海洋地质与环境重点实验室知识库

南海自海底扩张以来，在南北共轭大陆边缘发育了一系列的新生代碳酸盐台地，且碳酸盐台地分布面积广、厚度巨大。中新世时期，南海西北部陆缘发育有大量的生物礁碳酸盐岩台地，形成了重要的油气储层。但是由于资料限制对于西沙海域生物礁和碳酸盐台地的研究仍十分薄弱，因此总结南海西北部西沙海域生物礁碳酸盐岩台地的地震识别特征，并对其发育演化进行深入的分析，具有重要的理论意义和实际意义。
本论文主要通过对南海西北部西沙海域地震资料进行分析并结合区域地质资料，重点研究了西沙海域生物礁及广乐碳酸盐岩台地的发育特征与沉积演化模式，并取得了如下成果：研究区内生物礁在地震剖面上主要表现为顶、底强振幅反射内部为杂乱或强弱相间反射的地震响应特征，主要发育了台地礁、台地边缘礁、泻湖礁、盆地礁和滩礁五种类型的生物礁；生物礁的沉积演化经历了早中新世、中中新世早期、中中新世晚期、晚中新世和上新世-现今五个阶段。在地震响应特征识别基础上分析了广乐碳酸盐台地的结构特点，具有分段性：早中新世碳酸盐台地沉积在地震剖面上呈现为断续的强振幅反射，内部碳酸盐岩地层近平行发育，中-晚中新世碳酸盐台地沉积地震反射特征则表现为平行的中-弱振幅反射；广乐碳酸盐台地顶部普遍发育溶蚀塌陷特征，溶蚀陷落形成管状通道，通道内部地震反射特征较明显：顶部多为强振幅反射，同相轴表现出下凹特征；底部地震反射能量较弱或为空白反射，同相轴特征表现为向上弯曲凸起的特点；在碳酸盐台地沉积特征研究基础上总结分析了广乐碳酸盐台地的沉积演化史：广乐碳酸盐台地自早中新世开始广泛发育，在构造作用控制下，一直持续发育至晚中新世，发育过程中表现出自西向东迁移的特点，最终由于相对海平面上升，加上源于西部中南半岛的陆缘碎屑物质注入导致水体环境改变，广乐碳酸盐台地被淹没。

The huge thickness of Cenozoic carbonate platforms is widely developed in the conjugate continental margins of South China Sea. During Miocene, large quantity of reefs and carbonate platforms developed in the northwestern margin of South China Sea, which can be a potential reservoir in the future. As the exploration in this area is inchoate and lack of enough well data, study about development of reefs and carbonate platforms in this area was weak. Therefore, the study about the evolution process and main controlling factors about the reefs and carbonate platforms in Xisha area based on seismic analysis and shallow well drilled in Xisha Islands is necessary, and these research have academic and practical importance.
Combined with the pre-study, and the latest drilling and seismic data in this area, the paper focused on the reefs and carbonate platforms in Xisha area, summarized the seismic characteristics of reefs and carbonate platforms in study area, and proposed the development model of reefs and carbonate platforms. Some innovated results have achieved: 1) Identify the reefs based on analysis on seismic data in the study area, seismic characteristics show mounded reflections, apparent amplitude anomalies, intermittent internal reflections; five types of reefs were identified : patch reef, platform edge reef, pinnacle reef, atoll reef and lagoon reef; 2) The evolution of reefs experienced 5 stages: Early Miocene, early Middle Miocene, late Middle Miocene, Late Miocene and Pliocene to present; 3)The seismic characteristics of Guangle carbonate platform was difference between the upper and lower half: a wavy and discontinuous reflection pattern together with the often relatively high reflection amplitudes characterized much of the Lower to lower Middle Miocene succession and distinguished the lower half from the younger part; 4) Peculiar depressions formed up to a few hundred meters deep scars in the upper surface of Guangle carbonate platform, the features overly poorly to chaotically reflected columns of platform carbonates, the upper part of these columns frequently contained very strong reflections, concave upwards reflectors frequently signified the upper part of these columns whereas up-warping of deeper platform reflectors occured along the flanks of the deeper parts of the columns. 5) Studied the spatio-temporal distribution of Guangle carbonate platform then established the development model of carbonate platform in this area; the composition and development of Guangle carbonate platform experienced four main stages: start up within early carbonate Miocene, build up and build out during early, middle Miocene, back stepping to decrease its distribution scope in near top of middle Miocene, the platform carbonate stage transfers to downgrade stage and to drowning in the top of middle Miocene and covered then by younger sediment deposited from late Miocene to present.