Geochemical Characteristics and Gas‐to‐Gas Correlation of Two Leakage‐type Gas Hydrate Accumulations in the Western Qiongdongnan Basin, South China Sea

Abstract

AbstractIn recent years, a series of highly saturated leakage‐type gas hydrates have been discovered in the western Qiongdongnan Basin (QDNB), South China Sea. Based on the molecular compositional and isotopic characteristics of the gas samples relevant to the gas hydrates collected from the two leakage‐type gas hydrate accumulations in the GMGS5 and GMGS6 drilling zones, a detailed geochemical gas‐to‐gas correlation was conducted in this study, in order to further understand the geochemical characteristics and possible hydrocarbon sources of these gas hydrates. The natural gas relevant to the gas hydrates in the GMGS5 block is characterized by wet gas (67.96% < %C1 < 98.58%, C1/C1+ < 0.9) and significant molecular and carbon isotope fractionation within the depth profile, whereas the gas samples from the GMGS6 block exhibit the characteristics of dry gas (99.25% < %C1 < 99.81%, C1/C1+ > 0.9) and lack molecular and carbon isotope fractionation. Approximately 40%–60% of the methane within the gas hydrate is of microbial origin, while the C2+ gas components are typical coal‐type gas that are derived from thermogenic source rocks or deeply‐buried natural gas fields. In addition, typical in situ primary microbial methane (–80.6‰ < δ13C‐C1 < −67.7‰) was discovered in well Wb, which was applied to estimate the contribution of the microbial gas to the gas hydrates. The gas‐to‐gas correlation results show that the hydrate gases within the two leakage‐type gas hydrate accumulations in the GMGS5 and GMGS6 blocks are geochemically different, suggesting that they may have been derived from different source kitchens. Our results further indicate that the deeply‐buried thermogenic gas significantly contributed to the shallowly‐buried gas hydrates in the western QDNB and multiple effective thermogenic source kitchens provided the hydrocarbon gas in the gas hydrate accumulations.