The formation of marine gas hydrates is controlled by gas migration and accumulation from lower sediments and by the conditions of the hydrate stability zone. Permeability and porosity are important factors to evaluate the gas migration capacity and reservoir sealing capacity, and to determine the distribution of hydrates in the stable region. Based on currently available geological data from field measurements in the Shenhu area of Baiyun Sag in the northern South China Sea, numerical simulations were conducted to estimate the influence of heterogeneities in porosity and permeability on the processes of hydrate formation and accumulation. The simulation results show that: (1) The heterogeneity of the hydrate stability zone will affect the methane migration within it and influence the formation and accumulation of hydrates. This is one of the reasons for the formation of heterogeneous hydrates. (2) When the reservoir is layered heterogeneously, stratified differences in gas lateral migration and hydrate formation will occur in the sediment, and the horizontal distribution range of the hydrate in a high porosity and permeability reservoir is wider. (3) To determine the dominant enrichment area of hydrate in a reservoir, we should consider both vertical and lateral conditions of the sedimentary layer, and the spatial coupling configuration relationships among the hydrate stability region, reservoir space and gas migration and drainage conditions should be considered comprehensively. The results are helpful to further understand the rules of hydrate accumulation in the Shenhu area on the northern slope of the South China Sea, and provide some references for future hydrate exploration and the estimation of reserves.
Submarine cold seep and its associated authigenic minerals in sediment are meaningful to indicate the existence of underlying natural gas hydrate. The anaerobic oxidation of methane (AOM) is coupled with sulfate reduction (SR) and influences the dissolution and precipitation of barite. However, the forming mechanism of barite is not yet clearly understood. In order to investigate the forming process of authigenic barite and its relationship with methane leakage flux, based on the measured data of the Qiongdongnan Basin in the Northern slope of the South China Sea, we constructed a 1D model of a sedimentary column to reproduce the formation of barite using the numerical simulation method. The results show that the original equilibrium of barite was broken by the cold seep fluids and Ba2+ was carried upward to the sulfate-rich zone leading to the formation of barite front. When there is no flux of methane from the bottom of sediment, the barite front disappears. The relationship between methane leakage flux and authigenic minerals was also discussed. It can be concluded that high methane flux corresponds to a shallow barite front in the sediment, furthermore, the barite content first increases and then decreases as the methane flux increases. At the same time, an inverse relationship between the ratio of authigenic barite to calcite and methane flux was obtained.
In 2007, the first marine gas hydrate
drilling expedition of the
Guangzhou Marine Geological Survey (GMGS-1) was carried out in the
Shenhu area on the northern continental slope of the South China Sea,
collecting geophysical and drilling data from eight different sites.
No hydrate sample was recovered at site SH5, even though it locates
close to site SH2 where the hydrate was drilled. The reason for the
different distribution of hydrate is not thoroughly understood. On
the basis of the actual exploration data, the key factors controlling
hydrate formation in the Shenhu area were analyzed by employing the
numerical modeling. To evaluate the key controlling factors among
gas flux, water depth, temperature gradient, and cap rock location,
sensitive studies were conducted. The results show that the gas flux
is the prerequisite factor determining the hydrate quantity. The seawater
depth does not affect the hydrate formation significantly. The temperature
gradient plays an important role in hydrate quantity and saturation.
The cap rock location controls the gas migration to the gas hydrate
stability zone (GHSZ). According to the sensitivity analyses and drilling
data, we proposed a mechanism of hydrate differential accumulation
in the Shenhu area. The relatively high geothermal gradient and the
cap rock located below the GHSZ cause the absence of hydrate at site
SH5, leading to the differential accumulation.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.