Origin of natural gas within the deep-sea uncompacted sediments of the Shenhu area, northern South China Sea: Geochemical and methanogenic cultivation results

Abstract

Large amounts of natural gas are dispersed at low concentrations or have accumulated to form highly saturated gas hydrates within uncompacted deep-sea (water depth usually >500 m) sediments worldwide. However, studies on their origin and microbial genetic mechanism are scarce. Using the gas hydrate prolific region of the Shenhu area in the northern South China Sea as our study area, we systematically investigated the genetic types and generation pathways of the natural gas in those sediments based on gas geochemical and anaerobic methanogenic cultivation results. Our results indicated that natural gas within the uncompacted deep-sea sediments in the Shenhu area is dominated by microbial methane (>95%) with a minor amount of biodegraded hydrocarbon (ethane to propane) (<1%). The microbial methane within the highly saturated gas hydrate reservoir was produced by hydrogenotrophic methanogens in the deeper buried optical methanogenic zone (25 °C < in-situ temperature <85 °C), rather than in the in-situ gas hydrate layer. However, the primary microbial gas generative potential of the upper part of the methanogenic zone (28–300 mbsf) in the Shenhu area is relatively poor because of insufficient methanogenic substrates. These findings suggest a strong vertical heterogeneity of methanogenesis potential in the methanogenic zone in the Shenhu area, which may be a common phenomenon in the worldwide marine sediment environments, especially in the regions with relatively low organic matter richness.