西沙群岛西科1井中新统-上新统白云岩微观特征及成因

Microscopic Features and Genesis for Miocene to Pliocene Dolomite in Well Xike-1,Xisha Islands

西科1井白云岩主要分布于上中新统黄流组,在上新统莺歌海组二段和中中新统梅山组有零星分布;主要的白云岩层段一般发育在褐色铁质矿物浸染的古暴露面之下.根据岩石铸体薄片观察、阴极发光及扫描电镜测试分析,西科1井白云岩中白云石总体上呈微晶及细粉晶双峰态结构,微晶白云石为灰岩基质经选择性白云石化的结果,呈平直晶面半自形晶,主要为泥微晶基质白云石化的结果;粉晶-细晶白云石呈平直晶面自形晶,为胶结物白云石或过度白云化结果,过度白云化雾心亮边白云石的＂亮边＂与胶结物白云石成分一致,阴极发光下二者显示同样的光性特征.微量元素测试及碳氧同位素测试表明：白云岩一般具有低铁、低锰含量,δ18 OPDB均为正值,变化于2.293‰~5.072‰之间,δ13 CPDB变化于1.214‰~3.051‰之间;西科1井白云岩与西琛1井白云岩具有相似的层位分布特征和碳、氧同位素特征,可能反映着相同或相似的成因.回流渗透模式可能适用于西沙地区白云岩,频繁的海平面升降、环礁内蒸发环境及与中新世末期构造运动有关的热流体上涌促进了西沙地区白云岩的形成,高渗透性礁相碳酸盐岩沉积为高Mg/Ca比值的蒸发水回流渗透提供了运移通道.

The dolomites in the well Xike-1 drilled in Xisha Islands, South China Sea, are mainly found in the Upper Miocene Huangliu Formation and secondly found in the 2nd member of Pliocene Yinggehai Formation and in the Middle Miocene Meis- han Formation. The major dolomite layers commonly develop below the exposed boundary that are dipped and dyed with brown ferric minerals. While the dolomite crystals are found to be bimodal with micrite and siltyffine dolomite by means of the casting thin sections, scanning electron microscope and cathodoluminescence. The hypidiomorphic micrite dolomites with flat crystal planes are the result of the dolomitization from the micrite matrix. Meanwhile, the silty-fine crystalline dolomites are clean and bright cement or dolomite crystals with misted heart and bright side, and the latter is generated from the so-called overdolo mitization. It＇s thought that the cement dolomites are with the same composition to the misted heart of the overdolomitization dolomites due to their alike optical properties from the cathodoluminescence. It is indicated by means of the microelement and C, O isotopic measurements that the dolomite is with less iron and manganese composition, positive values of δ18 OPDB and δ13CPDB. The δ18 OpDB values range from 2.293%o to 5.072N0, and the δ13CPDB are 1.214%0--3.051%0. The similar distribution of dolomite layers and the similar C, O isotopic values of dolomites in the wells Xike-1 and Xichen-1 might indicate the same or similar cause of the dolomites in these two wells in Xisha area. The dolomitization model of infiltration and circumfluence may be applied to that in the wells Xike-1 and Xichen-1. The highly permeable reef carbonates in Xisha area enabled the infiltration and circumfluence of the fluid with a high Mg/Ca ratio, which was affected by the frequent sea-level changes, the evaporitic environment in the atolls and the upwelling of the thermal fluid during the latest Miocene to Early Pliocene periods.