Gas sources and its implications for hydrate accumulation in the eastern Pearl River Mouth Basin

Pressure cores and void gas samples are collected by drilling from target layers at five coring sites in the eastern Pearl River Mouth Basin. Dissociation gas is also sampled in thirteen layers, while headspace gas samples acquired at all the five sites. All gas samples have been tested for chemical and isotopic compositions. The gas is methane dominated, which is up to 96.5%~99.8% in concentration. Trace amount of ethane is also detected in concentration of (175~554)×10^-6. No C₂₊ hydrocarbons have been found in all the gas samples. The isotopes of carbon and hydrogen of the methane suggest that δ¹³C₁ is in the range of -68.4‰~-71.2‰, with δDC₁ from -182‰ to -184‰, indicating a biogenic origin. The gas source of hydrate is not directly related to the occurrence of hydrate, while hydrate with multiple sources seems closely related to the evolution and morphological characteristics of reservoir. The main gas source area is located in the shallow strata less than 1000m in depth, suggesting that the hydrocarbon has migrated to the favorable stability zone to form gas hydrate accumulation by shallow lateral migration.