Integrated signatures of secondary microbial gas within gas hydrate reservoirs: A case study in the Shenhu area, northern South China Sea

A series of highly saturated pore-filled gas hydrate deposits were recently discovered in the Shenhu area of northern South China Sea. However, the main hydrocarbon sources of the gas hydrates remain controversial. The main hypotheses include a shallowly buried microbial gas source or a deeply buried thermogenic hydrocarbon source. In this study, an integrated geochemical-microbial-geological analysis of gas hydrate and sediment samples from sites SC1 and SC2 in the Shenhu area was conducted to investigate the possible hydrocarbon sources, and to identify the signatures of the secondary microbial gases in the gas hydrate reservoirs. The results show that the gas hydrates from sites SC1 and SC2 exhibit the geochemical characteristics of secondary microbial gas (C1/(C2+C3) >100, −50.2‰<δ13C–C1<−47.4‰), and the propane (C3) component and liquid thermogenic hydrocarbons (C15+) have been severely biodegraded (a strongly elevated δ13C–C3 value and a high abundance of 25-norhopanes). In addition, we discovered that the in-situ Quaternary marine sediments exhibit relatively low generation potential of primary microbial gas because of the extremely low organic matter richness (total organic matter content (TOC) < 0.3%) and low in-situ temperatures (5–18 °C). Furthermore, a relatively high abundance of methanogens (∼7%–15%) were detected within the gas hydrate reservoir, which may be attributed to the methane generation process of hydrocarbon biodegradation. Moreover, the geological-geophysical interpretations indicate that the distributions of the gas hydrate accumulations and the shallow gas pools are strictly controlled by the effective faults connecting them with the deeply buried thermogenic source rocks. These lines of evidence suggest a significant contribution of secondary microbial methane to sites SC1 and SC2, which further indicates that the main contribution was from deeply buried thermogenic hydrocarbon sources. Our results suggest that the secondary microbial methane within gas hydrates is an important terminal product that has been converted from thermogenic hydrocarbons via microbial degradation, which helps explain why the geochemistry of most of the hydrate-bound gas indicates a microbial origin even though the geological evidence indicates that the gas is mainly sourced from deeply buried thermogenic hydrocarbons.