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摘要:
西沙群岛新近纪以来广泛发育生物礁白云岩,在西永2井白云岩层段发现大量的铁白云石,关于该区的铁白云石研究较少。笔者主要通过X射线粉晶衍射数据,结合前人研究资料,对西永2井407~570 m白云岩-铁白云岩进行了分析。研究发现西沙群岛西永2井中新统铁白云岩主要集中在上中新统黄流组一段,其黄流组二段主要发育含铁白云岩。白云岩孔隙度分布与其铁白云石分布有较好的对应关系,孔隙度高的层位铁白云石分布更密集。从中新统白云石和铁白云石的晶胞参数可以得知,该区铁白云石和白云石的晶胞参数都较大,呈现出富钙铁的特征。西永2井白云岩-铁白云岩有序度受Fe2+的影响较大,Fe2+的加入使其有序度降低,因此有序度低的现象不能简单的认为是结晶速度快的准同生作用造成。西沙群岛晚中新世有大量火山活动的现象,岩浆热液提供了铁白云石形成所需要的Fe2+和热量来源。在成岩后期,孔隙性较好的黄流组一段受火山活动影响大,其热源及富Fe2+离子促使白云石向铁白云石转变,形成了大量的铁白云石;而孔隙性较差的黄流二组仅仅形成大量的含铁白云岩。
Abstract:Post-Neogene reef dolostone occurs widely in the Xisha Islands and a great amount of ankerite has been found in the well of Xiyong 2. However, the research on ankerite is rare in literatures. In this paper, the ankerite-dolomites in the interval of 407~570 m of the well Xiyong 2 are analyzed with X-ray powder diffraction upon the basis of previous research data. It is found that the Miocene ankerite mainly occurs in the first Member of the Upper Miocene Huangliu Formation of the well Xiyong 2, while the ferroan dolomite dominates the second member. The porosity of dolomite shows a positive relationship with the amount of ankerite. The higher the porosity of dolomite rock, the higher the ankerite content. The crystal cell parameters of the Mesozoic dolomite and ankerite in this region are large, suggesting that they are rich in calcium and iron. The order degree of the dolomite-ankerite in well Xiyong 2 is greatly affected by Fe2+ content and the increase in Fe2+ will reduce its order degree. Therefore, it is not appropriate to suppose that the dolomite in the well Xiyong 2 of Xisha is simply formed by penecontemporaneous dolomitization. There was a great deal of volcanic activity in the Xisha Islands during Late Miocene, and the magmatic hydrothermal fluid may provide enough Fe2+ and heat for the formation of ankerite. In the late diagenetic period, the first member of Huangliu Formation with good porosity was greatly affected by volcanic activities. The heat and rich Fe2+ input promoted the transformation of dolomite to ankerite, and a large number of ankerite was then formed. However, in the second Member of Huangliu Formation with poor porosity there formed only ferroan dolomites.
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Key words:
- well Xiyong 2 /
- ankerite /
- X-ray diffraction /
- degree of order /
- genesis analysis
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图 6 西永2井白云岩-铁白云岩随深度的变化关系[18]
Figure 6.
表 1 西永2井X射线粉晶衍射分析结果
Table 1. X-ray powder diffraction analysis results of dolomite in well Xiyong 2
样品编号 序号 深度/m 岩性 各矿物及质量分数 最强衍射峰d值/Å 晶胞参数 I(015) I(110) 有序度 a/Å c/Å xy-19 1 407~408 白云岩 白云石100% 2.904 3 4.832 6 16.167 1 1 375 2 681 0.512 9 xy-1 2 410~410.3 白云岩 白云石100% 2.900 6 4.829 4 16.110 3 1 133 2 573 0.440 3 xy-20 3 411.4~412 白云岩 白云石100% 2.902 4 4.829 4 16.147 3 1 361 2 388 0.569 9 xy-21 4 414~415 铁白云岩 铁白云石98.3% 2.907 9 4.836 8 16.195 7 1 217 2 363 0.515 0 方解石1.7% 3.035 6 xy-2 5 420~420.3 铁白云岩 铁白云石56.6% 2.906 1 4.833 8 16.187 6 332 810 0.409 9 方解石43.4% 3.027 6 xy-3 6 422.5~422.9 铁白云岩 铁白云石60.0% 2.908 0 4.837 6 16.163 7 472 1 146 0.411 9 方解石36.5% 3.033 4 磁铁矿3.5% 2.530 8 xy-4 7 425.2~425.7 铁白云岩 铁白云石49.6% 2.907 9 4.838 7 16.233 6 208 489 0.425 4 方解石42.0% 3.031 6 磁铁矿5.0%、石盐3.4% 2.520 6、2.822 4 xy-22 8 430~430.5 铁白云岩 铁白云石54.5% 2.908 0 4.836 5 16.200 8 278 643 0.432 3 方解石44.7% 3.031 3 磁铁矿0.8% 2.530 7 xy-5 9 432.75 白云岩 白云石99.2% 2.900 6 4.830 5 16.150 6 1 515 3 092 0.490 0 石盐0.8% 2.820 4 xy-6 10 435 铁白云岩 铁白云石100% 2.906 0 4.832 7 16.207 8 1 419 3 240 0.438 0 xy-7 11 446.5~447.2 铁白云岩 铁白云石97.9% 2.906 0 4.832 1 16.177 5 1 133 2 532 0.447 5 方解石2.1% 3.031 4 xy-8 12 451.2~451.7 铁白云岩 铁白云石98.6% 2.907 9 4.836 2 16.191 0 1 383 2 776 0.498 2 方解石1.4% 3.033 2 xy-9 13 452.96~456.93 铁白云岩 铁白云石98.1% 2.906 1 4.834 5 16.309 3 886 2 073 0.427 4 方解石1.3%、纤铁矿0.6% 3.033 5、3.296 9 xy-10 14 457.5~458 铁白云岩 铁白云石97.7% 2.906 1 4.835 5 16.201 2 1 192 2 642 0.451 2 方解石1.7%、石盐0.6% 3.027 5、2.818 8 xy-11 15 460.5~461 铁白云岩 铁白云石98.8% 2.906 1 4.834 4 16.184 6 1 386 2 930 0.473 0 方解石1.2% 3.029 6 xy-23 16 462.8~463.1 白云岩 白云石99.1% 2.904 3 4.832 3 16.170 5 1 360 2 598 0.523 5 方解石0.9% 3.029 3 xy-24 17 464.6-464.8 白云岩 白云石100% 2.900 7 4.830 6 16.114 3 1 305 2 533 0.515 2 xy-12 18 466.3~466.6 白云岩 白云石99.0% 2.902 4 4.833 7 16.145 5 1 286 2 413 0.532 9 方解石1.0% 3.029 3 xy-13 19 469.87~470.91 铁白云岩 铁白云石97.0% 2.906 1 4.834 1 16.163 7 1 179 2 951 0.399 5 方解石3.0% 3.031 2 xy-14 20 473.55~477.15 铁白云岩 铁白云石95.5% 2.908 0 4.835 9 16.236 6 735 1 822 0.403 4 方解石3.0%、石盐0.9% 3.031 7、2.823 9 磁铁矿0.6% 2.522 5 xy-25 21 487.9~488.05 白云岩 白云石100% 2.900 6 4.829 9 16.167 5 1 566 2 591 0.604 4 xy-26 22 492.2~492.5 铁白云岩 铁白云石100% 2.906 1 4.834 1 16.168 7 1 159 2 453 0.472 5 xy-27 23 495~495.3 白云岩 白云石100% 2.895 2 4.829 1 16.104 9 1 384 2 291 0.604 1 xy-28 24 497.5~498.3 白云岩 白云石100% 2.904 2 4.832 1 16.127 5 1 290 2 312 0.558 0 xy-29 25 500.7~501.2 白云岩 白云石100% 2.897 0 4.827 6 16.131 2 1 282 2 329 0.550 5 xy-30 26 502.1~502.3 白云岩 白云石100% 2.9025 4.835 1 16.190 4 1 350 1 755 0.769 2 xy-31 27 502.9~503.2 白云岩 白云石100% 2.886 0 4.812 4 15.896 6 1 022 2 173 0.470 3 xy-32 28 504.1~504.3 白云岩 白云石100% 2.900 6 4.831 0 16.144 5 1 445 2 581 0.559 9 xy-33 29 507.7~507.8 白云岩 白云石100% 2.904 4 4.835 1 16.214 2 1 460 2 237 0.652 7 xy-34 30 517.5~518 白云岩 白云石100% 2.902 4 4.832 2 16.169 1 1 109 2 192 0.505 9 xy-15 31 517.5~518 白云岩 白云石100% 2.902 5 4.829 9 16.164 9 1 196 2 377 0.503 2 xy-16 33 527.3~527.5 白云岩 白云石100% 2.895 2 4.823 9 16.079 7 1 340 2 160 0.620 4 xy-17 34 555.2~555.4 白云岩 白云石100% 2.898 9 4.827 1 16.117 7 1 328 1 810 0.733 7 xy-18 35 570.07~573.94 灰岩 白云石37.3% 2.904 2 方解石62.7% 3.027 5 表 2 西永2井中新统白云岩有序度平均值
Table 2. Average order degree of Miocene dolomite in well Xiyong 2
铁白云石 白云石 铁白云石-白云石 黄流组一段 0.441 0 0.512 1 0.465 9 黄流组二段 0.472 5 0.594 3 0.585 0 上中新统 0.443 2 0.564 0 0.512 8 -
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