Joint control of pressure and gas hydrate stability zone on hydrate occurrences

Abstract

Most fracture-filling hydrates are discovered in the clay-domain sediments shallower than 200 meters below seafloor, while pore-filling hydrates were found in deeper sediments except for shallow sands worldwide. The controlling mechanisms for the occurrences of different types of hydrates are universally unrecognized. In this study, the ubiquitous hydrate reservoirs recovered in the northern South China Sea are categorized into three types based on the depth of hydrate-bearing sediments and hydrate morphology, including 1) fracture-filling hydrates in shallow clay sediments, 2) shallow fracture-filling hydrates and associated hydrates in coarse grained sediments, and 3) pore-filling hydrates in deeper clay-domain sediments. By analyzing the in-situ pressures, a critical depth range (CDR) is found where capillary pressure almost equals to the sum of the minimum principal stress and tensile strength, and hydrate types are determined by the coupling relationship between the depth of bottom of gas hydrate stability zone (BGHSZ) and the depth of CDR. Above CDR where the capillary pressure is greater, gas first migrates by inducing fractures, and a BGHSZ shallower than CDR must be matched to produce fracture-filing hydrates. Below CDR where capillary pressure is basically less than fracturing pressure, gas prefers migrating between pores. If BGHSZ is deeper than CDR at the same time, pore-filling hydrates are forming below CDR. The formation of hydrate further reduces the permeability of sediments, blocking fluids to migrate into the sediments above CDR. This study reveals the controlling mechanisms of hydrate occurrences, and provide scientific implications for hydrate research worldwide.