Origin and microbial degradation of thermogenic hydrocarbons within the sandy gas hydrate reservoirs in the Qiongdongnan Basin, northern South China Sea

Large-scale natural gas hydrate accumulations (generally over one billion cubic meters of natural gas) in coarse-grained marine sediments are the most valuable types for commercial exploration and exploitation. The formation of such a gas hydrate deposit generally requires a sufficient natural gas supply such as deeply buried thermogenic gas source. However, the worldwide discovered thermogenic gas hydrate deposits are rare, leaving relatively a poor understanding of their formation and evolution. Based on the geochemical and microbial analyses of eleven hydrate-related gas and five sediment samples from the thermogenic gas hydrate deposits in the Qiongdongnan Basin in northern South China Sea, we systematically investigated the origin and evolution of thermogenic hydrocarbons within sandy gas hydrate reservoirs. Results show that natural gas within high saturated gas hydrate-filled sandy sediments is predominately of thermogenic (the proportion is 57.1‒70.5%), characterized by “dry gas” (99.38‒99.7% C1, 0.26‒0.47% C2 and <0.5% C3+), relatively lower C1/(C2+C3) ratio (187‒376), and heavier δ13C-C1 values (-55.0 ‒ -31.3‰). In addition, typical biomarkers (including the alkanes, tricyclic terpanes, hopanes and steranes) generated from mature source rocks were detected in the hydrate-filled fine sand reservoir, as well as the biomarker indicators (including the unresolved complex mixtures (UCMs) hump and the 25-norhopane homologous series) for severe hydrocarbon biodegradation. Furthermore, large amounts of hydrocarbon degrading bacteria (50.5% of bacteria) and methanogenic archaea (34.3% of archaea) were detected within hydrate-filled sandy sediment. Our results indicated that the deeply buried thermogenic source can indeed provide sufficient gas supply for the formation of a large-scale gas hydrate deposit, but thermogenic liquid hydrocarbons within gas hydrate reservoirs are very susceptible to be degraded and consumed by hydrocarbon degrading bacteria. In addition, we also found that the clay components of argillaceous gas hydrate reservoirs can adsorb and store large amounts of in situ immature organic matter, which can significantly obscure or cover up the original geochemical signatures of thermogenic hydrocarbons therein. These findings suggest that the relative contribution of deeply buried thermogenic source for gas hydrate accumulation worldwide may be underestimated, because we have previously ignored the fact that thermogenic hydrocarbons within gas hydrate reservoirs would be significantly consumed by hydrocarbon degrading microbes or diluted by in situ immature sedimentary organic matter. Our findings are highly significant for the evaluation and exploration of thermogenic gas hydrate accumulations worldwide.