Comparative Sedimentology of Late Tertiary Organic Reefs in Xisha-Northern South China Sea

Key words: 
• Xisha-Northern South China Sea /
• Miocene organic reef /
• comparative sedimentology /
•  /

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.