西沙群岛白云岩-铁白云岩岩相学特征

西沙群岛永兴岛西永2井和石岛西科1井均发现铁白云岩。主要通过X光粉晶衍射、偏光显微镜和扫描电镜技术完成测试，分析白云岩-铁白云岩分布及岩相学特征。X光粉晶衍射测试结果及综合分析表明，西科1井发育11层铁白云岩，西永2井发育8层铁白云岩。通过偏光显微镜鉴定，发现西永2井中新世白云岩主要为生物碎屑白云岩，白云石晶体呈粉晶—细晶结构；某些层段含较高铁白云石，呈微晶—粉晶状。通过薄片观察分析发现，西科1井发育大量物碎屑白云岩，多细晶—粉晶结构，自形—半自形为主，部分样品可见黑色铁质氧化物或者胶结物。扫描电镜观察发现2个井多数白云石为典型菱形多面体结构，白云化程度高。

PETROGRAPHIC CHARACTERISTICS OF DOLOMITE AND FERROAN DOLOMITE IN XISHA ISLANDS

Ferroan dolomite is found in well Xiyong 2 of Yongxing Island and well Xike 1 of Shidao Island in Xisha Islands. Through three methods including X-ray powder diffraction quantification, polarized light microscopy and scanning electron microscopy, distribution and petrographic characteristics of ferroan dolomite in the aboved two wells were discussed. The X-ray powder crystal diffraction analysis and comprehensive study show that eleven layers of ferroan dolomites are developed in the well Xike 1 of Shidao Island, and eight layers ferroan dolomites in the well Xiyong 2 of Yongxing Island. It was found by polarized light microscopy that the Miocene dolomite in the well Xiyong 2 of Yongxing Island is dominated by bioclastic dolomite, and the dolomite crystals are powder-fine grained; the content of ferroan dolomites is higher in some layers, and the crystal is mainly microcrystal-powder crystal dolomite. Through observing and analyzing the casting thin slices , it is found that a large amount of clastic dolomite is developed in well of Xike 1 and the crystals are mostly fine-grain crystal structure, self-formed to semi-self-shaped, and some samples are black ankerite oxide or cement. Under the scanning electron microscopy, samples from well Xike 1 in Shidao Island and well Xiyong 2 in Yongxing Island show that the dolomite crystals present rhombohedral polyhedral structures, with a high degree of dolomification.