸盐的沉积作用与微生物活动之间存在直接联系。以上研究表明纹层状泥晶碳酸盐形成过程中受微生物活动的影响,为湖相环境中生物活动对碳酸盐矿物沉积作用的影响研究提供了重要的实例。

The black organic-rich mudstones of the Paleogene Shahejie Formation are interbedded with some lamellar micrite. There is a positive correlation between the content of micritic carbonate minerals and organic matter. The petrological and geochemical features of micrite are studied by means of core and thin section observation, fluorescence microscope, SEM and carbon iso- tope analysis so as to understand the genesis of the micrite in the continental lacustrine environment. It shows that nanoscale spherical and filamentous carbonate minerals exist in the lamellar micrite and they are similar to the extracellular polymer and filamentous bacteria reported in other research. These carbonate minerals are probably formed by changes of physical and chemical conditions of extracellular microenvironment resulted from the microbial activity. According to the carbon isotopes analysis, non-organic carbon isotope value ranges from 2.1%0 to 4.8%0 and the organic carbon isotope value from -27.9‰ to -22.6‰. There is a negative correlation between organic carbon isotopes and non-organic carbon isotopes,indicating that the sedimentation of the micrite has a direct link to microbial activities. Therefore, the formation of lamellar micrite in the black mudstone in the Shahejie Formation is closely related to the microbial activities. This study provides important evidence for the effect of microbial activities on the carbonate mineral deposition in the continental lacustrine environment.