Title
Characteristics and origin of dolostones in the Miocene Meishan Formation in Well Xike 1, Xisha Islands, South China Sea
摘要
南海新生代碳酸盐台地发育，在西沙地区碳酸盐岩层序中白云岩常见。通过对西科1井白云岩的宏观形态、微观特征以及碳、氧同位素、主微量元素测试分析，描述了中新统梅山组白云岩的特征。西科1井梅山组主要发育4套厚度不一的白云岩层，颜色各异，孔隙发育。厚层白云岩中可见砂糖状白云岩，且往往分布在古暴露面之下；薄层白云岩表现为弱固结，白云岩化不彻底。白云石晶体以平直面半自形-自形晶为主，部分粉、细晶白云石发育雾心亮边。梅山组碳酸盐岩的碳、氧同位素值整体呈现一定的相关性，而白云岩的碳、氧同位素值相关性不明显。白云岩沉积时的古盐度值略高正常海水，且白云岩层中氧同位素值均为正值，说明其不受岩浆热液流体及有机酸溶液的影响。梅山组薄层白云岩多为灰质云岩，白云岩整体具有低铁、低锶、低锰的特征，说明其镁离子来源主要为海源。稀土元素具有低Ce、低Eu的特点，综合认为梅山组白云岩以渗透回流成因为主，且受到地热增温的影响，具有高孔隙度、高渗透率的特点，具有良好的储层潜力。
Abstract
Cenozoic carbonate platform was well developed in South China Sea, and dolostones are quite common in carbonate sequences of Xisha Islands. The characteristics of the Miocene Meishan dolomite is described by studying the macro-and micro-scopic features of dolostones, the carbon and oxygen isotopes, and main/trace elements in Well Xike 1. Mainly four categories of dolostones were developed in the Meishan Formation in Well Xike 1, which are different in thickness, color and porosity. Sucrosic dolomites, commonly distributed under the ancient exposed surface, can be seen in thick-bedded dolostones. While thin-bedded dolostones feature loose consolidation and incomplete dolomitization.The dolomite crystals are dominated by subhedral-idiomorphic flat surfaces, and bright edge and foggy center are seen in some dolomites of very fine to fine-crystallized crystals. The carbon and oxygen isotopes of carbonate rocks in the Meishan Formation show certain correlation, while the correlation of carbon and oxygen isotopic values in the dolostones is insignificant. The paleosalinity value of dolostones is calculated to be slightly higher than that of contemporaneous seawater. The oxygen isotopic value of dolostones is positive, indicating that the dolomitization was not affected by magmatic hydrothermal fluids and organic acid solution.Most thin-bedded dolomites in the Meishan Formation are calcareous dolostones, low in Fe, Sr and Mn concentrations, suggesting that the source of magnesium ions is mainly seawater. The rare earth elements (REE) are characterized by low Ce and Eu. In conclusion, the dolomitization in the study might have been caused by seepage reflux, and is characterized by high porosity and permeability as a result of geothermal warming, implying good reservoir potential.
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