Comparative Sedimentology of Late Tertiary Organic Reefs in Xisha-Northern South China Sea
摘要: 在西沙群岛琛航岛一井晚第三纪生物礁中发现了９属４３种造礁钙藻和９个生物化石群落，与在南海北部陆架珠江口盆地惠州３３－１－１井发现的７属１６种造礁钙藻，７个生物化石群落() 及在莺歌海—琼东南盆地区莺６井发现的５属２１种造礁钙藻和５个生物化石群落() 一起，并列为迄今在研究区发现生物礁的三大典型研究剖面。本文较详细地从比较沉积学的角度讨论了这三个生物礁的异同点。生物礁最为重要的鉴定标志就是它具备典型的生物骨架结构。利用有关三个生物礁化石群落的鉴定成果、分布特征、组合关系、剖面演变规律的资料，指出皮壳状与坚硬分枝状珊瑚藻（Ｍｅｌｏｂｅｓｉａｅ）、直立分节状珊瑚藻（Ｃｏｒａｌｉｎａａ）和仙掌藻（Ｈａｌｉｍｅｄａ）在成礁过程中形成格架，扮演了“开路先锋”、“主力军”、“收容队的角色；尤其绿藻门仙掌藻作为其中最为重要的骨架生物之一在西沙生物礁中形成剖面序列，对于完善成礁理论和探索中国的“Ｍｅｓｓｉｎｉａｎ”事件—“南中国海”事件具有重要的古海洋学意义。三地生物礁因陆架海与陆缘海的区别而在沉积环境方面有所不同；成礁基有碳酸盐台地、古老的前第三纪基岩或“海台”；主要成礁期为中新世不同阶段；重要的岩石类型包括珊瑚藻礁灰岩与白云。Abstract: genera and 43 species of reef-building calcareous algae (belonging to 9 fossil communites) have been discovered in Late Tertiary organic reefs from the Chen-1 hole core of Xisha Islands. They are considered as three example organic reef sections together with 7 genera and 16 species of reef-building calcareous algae (belonging to 7 fossil communities, Chen Shizhong, 1987) discovered in the Huizhou 33 1 1 hole of the pearl river mouth basin and 8 genera and 21 species (belonging to 5 fossil communities, Zhengdingqian, 1981) in the Ying-6 hole of the Yingge Sea-Qiongdongnan Basin in the study area so far. Detailed discussion is presented in this paper on the forming machanic characteristics of the three organic reefs on the basis of comparative sedimentology. The most important identification mark of organic reefs is its typeical skeleton structure. On the basis of the data of identification results, distribution characters, assemblage relation and section evolution regularity concerning the fossil communities of the three organic reefs, the paper proposed that Melobesiae, Corallinae and Halimeda played a part of "pathbreaker", "main force" and "taking in team" in the reef-building process, espeically Halimeda (Chlorophyta), which is one of the most important skeleton organisms to form the section in Xisha organic reefs and is of great paleao oceanographic significance in exploring Chinese "Messinian" events (the Mediterranean salinity crisis)-" South China Sea" events. It is suggested in the paper that epicontinental sea is different from shelf sea in sedimentary environment in the Xisha Islands and northern South China Sea. The reef-building bases are carbonate platform, Pre-Tertiary basement rocks or old platfrom. The main reef-building phase occurred at different stages of Miocene and important rock types are represented by Corallinaceae reef dolomite rocks, Halimeda reef dolomite rocks and Corallinaceae reef carbonate rocks. The reservoir property of organic reefs in the three areas was controlled by the transgression and dolomilization. The Huizhou 33 1 1 hole recorded several transgression processes. In the Xishen 1 hole, organic reefs had two dolomitization mechanisms: 1) the adjustment dolomitization; 2) seepage refluxation dolomilization. The reservoir property of organic reefs was strengthened by dolomitization effect. At least 3 types and 12 kinds of pores and over 5 kinds of pore combinations have been found and they have made up different reservoir grades. In fact, this paper pointed out that the organic reef formation took plants as dominant sources nor animals. The establishment of the three algae reef sections is the best judgement. The above-mentioned understandings, especially the systematic testing data on rock physical properties are expected to be applied as reference to oil and gas explorations in organic reefs in the Wanan-Zengmu basin in the south part of the South China Sea.
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