Geochemical Characteristics of Reef Carbonate Rocks in well Xike-1 of Shidao Island, Xisha Area

Reef carbonate rocks of well Xike-1 are analyzed for major constituents, trace and rare earth elements in this study, aimed to explore the formation, evolution and environmental change recorded in reef carbonates. The results show that there is an important interface of formation or environmental mutation at depth of 36 m, where geochemical parameters change significantly. The major constituents can be divided into following three components: the lithogenic carbonate components, consisting of CaO and K₂O, corresponding to calcites; the enriched components by dolomitization, including MgO, Na₂O and P₂O₅, which indicates restricted lagoon environment; the enriched components by reef-builders, composed of SiO₂, Al₂O₃, TiO₂, Fe₂O₃ and MnO, potentially including a small amount of volcanic components. Contents of most trace and rare earth elements in well Xike-1 are low compared to those of other endmembers such as carbonate rocks of Quaternary around the world, upper crustal, and shales. The contents of redox-sensitive trace elements (RSE) in well Xike-1 are low, indicating oxic environment through most of development process of reefs. There are alternations of oxidation and reduction environment, with wide fluctuations of RSE contents. In addition, there is no obvious correlation between mineral compositions and geochemical parameter distribution, such as trace elements, REEs and their characteristic parameters, indicating that dolomitization and diagenesis did not result in the significant changes of trace elements, REEs, and their eigenvalues.