Carbon dioxide capture, utilization and storage technology is considered to be one of the most effective strategies to mitigate CO₂ emissions. In this process, CO₂ that is injected into seabed sediments under specific temperature and pressure conditions is sealed in the form of CO₂ hydrate, known for its high gas storage density and exceptional security features. This method has significant advantages compared with onshore geological storage schemes. Thus far, however, there has been no industrial demonstration of CO₂ hydrate storage, and the CO₂ hydrate storage potential in the South China Sea remains underexplored without targeted evaluations. In this study, the phase equilibrium mechanism is combined with the volumetric method to describe and evaluate the CO₂ hydrate storage distribution range, effective thickness, and potential volume available for CO₂ hydrate storage. Based on the latest exploration and development data from the Qiongdongnan Basin, along with geological structure data, multibeam bathymetry, local high-resolution three-dimension multichannel seismic reflection data, logging data, and submarine heat flow data, the distribution of the CO₂ hydrate storage stability zone is determined. The results show that the effective thickness and regional scope of CO₂ hydrate storage in the concerned area can be determined by virtue of the local water depths and the submarine temperature and pressure of 18 virtual wells. The minimum water depth in the Qiongdongnan Basin that satisfies the temperature and pressure conditions needed for CO₂ sediment storage is established as 415 m. The theoretical geological storage capacity of CO₂ hydrate in the Qiongdongnan Basin is determined as 5.75×10¹¹ to 8.73×10¹¹ t, where the value range of E is between 0.56 and 0.85. These findings offer a solid foundation for China to create, advance and execute a viable strategy for CO₂ hydrate storage.

Hydrate reservoirs are different from the host reservoirs of all other fossil energy sources because the characteristics of hydrate reservoirs are generally controlled by deep-sea fine-grained sedimentation. In such reservoirs, the reliability of the classical logging evaluation models established for diagenetic reservoirs is questionable. This study used well W8 in the Qiongdongnan Basin to explore the clay content, porosity, saturation, and hydrate-enriched layer identification of a logging-based hydrate reservoir, and it was found that considering the effect of the clay content on the log response is necessary in the logging evaluation of hydrate reservoirs. In the evaluation of clay content, a method based on the optimization inversion method can obtain a more reliable clay content than other methods. Fine-grained sediment reservoirs have a high clay content, and the effect of clay on log responses must be considered when calculating porosity. In addition, combining density logging and neutron porosity logging data can obtain the best porosity calculation results, and the porosity calculation method based on sonic logging predicted that the porosity of the studied reservoir was low. It was very effective to identify hydrate layers based on resistivity, but the clay distribution and pore structure will also affect the relationship between resistivity, porosity and saturation, and it was suggested that the factors effecting the resistivity of different layers should be considered in the saturation evaluation and that a suitable model should be selected. This study also considered the lack of clarity of the relationships among the lithology, physical properties, hydrate-bearing occurrence properties, and log response properties of hydrate reservoirs and the lack of specialized petrophysical models. This research can directly help to improve hydrate logging evaluation.

Nan′an Basin is a giant hydrocarbon basin, but its tectonic division scheme and associated fault systems has not been well understood. Based on newly acquired seismic data from the southwestern margin of the South China Sea, this study analyzed the structural units, tectonic feature and geodynamics of the sedimentary basin. The new data suggests that the Nan′an Basin is a rift basin oriented in the NE-SW direction, rather than a pull-apart basin induced by strike-slip faults along the western margin. The basin is a continuation of the rifts in the southwest South China Sea since the late Cretaceous. It continued rifting until the middle Miocene, even though oceanic crust occurred in the Southwest Subbasin. However, it had no transfer surface at the end of spreading, where it was characterized by a late middle Miocene unconformity (reflector T3). The Nan′an Basin can be divided into eight structural units by a series of NE-striking faults. This study provides evidences to confirm the relative importance and interplay between regional strike-slips and orthogonal displacement during basin development and deformation. The NE-SW-striking dominant rift basin indicates that the geodynamic drivers of tectonic evolution in the western margin of the South China Sea did not have a large strike-slip mechanism. Therefore, we conclude that a large strike-slip fault system did not exist in the western margin of the South China Sea.