Basal shear zones of recurrent mass transport deposits serve as potential reservoirs for gas hydrates in the Central Canyon area, South China Sea

Abstract

Recurrent mass transport deposits (MTD) not only act as competent seals trapping fluids, but their basal shear zones connected by pipes also serve as potential reservoirs for gas hydrate within the gas hydrate stability zone. However, this has seldom been addressed in previous studies. Using conventional 3D seismic reflection data and well data collected from the Central Canyon area of the northwestern South China Sea, we find that gas hydrates mainly lie in the fractures within MTDs. These fractures can be subdivided into two groups. The first group is composed of the horizontal and low-angle oblique fractures characterized by pseudo-SC structures, the majority of which occurred within the basal shear zones of MTDs. They probably resulted from the strong shear stresses during mass transport. The second group comprises the nearly vertical fractures within pipes 1 and 2. They are attributed to the result of hydraulic fracturing during the generation of pipes. It is important to note that the basal shear zones of MTDs 2–4 were connected by the pipes in the vertical direction and have good lateral continuities (with the area of hundreds of km2), and thus they probably provide larger reservoir spaces for gas hydrates, compared with the hydraulic fractures merely confined within pipes 1 and 2 (with the area less than 1 km2). Therefore, in the future gas hydrate exploration, much more attention should be paid to basal shear zones of recurrent MTDs that are connected by pipes in the vertical direction. Hydrocarbon gas that formed gas hydrates with water molecular, were mainly supplied by three large-scale gas chimneys above the basement highs. We suggest that the single overpressure configuration and effective seals provided by the deep-seated MTD1 were responsible for the continuous gas accumulations above the basement highs. Hence, hydraulic fracturing took place and these gas chimneys were formed. We also propose a 3D conceptual model to show how MTDs influence fluid migration and accumulation, and prevent methane seepages.