## 4 Results  
### Sedimentary Characteristics from Drilling Wells  
Previous studies on drilling wells in the Xisha Islands have mostly used the sedimentary facies of a single well and the unique combination of fossil communities to divide the sequence, analyze the sedimentary environment, and discuss diagenesis [6; 20]. This paper uses the drilling data from Wells XY-1 and XC-1 on the uplift, combined with the latest data from Wells XX-1 and 120-CS-1X [21] on the Gungle Uplift. A detailed analysis of the sedimentary facies of the two uplifts was carried out, as shown in Figure 4. In addition, a comparison of wells in the deep-water area in Figure 5 shows that different structural units have different depositional environments during the same period. In the Early Miocene, the carbonate rocks of the Sanya Formation were directly deposited on the metamorphic rock basement, and the fossil content was relatively low and contained mainly algae and coral fragments. These carbonate rocks were domitized in the later period. Well YL2-1-1 in the deep-water area shows that the Sanya Formation is dominated by sandstone and marl, which are littoral clastic shoal deposits. Wells LS33-1-1 and YL19-1-1 show that the Sanya Formation is dominated by mudstone and marl deposits. Well YC35-1-1 also contains large amounts of marl, sandstone, and sandy limestone. These results indicate that the peripheries of the three wells are sedimentary slope facies mixed with carbonate fragments and mudstone. The above phenomena indicate that carbonate platforms developed during the Early Miocene, but most of them were modified by late deformation and partly dolomitized. The Xisha Uplift and Gungle Uplift continuously received carbonate sediments during the Middle Miocene. Wells XX-1 and XY-1 reveal that algae dominate the reef-building organisms in this section. In the Late Miocene, almost all of the carbonate rocks in the Xisha Island reef area were domitized, as shown in Figure 4, which may have been caused by the evaporative environment [22]. During this period, the water bodies on the peripheral slopes of the Xisha Uplift were relatively deep, making them unsuitable for the development of reefs and carbonate platforms, as shown in Figure 5. In the early Late Miocene, the carbonate platform of the Gungle Uplift was exposed and eroded, possibly due to the structural uplift of the regional basement [21]. In the Late Miocene, the rapid rise in the relative sea level resulted in no more carbonate platform development on the Gungle Uplift. From the Late Miocene to the present, the water bodies in the area deepened further, causing the carbonate platform to enter the submerged stage. Only the islands on the Xisha Uplift have exposed carbonate platforms, represented by the Xuande Islands and Yongle Islands [23].  
Figure 4: The cross-section of wells from Well 120-CS-1X to Well XX-1 for sedimentary facies and lithological combinations from the Early Miocene to the Holocene on the uplifts at the Xisha sea area. The well locations are shown at the bottom of the scaled map in Figure 1B.  
Figure 5: The cross-section of wells from Well YC35-1-1 to Well YL2-1-1 for sedimentary facies and lithological combinations in the deep-water zone of the Xisha sea area. The well locations are shown in the left corner of the scaled map in Figure 1B.