Microscopic Features and Genesis for Miocene to Pliocene Dolomite in Well Xike-1, Xisha Islands
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摘要: 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, δ18OPDB均为正值, 变化于2.293‰~5.072‰之间, δ13CPDB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.Abstract: 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 Meishan 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 silty-fine 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 overdolomitization. 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 δ18OPDB and δ13CPDB. The δ18OPDB values range from 2.293‰ to 5.072‰, and the δ13CPDB are 1.214‰-3.051‰. 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.
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Key words:
- Xisha /
- dolomitization /
- Miocene /
- Pliocene /
- South China Sea /
- petrology
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图 2 西科1井上新统-中中新统岩性及碳、氧同位素柱状图
Fig. 2. Lithologic column and oxygen and carbon isotopes of the Pliocene-Middle Miocene of the well Xike-1
图 3 西科1井白云岩宏观特征
a.上新统莺歌海组二段顶部(289.2 m)白云岩中有褐色金属矿物浸染; b.莺歌海组底部发育浅褐色白云质颗粒灰岩(375.9~376.0 m)，其沉积时受到中新世末构造运动造成的古暴露面的影响; c.中新统黄流组顶部(376.1 m)白云岩受到褐色铁质矿物浸染; d.黄流组二段近顶部(470.2~470.6 m)见铁质浸染脉; e.黄流组二段近顶部(470.64 m)见铁质矿物结核; f.中中新统梅山组近顶部(582.4 m)白云质灰岩中见白云石结核及厘米级的方解石巨晶，二者在显微镜下均可识别
Fig. 3. The macroscopic variables of dolomite in the well Xike-1
图 4 西科1井白云岩微观特征(均为单偏光)
a.莺歌海组一段下部(284.69 m)含云微晶生屑灰岩，仅棘皮类生物碎屑发生不完全的白云石化，茜素红染色，对角线长1.6 mm; b.莺歌海组二段顶部(288.91 m)微晶生屑云岩，照片中间的棘皮类生屑完全白云石化，对角线长4.0 mm; c.黄流组一段(457.21 m)藻云岩，藻类由微晶白云石组成，孔隙内见雾心亮边白云石，对角线长4.0 mm; d.黄流组二段(534.03 m)粉晶云岩，基质见大量不彻底的过度白云化白云石，孔隙内见雾心亮边白云石，对角线长4.0 mm; e.黄流组二段(542.29 m)微晶-粉晶云岩，微晶基质边缘及孔隙内见雾心亮边粉晶白云石，对角线长1.6 mm; f.黄流组二段(568.20 m)粉晶云岩孔隙内沉淀粉晶自形晶胶结物白云石，对角线长0.8 mm; g.黄流组底部(576.70 m)灰质珊瑚云岩，隐晶质珊瑚未发生白云化(或发生去云化)，生物骨架孔内主要填充粉晶胶结物白云石，对角线长1.6 mm; h.梅山组一段(621.97 m)生屑云岩，胶结物白云石发育在颗粒边缘，粒间孔内见雾心亮边白云石，对角线长1.6 mm; i.梅山组一段(628.86 m)粉晶残余珊瑚云岩，过度白云化“雾心亮边”粉晶白云石含量高，对角线长4.0 mm
Fig. 4. The micro characteristics of dolomite in the well Xike-1
图 5 白云岩阴极发光、扫描电镜显示特征及去云化现象
a, b.黄流组二段残余生屑云岩(482.85 m)单偏光及阴极发光照片，白云石不发光; c, d.黄流组二段粉晶云岩(504.44 m)单偏光及阴极发光照片，微晶生屑及雾心亮边白云石中心发暗色光性; e.黄流组二段粉晶云岩(505.00 m)扫描电镜下特征：生物体内部为泥微晶白云石，外部为晶粒白云石; f.梅山组一段上部(585.18 m)白云石被方解石不完全交代，大部分白云石晶体边缘被首先交代，一部分白云石晶体内部被首先溶解交代，单偏光，茜素红染色，对角线长1.6 mm; g.梅山组一段含云灰岩(648.45 m)中见菱形自形晶白云石(黄色三角所指)内部发生方解石交代作用，单偏光，茜素红染色，对角线长1.6 mm; h.梅山组一段云质灰岩(652.93 m)中自形晶粉晶白云石(黄色三角所指)普遍发生不完全去云化，局部较小的晶体(蓝色三角所指)已完全被方解石交代，绿色三角所指晶体为近乎完全去云化的白云石晶体，单偏光，茜素红染色，对角线长1.6 mm; i.梅山组一段含云灰岩(712.01 m)中见白云石结核及菱形自形晶白云石(黄色三角头所指)，蓝色三角所指区域疑为白云石结核发生完全去云化所致，单偏光，茜素红染色，对角线长0.8 mm
Fig. 5. The characteristics of dolomite indicated with CL, SEM and the dedolomitization
图 6 西科1井碳酸盐岩碳、氧同位素特征
Fig. 6. The oxygen and carbon isotopic values of the carbonate rocks in the well Xike-1
表 1 西科1井各组段白云岩碳、氧同位素统计
Table 1. The oxygen and carbon isotopic values of dolomite in the formations of the well Xike-1
地层层位 数值点(个) CaO(%) MgO(%) δ18OPDB(‰) δ13CPDB(‰) 莺歌海组二段 2 33.708~33.933 18.57~18.73 4.284~4.854/4.565 1.762~1.960/1.861 黄流组一段 22 32.697~34.382 17.93~19.53 3.121~5.072/3.194 1.792~2.565/2.135 黄流组二段 55 31.686~36.866 16.27~19.78 2.473~4.719/3.519 2.115~2.801/2.410 梅山组一段 10 34.057~37.191 15.58~18.66 2.293~4.619/3.984 2.626~3.051/2.789 注：CaO与MgO含量显示了数值区间，δ18O与δ13C显示为数值区间/平均值. 
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