南沙群岛南科1井白云岩发育特征及铁白云石成因机制的初步研究

Preliminary studies on the development characteristics of reef dolostones and the formation mechanism of iron dolomite in the Well Nanke 1,Nansha Islands

南海自新生代以来发育了大量的碳酸盐岩台地和生物礁,并普遍发生白云岩化作用,具有广阔的油气勘探前景。文章通过开展岩相学观察、矿物学分析、常微量元素和碳氧同位素分析,同时结合Fe组分及Fe同位素地球化学分析,对南沙群岛南科1井白云岩的发育特征和成岩环境进行了系统研究。结果表明:南科1井上中新统—下更新统白云岩主要形成于近地表环境中,溶蚀孔隙发育,残余结构明显。白云岩层中发育有多个与暴露成因有关的界面,未发现石膏层的存在。同时,白云岩普遍具有低Fe、Mn和Sr的特征以及与现代海水相似的REE分布模式,δ^(13)C和δ^(18)O多为正值,但不存在相关性。整体来看,白云岩可能形成于轻微蒸发海水的渗透回流作用,还受到了与古气候变冷有关的海平面下降的控制。此外,南科1井岩芯中多个层位发育铁白云石,并且集中分布在暴露面附近。Fe组分和Fe同位素组成结果显示,白云岩中Fe主要来源于海水中碳酸盐的沉淀,成岩过程中基本不存在额外的陆源或热液来源的Fe混入。铁白云石主要形成于低温和浅埋藏环境中,大气淡水对生物骨架、生物碎屑以及自生碳酸盐矿物的淋滤-溶解作用为其提供了重要的物质来源。作为生物礁体暴露地表期间的产物,铁白云石的发育层位可能指示着低海平面时期。

Carbonate platforms and biological reefs are widely developed in the South China Sea(SCS)since Cenozoic,and the dolomitization is a common phenomenon,which has led to a broad prospect for oil and gas exploration.In this study,the development characteristics and diagenetic environments of reef dolostones in the Well Nanke 1 from Meiji Island were investigated,mainly based on the petrographic observations,mineralogical analysis,major and minor elements analysis,carbon and oxygen isotopes analysis,and the geochemical analyses of Fe speciation and Fe isotope,etc.The results showed that the reef dolostones of the upper Miocene to Lower Pleistocene in the Well Nanke 1 were mainly formed in the near-surface environments,and the dissolved pores were ubiquitous,which were accompanied with obvious residual structures.Several exposed surfaces were found in the dolomite layers,but there was no existence of the gypsum layer.Meanwhile,the reef dolostones were characterized by relatively low concentrations of Sr,Fe and Mn,with the similar REE distribution pattern to that of modern seawater.The δ^(13)C and δ^(18)O values were mostly positive,but there was no correlation between them.It indicated that the dolomitization could largely attributed to the seepage-reflux of slightly evaporated seawater and the sea level fall related to the paleoclimate cooling.In addition,iron dolomite was found in several layers,mainly near the exposed surfaces in the Well Nanke 1.The results of Fe speciation and Fe isotopic composition showed that the iron of reef dolostones was mainly derived from the chemical precipitation from the seawater,and no significant additional sources(e.g.,terrigenous or hydrothermal)were present in the diagenetic environments.In general,the iron dolomite was mainly formed in the shallow burial conditions with low temperature.The vital material sources of iron dolomite were mainly from the leaching and dissolution of bioskeletons,bioclasts and authigenic carbonate minerals by meteoric freshwater.As a product of reefs exposure,the positions where the iron dolomite prevalently developed in the core could be an indicator of low sea-level stages.