CHARACTERISTICS OF SHALLOW GAS HYDRATES ACCUMULATION IN THE SEA

Based on the migration ways of the gas responsible for the gas-hydrate formation, the leaking-type accumulation system is the major model for the formation of the shallow gas-hydrate reservoirs, and it would be influenced by multiple critical controlling factors including temperature-pressure field, gas source, et al. The temperature-pressure field mainly influences the gas-hydrate distribution in plane view and vertical section. The area with low heat flow is favorable for the formation of gas hydrates, while the deep-sea area with very high heat flow and high-flux methane can also deposit dense, thick-bedded (up to several meters) shallow gas-hydrate if there are abundant gas supply, and this type of gas-hydrate reservoirs are generally accompanied with special geological bodies, such as mud volcanoes and gas chimneys. The gas responsible for the shallow gas-hydrate formation is dominated by organic pyrolysis gas, since shallow gas-hydrate reservoirs are generally presented above mature oil- and gas-bearing basins, in which source rocks are widespread and characterized by the kerogen having defined hydrocarbon generation processes, and the generated pyrolysis methane would migrate upwards along the broken zones like faults, gas chimneys, and then form gas-hydrate at the shallow layers where temperature-pressure fields are appropriate for the gas-hydrate deposition, thus the deep-sea basin with very high flow can also be favorable for the formation of the shallow gas hydrates.