Fluid migration patterns in shallow horizontal sand bodies pierced by vertical gas seepage in the Qiongdongnan Basin, South China Sea

Seafloor seepage, caused by vertically concentrated fluid flow, is a global phenomenon that mainly occurs in the structurally highest part of the sealing layers. Gas buoyancy resulting from the vertical height of the gas column acts as the primary driving force. This study deals with the seepages caused by the overpressure due to increased lateral migration of gas/hydrate away from the seeping center in horizontal sand bodies. Fluids migrate horizontally to larger radii in deeper sands than shallower sands, resulting in the formation of widely distributed christmas-tree shape structures in the Qiongdongnan basin. However, the fluid migration patterns and geological controls have not been well studied. In this paper, we studied the fluid migration by analyzing stress, hydrate morphology, and permeability variation of gas/hydrate-bearing sands. Below the gas hydrate stability zone (BGSHZ), gas can be laterally transported to large radii away from the center in deep sands because of large fracture pressures of the overlying clay sediments. Within the BGHSZ, hydrates can still grow laterally to almost the same radii as the gas does due to the comparable permeability of gassy sand and hydrate-bearing sand under the gas excess condition. If the fluid flux is high, the fluid will be transported laterally in the sand for a shorter distance, resulting in the formation of a narrow christmas-tree structure. Therefore, shallow horizontal sand, which stores gas and hydrates, plays a blocking role in vertical seepage and their blocking capacity depends on its burial depth and the fluid flux of the seepage.

Automated summary

Seafloor seepage is a global phenomenon that mainly occurs in the structurally highest part of the sealing layers, driven by gas buoyancy resulting from vertical height. This study focuses on the seepages caused by lateral migration of gas/hydrate in horizontal sand bodies, analyzing fluid migration patterns and geological controls. It was found that shallow horizontal sand blocks vertical seepage, and its blocking capacity depends on burial depth and fluid flux.