Development characteristics and main controlling factors of sediment pores in the cold seep area of Qiongdongnan Basin

Methane rich cold seeps are actually direct outcrops of natural gas hydrates on the seabed, and are also important indicators and clues for exploring deepwater oil and gas resources. So far, although significant progress has been made in the investigation of natural gas hydrates in the Qiongdongnan Basin, research on the characteristics of sedimentary pore development is still relatively weak. Therefore, in-depth analysis of sedimentary reservoir characteristics is of great economic and scientific significance for a comprehensive understanding of the accumulation of basin hydrates, and for promoting the exploration and development of hydrate resources in the northern South China Sea. This article conducted a detailed study on the mineral composition, reservoir space types, microstructure, and physical properties of seabed sediments and cold seep carbonate samples in the cold seep leakage area of the Qiongdongnan Basin based on scanning electron microscopy, RoqSCAN testing, micrometer CT scanning imaging, and sediment particle size experimental data. The following important insights have been obtained: (1) Autogenic carbonates, sulfates, and sulfides that are indicative of natural gas hydrates have been detected in cold spring carbonate rock samples; (2) The storage space is mainly composed of dissolution pores, followed by fractures. The effective porosity calculated by micron CT scanning imaging ranges from 5.18% to 9.54%, with an average value of 8.14%, indicating a medium to low porosity reservoir. The pore size is mainly composed of fine and coarse pores. Overall, the surface sediment and cold seep carbonates in the Qiongdongnan Basin have reservoir conditions; (3) The contents of carbonate minerals and mud is an important factor affecting the reservoir properties. The higher the content of carbonate minerals, the lower the mud content, and the better the reservoir properties. In summary, the factors that affect the development and evolution of reservoir pore structure not only include sedimentation, but also the alteration of diagenesis. The reservoir properties characteristics of different types of reservoirs are controlled by their main mineral component types.