| Abstract: CNOOC (China) Co., Ltd. Zhanjiang Branch designed and drilled the“SK-1well” in Shidao of Xisha Archipelago in2012. The well was designed to be1350m in depth. By the end of2013, the length of drilled core had reached748m witha coring recovery rate of78.4%. This thesis is based on National Major TechnologySpecial Project,“The Geochemical Characteristics of Paleoclimate andPaleoenvironment since Neogene”. By identifying and analyzing on the mineralcomposition of carbonate samples selected from the core of the SK-1well, combinedwith geochemical characteristics of major and trace elements of the core samples, aswell as previous fruits about another associated four cores, We divide sedimentarysequence and sedimentary facies, and discuss the diagenesis of Xisha biological reefand the dolomitization mode. These could provide scientific guidance for the resourceevaluation and exploitation of oil and gas resources of reef carbonate layers in theSouth China Sea.The core depths of the bottom boundaries of Holocence-Pleistocene, Pliocene,upper Miocene are216m,373m, and577m, respectively. Judging from the time incompletely transforming high magnesian calcite into low magnesian calcite as well asother mineralogical features, we conclude that the upper boundary of Late Pleistocene,i.e., the boundary between Pleistocene and Holocene (about11500years ago) islocated near35.4m of the core depth. The lithology of drilling cores mainly includesreef limestone, bioclastic limestone, bioclastic dolomite and reef dolomite. Severalsubfacies (e.g., reef frame facies, reef platform facies, gray sand island facies, backreef lagoon facies and bioclastic bank), several structural components (e.g., organic framework, micrite, sparite, grained clastics), and several structures (e.g., reef framestructure, bonding structure, and product structure), have been identified in thedrilling cores.According to primarily analysis, carbonate minerals in the SK-1well mainlyconsist of low magnesian calcite, high magnesian calcite, aragonite and dolomitewhile clay minerals only appears in a couple of layers. High magnesium calcite andaragonite are mainly distributeed in the top part of the core. The stratigraphy sequenceof reef facies includes5segments: the upper Pliocene dolomite (layer5), upperMiocene dolomite（layer2,3and4），and Middle Miocene dolomite（layer1）.The content of Ca, Mg, Sr, P and other elements shows a distinct segmentation. Pelement, mainly coming from guano, is enriched in the uppermost of the core, and thehigh P2O5content，appearing in the form of spikes, indicats the gray sand island facies.The content of Sr element changes largely in the top and middle layers, where thereexists aragonite, but is relatively stable at100~200μg/g with depth where existsdolomite. The distributions of the four elements of Ga, Ba, Hf and Zr are similar, andare not enriched in the dolomite section.The development of reef was partially controlled by ancient oceanic environment.The formation of reef carbonates was mainly controlled by the development andevolution of the marine environment, such as pressure, temperature, seawatergeochemical characteristics (e.g., Sr, Mg and Ca content), the diagenesis, and so on.The changes of ancient sea level also had an effect on the reef development. When thesea level dropped, exposed erosion surface was easy to develop, leading to furthermutual transformation among mineral phases. Three ancient marine incidents, theformation of the Arctic ice cap in Pliocene, the late Miocene Messinian events and amajor expansion of the Antarctic ice sheet in the Miocene events, gave rise to theLagoon and dolomite. The dolomitization of carbonate rocks in organic reefs mainlytook place in three models, i.e., lagoon vaporization, infiltration and circumfluencemodel, the mixing water model, and the diagenesis model under burial compaction. |
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