Abstract
In this paper, the gas-hydrate-bearing cores were analyzed for giving a good insight into the exact reserves of natural gas hydrate. The cores were collected from the China National Offshore Oil Corporation (CNOOC) drilling expeditions in 2015 at the LW3-1 area, closed to China's first trial production of natural gas hydrate in 2017. The gas composition of natural gas hydrate and their origin were mainly investigated. The results showed that the gas is mainly methane, up to 98.30%–99.20%. It also contains a small amount of carbon dioxide and other hydrocarbons, which are 0.58%–1.31% CO2, and trace of C2 ~ C4 (less than 0.5%). Gas is generated by mixed-genetic actions dominated by microbial origin. C1 is generated by microbial actions, while C2 is generated by thermogenic actions. Meanwhile, the surface morphology, composition, and distribution of grain size of the host sediments were also studied. The results showed that, the percentage of silty sand is up to 83.57%, where the content of coarse silts is up to 25.12%. There are a preponderance of microfossils and pore structures in the host sediments which consists of silica (SiO2), calcite (CaCO3) and illite, providing a space for the accumulation of gas hydrates. The chloride ion concentration in pore water is 349.09–364.41 mmol/L, which suggested that the hydrate saturation is in a range of 42.81%–45.58% with a good exploitation potential.
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