【作者】 魏喜；

【导师】 邓晋福；

【作者基本信息】 中国地质大学（北京） ， 矿物学、岩石学、矿床学， 2006， 博士

【摘要】 西沙海域位于南海西北部陆坡区,按照基底构造和上覆沉积地层特征,可分为东部岛礁区和西部盆地区。东部岛礁区为西沙隆起的较高部位,海水较浅,基底之上直接覆盖礁相沉积地层。西部盆地区处于西沙隆起西侧向琼东南盆地延伸部位,海水较深,地质历史时期曾发育生物礁,但目前多被淹没和/或覆盖。 本文利用西沙海域西部盆地区的二维地震资料和东部岛礁区的西琛1井岩心的薄片、元素、同位素、包裹体、X-衍射、电子探针、背散射、扫描电镜、图像分析、压汞、岩石物性等测试资料,对西沙海域礁相碳酸盐岩进行了研究。取得以下主要结论和认识: ① 西沙海域东部岛礁区前寒武纪基底之上地层全部是较纯净的礁相碳酸盐岩,厚度达1250m,未被陆源碎屑岩覆盖;西部盆地区礁相碳酸盐岩面积达2549km²,累计厚度达1183～1438m。不同礁相碳酸盐岩被砂泥质沉积物分隔或覆盖。西沙海域礁相碳酸盐岩共有6期,时代分别为渐新世、早中新世、中中新世、上中新世、上新世和第四纪。 ② 西沙海域晚新生代生物礁主要是植物藻礁,其次为珊瑚礁。后者主要出现在第四纪。沉积相包括礁核（礁格架）相、礁前相和礁后（泻湖）相等。岩石成分类型包括石灰岩和白云岩,确定7个岩性段,包括:3个白云岩段和4个石灰岩段。 ③ 岩石化学分析结果表明,西沙海域礁相碳酸盐岩中主要元素、微量元素和同位素具有明显的分段性。表现在:（a）CaO有四个高值段,三个低值段;相应地,MgO有三个高值段和四个低值段。（b）Sr元素在地层中含量变化在139×10^-6～2180×10^-6之间,整体上可分七段;Na、K、P三个元素分布特征近于相似,具有上高下低的两段性;Fe、V、B三个元素分布特征近于相似,总体上可分为两段,但富集趋势与Na、K、P元素相反。（c）碳、氧同位素可划分为七个带;⁸⁷Sr/⁸⁶Sr比值可划分为四段。西沙海域礁相碳酸盐岩的这些分段性特征是在不同侧面对古气候、青藏高原隆升、幔源火山活动和古海洋事件的记录和表现。其中,中新世南极冰盖的形成、岩石圈减薄引起的幔源岩浆活动和由印度—欧亚板块碰撞引起的大陆隆升对南海古海洋特征影响巨大。中—晚中新世南海曾一度是一个以巴士海峡为开口,与菲律宾海及太平洋时而相连,时而断开的封闭的陆缘海。因此,除中央海盆水体较深外,东沙、中-西沙和南沙等海域水体均较浅,浅滩和泻湖发育。这为礁相碳酸盐岩的形成和同生白云岩化作用创造了条件。 ④ 南海盆地形成演化构造背景和由周围大陆入海的大型河流三角洲在空间上控制着礁相碳酸盐岩的形成、发育和分布。海平面升降决定生物礁类型和规模。 ⑤ 古海洋环境决定礁相碳酸盐岩的岩石类型和原始储集性能。而自生矿物胶结、重结晶、淡水淋滤溶蚀和白云岩化等成岩作用决定了该类储层的最终储集性能。根据储层特征参数及其影响因素分析,认为中中新统一上中新统是最有利的礁相碳酸盐岩储层发育带。 ⑥ 西沙海域东部岛礁区,礁相碳酸盐岩不具备形成圈闭的条件,不能形成油气藏。而西沙海域西部盆地区,礁相碳酸盐岩具备形成油气藏的生油、储层、盖层、圈闭、运移和后期保存等条件,因此,具有广阔的油气勘探前景。南海盆地其它海域具有类似的规律。更多还原

【Abstract】 Located at the northwestern continent slope of the South China Sea, based on the characteristics of basement structure and the covering sedimentary strata, Xisha sea area consists of eastern reef island area and western basin area. The eastern reef island area, with shallow seawater, is higher parts of Xisha apophysis, where the basement is directly covered with reef facies carbonate rocks. The western basin area, stretching into Qiongdongnan basin and with deep seawater, is the western slope of Xisha apophysis, where reefs developed in geological history, but were covered with clastic rocks or seawater now.By means of 2D seismic data in western basin area and data, in well Xichen 1 of eastern reef island area, of rock flake, major element, trace element, isotope, liquid enclosure, X-ray diffraction, electron probe, back dispersion, SEM, image analysis, capillary pressure curve analysis, porosity, permeability, and so on, reef facies carbonate rocks were studied, and the following understanding and conclusions were acquired.1. All basement rocks in eastern reef island area is covered with pure reef facies carbonate rocks in the thickness of 1250m. The carbonate rock area ascertained by 2D seismic data in western basin area is up to 2549km2 with the thickness of 1183 to 1438m. The reef facies stratum sequence include 6 stages, those are, Oligocene, early-Miocene, middle-Miocene, late-Miocene, Pliocene, and Quaternary.2. The Late-Cenozoic reef in Xisha sea area is mainly plant algae one, and secondly coral one, and the latter are mostly found in Quaternary. Reef facies include reef core （framework or skeleton part）, reef front, and reef back （lagoon）, and the like. Rock component types include limestone and dolostone. According to Well Xichen 1 in Xisha sea area, 7 lithology segments are confirmed in reef facies stratum sequence, including 3 dolostone segments and 4 limestone segments.3. Petrochemical analysis results of reef facies carbonate rocks in Xisha sea area indicate that the major element, trace element, and isotope in the rocks represent obvious subsection characteristics in stratum profile. These manifest that, （a） CaO, with 4 high value sects and 3 low value sects, and MgO, with 3 high value sects and 4 low value sects, compensate each other. （b） Sr content, varying largely from 139×10^-6 to 2180×10^-6, can be marked off 7 sects in strata. The variations of Na、 K、 P are similar, with high value in the upper part of, and low value in the lower part of, this set of strata in well Xichen 1. Fe、 V、 B have similar variation, but the value varies the other way round with Na、 K、 P. （c） Carbon and Oxygen isotopes can be divided into 7 sects in the stratum profile of Well Xichen 1.⁸⁷Sr/⁸⁶Sr value, increasing by degrees from bottom to top, can be carved up to 4 stages. These subsection characteristics are the responses or track records, in different aspect, of palaeoclimate, Qinghai-Tibet Plateau upheaving, volcano activities, and palaeo-ocean events. Thereinto, South Pole icecap formation in Miocene, mantle magma activity caused by lithosphere thinning, and continent upheaving brought byIndian-Eurasian plate collision influence the ancient South China Sea evidently. The middle- to late-Miocene South China Sea might be occlusive continent edge sea which communicates with the Philippine sea and Pacific ocean only by Bashi Channel, which sometimes opened and sometimes closed, and when, apart from deep sea in central South China Sea basin, all Dongsha, Zhong-Xisha, and Nansha were shallow sea, bank, or lagoon.4. The formation, development, and distribution of reef facies carbonate rocks were controlled in extensity by the South China Sea evolution tectonic setting and great deltas coming from continents. The reef types and dimensions were determined by sea level rising and falling.5. Rock types and original reservoir capability were controlled by palaeo-ocean environment, while the final reservoir performance was determined by mineral cementation, recrystaline, eluviation by freshwater, and dolomitization. The middle- to upper-Miocene is the most favorable reservoir based on reservoir character parameters and the controlling factors mentioned above.6. There exists no enclosure forming condition in eastern reef island area of Xisha sea area. No oil and gas reservoirs there. However, with fecund source rocks, advantaged reservoirs, favorable cap rocks, good trap condition, good oil and gas motion condition, and preservation condition, western basin area of Xisha sea area has a wide oil and gas exploration potential.更多还原