【作者】 房殿勇；

【导师】 赵泉鸿；

【作者基本信息】 同济大学 ， 海洋地质学， 2002， 博士

【摘要】 本文选择南海ODP184航次1148站渐新统泥岩为研究对象，利用扫描电镜、X射线衍射、化石分析、气相色谱-质谱、稳定同位素等多种手段，全面探讨了中国海区首次发现的深水相渐新统沉积特征、化石成岩作用以及油气前景。1148站位于南海北部陆坡最下部(18°50.17’N，116°33.94’E，水深3294 m，进尺859.45m)，靠近陆壳与洋壳的界线，是184航次惟一钻遇渐新统的站位。岩性主要为钙质超微粘土。井深460 m以下为渐新世沉积，早晚渐新世界线位于井深488 m，井底年龄不超过32.8 Ma。分析表明，南海扩张初期已处于深海环境。根据岩性、全岩稳定同位素、化石组合和地球化学组分变化，将1148站渐新世分为32 Ma～＜32.8 Ma、25.5 Ma～32 Ma和24.0 Ma～25.5 Ma三个阶段：32 Ma～＜32.8 Ma(710 m～859.45 m)，堆积速率高，浊流沉积明显，可能代表了南海早期扩张的快速扩张；25.5 Ma～32 Ma (475 m～710 m)，岩性较为单一，早期28.5 Ma～32 Ma(488 m～710 m)沉积物堆积速率相对稳定，地球化学组份、孢粉浓度、沟鞭藻含量等总体变化不大，可能为南海平稳扩张阶段，但晚期25.5 Ma～28.5Ma(475 m～488 m)堆积速率降低，并在488 m与478 m出现沉积间断；24.0 Ma～25.5 Ma(460 m～475 m)，堆积速率低，滑塌沉积明显，本阶段和第二阶段晚期可能代表了海底扩张加剧阶段。全岩稳定氧、碳同位素突变界线与沉积间断、地层界线相一致。全岩碳同位素在井深475m(25.5Ma)和710m(32Ma)出现明显变轻，并与地震剖面的强反射界面相对应。通过与珠江口盆地化石对比、参考本站全岩碳同位素事件，提出珠江口盆地珠海组的顶界年龄为25.5Ma，恩平组顶界年龄为32Ma。与低纬度开放性大洋氧同位素变化明显不同，1148站数值模拟计算以及浮游有孔虫压扁壳、充填壳和剥离次生方解石氧同位素比较都证实，受成岩作用影响，钙质壳体稳定氧同位素值将变轻。同位素比较结果还表明，成岩作用过程中，钙质壳体碳同位素将变重，而并非原来认为的基本不受成岩作用影响。观察发现，井深475 m-600 m之间，硅质化石主要表现为蛋白石-CT。随着深度增加，硅质破壳率增大，蛋白石-CT的X射线衍射峰强度减弱，自生石英的衍射峰则相应增强，反映了蛋白石-CT向石英的转变。井深620 m附近，硅质化石完全溶解，蛋白石-CT衍射峰消失，蛋白石-CT全部转变为石英。X射线衍射定量也同样表明了这一转变过程。样品取芯率在470 m处从近100％降到485 m处的8％，可能是硅质成岩作用，尤其是蛋白石-A向蛋白石-CT的转变影响所致。而取心率的降低，则是此井未能观察到蛋白石-A／蛋白石-CT转变界线的部分原因。渐新统有机质含量普遍较高，670 m之下多超过0.5％，710 m以下平均含量超过0.59％。镜下观察证实，壳质体与孢质体组成的Ⅱ类有机质与主要来自陆地植物的木质体和煤质体组成的Ⅲ类数量相当，来自水生生物的壳质体也占相当比例。但岩石热解、GC／MS等分析表明，有机质成熟较低。尽管如此，由于1148所揭示的深海相渐新统厚度超过400 m，有机碳丰度大于0.5％的沉积厚度也逾100 m，因此，我们认为南海北部的这套深水相渐新统具有较强的生烃能力。更多还原

【Abstract】 In the past decade, many oil companies have shifted their exploration from the continental shelf to deep-water area. Inspirited by the giant reservoirs discovered in the Cenozoic passive margin submarine fans, particularly in the Gulf of Mexico and Angola, oil companies focus on the exploration in the turbidity and related systems.The paper has dealt with the Oligocene nannofossil clay in the ODP Site 1148 by means of scanning electronic microscopy, X-ray diffraction, paleontological analysis, gas chromatograph-mass spectrometry, Rock-Eval pyrolysis and stable isotopic analysis, with a purpose to understand the sedimentological characteristics of the deep-water Oligocene that was discovered for the first time in China Seas, and to discuss its hydrocarbon potential.ODP Site 1148, located on the northern slope of the South China Sea (18°50.17’N, 116°33.94’E, water-depth 3294 m, and core length 859.45 m), reveals a late Cenozoic sequence from the earliest Oligocene (ca. 32.8 Ma) to the Holocene (Wang et al. 2000). The upper boundary of the Oligocene is at the depth of 460 m. The micropaleontological results indicate consistent deep-water condition at Site 1148 even since the earliest Oligocene. Based on the data of lithology, fossil assemblages, sedimentation and accumulation rates, bulk rock stable isotope and geochemistry, the Oligocene depositional history can be subdivided into＜32.8～32 Ma (859.5-710 m), 32～25.5 Ma (710-475 m) and 25.5～24.0 Ma (460-475m) three stages. Stage 1 represents the incipient spreading of the South China Sea Basin with rapid accumulation and distal fine-grained turbidites, stage 2 the relative stable spreading with lower accumulation and homogenous nannofossil clay, in its earliest and middle period (32-28.5 M), and the later stage 2 and stage 3 the accelerated spreading with lowest accumulation, convolute bedding and slumping.There exist two negative excursions in bulk carbon isotope at 475 m (～25.5 Ma) and 710 m (～32 Ma) respectively, that are coincident with the statigraphic boundaries and seismic reflection profiles. Based on the comparison of the Oligocene sequences between Site 1148 and the Pearl Mouth Basin, the age of the upper boundaries of Zhuhai Formation and Enping Formation are suggested as～25.5 Ma and～32 Ma B.P. respectively.The Oligocene oxygen isotope of at Site 1148 shifts lighter during the diagenesis. It is opposite to the diagenetic effect happened in the low latitude open sea where the oxygen isotope shifts heavier. Our results also show that the carbon isotope of carbonate test shifts heavier, that differs from the previous work in which the carbon isotope is considered to be lesser sensitive to the diagenetic effects.X-ray diffraction and scanning electron microscopy analyses were performed and the resultsshow that the opal-A of siliceous fossils has been transformed into opal-CT in the interval between 475 m and 600 m. With the depth increasing, the fragmentation of siliceous fossils increased, the X-ray peaks of authigenic quartz became more prominent and the X-ray peaks of opal-CT reduced in intensity, indicating the occurrence of the opal-CT in quartz transformation in this interval. The opal-CT intensity peak disappears in samples below 600 m, suggesting that the opal-CT had been completely transformed into quartz. The abrupt decrease in core recovery from nearly 100% above 470 m to 8% in the interval of 470 m～485 m at Site 1148 is likely to be related to the silica diagenesis, particularly to the transformation of opal-A to opal-CT. The opal-A/opal-CT boundary, however, has not been determined in this work mainly due to the poor core recovery.The Oligocene sediments at Site 1148 contain a relatively abundant organic matter, with a content more than 0.5% below depth 670 m and more than 0.59% below depth 710 m. Rock-Eval pyrolysis and observation under microscope reveal that the typesⅡandⅢof Kerogen is mainly from terrestrial higher plants, and partly from the aquatic. However, the pyrolysis Tmax values and GC/MS data indicate the gross thermal immaturity. Nevertheless, the 400 m-thick Oligocene and the 100 m-thick sediments with TOC＞0.5% are undoubted facts from that the deep-water Oligocene of the northern South China Sea is considered as a higher potential in the hydrocarbon generation.更多还原