Effects of differential tectonic activities on overpressure evolution in the deep-water area of the Qiongdongnan Basin: implications for gas hydrate accumulation

Abstract

The lack of data on the complex tectonics of the Qiongdongnan Basin has thus far restricted the understanding of overpressure, deep hydrocarbon, and shallow gas hydrate distribution. In this study, integrated seismic, well, and borehole-test data were combined with basin models to clarify the relationship between tectonic activity and overpressure evolution, as well as potential effects of sub-surface pressure on gas hydrate accumulation. The results show differences in tectonic activity and pressure characteristics between the eastern and western basins. Since the Late Miocene (∼10.5 Ma), large amounts of faults and magmatic intrusions developed in the post-rift layer of the eastern basin (Baodao to Changchang sub-basins), while they seldom formed in the western basin (Lengdong to Lingshui sub-basins). Correspondingly, the eastern basin was characterized by lower overpressures (up to 40 MPa) and the western basin displayed higher overpressures (up to 110 MPa). The results indicated that the tectonic activity since ∼10.5 Ma caused the overpressure relief in the eastern basin. In stark contrast, the overpressure in the western basin increased significantly due to the lack of structural conduits and rapid sedimentation of the thick Pliocene and Quaternary. Numerical models reveal that faults and gas chimneys associated with intrusions constituted shallow plumbing systems through which deep natural gas was able to migrate upward. The formation of gas hydrates relied on the migration of deep gas to the hydrate stability zone below the seafloor. Therefore, the relatively lower overpressured zones due to pressure relief in the eastern basin are considered the next areas potentially favorable to the formation of gas hydrates. Thematic collection: This article is part of the Emerging knowledge on the tectonics of the South China Sea collection available at: https://www.lyellcollection.org/topic/collections/south-china-sea