Geochemical Significance of Biomarkers in the Methane Hydrate-Bearing Sediments from the Shenhu Area, the South China Sea

Abstract

Biomarkers from methane hydrate-bearing sediments can provide vital evidence for microbial activities associated with methanogenesis and their relation to the formation of methane hydrates. However, the former mainly focus on intact polar lipids from these microorganisms, and rarely investigate molecular hydrocarbons such as acyclic isoprenoids and hopanes so far. In this work, the composition of biomarkers in the methane hydrate-bearing sediments in cores SH2B and SH7B from the Shenhu area, the South China Sea (SCS) were identified by gas chromatography-mass spectrometry (GC-MS) and comprehensive two-dimensional gas chromatography/time-of-flight mass spectrometry (GC×GC-TOFMS). The occurrence of unresolved complex mixtures (UCMs) and 25-norhopane indicate that the organic matters in methane hydrate-bearing sediments underwent a high degree of biodegradation. Although specific biomarkers for methanogens were not identified, the UCMs, 25-norhopane, pristane, phytane, and hopanes can still indicate the microbial activities associated with methanogenesis. These molecular signals suggest that diverse microorganisms, particularly methanogens, were quite vigorous in the methane hydrate-bearing sediments. Further, the biomarkers identified in this study can also be steadily detected from deep oil/gas reservoirs. Considering numerous adjacent oil/gas reservoir systems, fault systems, and mud diapers occurred in the SCS, it can be inferred that microbial activities and deep oil/gas reservoirs may have jointly contributed to the formation of methane hydrate deposits in the SCS.

Source Indication and Geochemical Significance of Sedimentary Organic Matters from the Xisha Area, the South China Sea

“…Unfortunately, pristane and phytane are usually below detection lines, and only recognized in a few samples but with low abundances. It is noteworthy that terpanes and steranes are undetectable, which is similar to the previous result that the conventional GC-MS could not clearly identify these biomarkers due to their low abundances [7]. Therefore, indicators of thermal maturity such as Ts/(Ts + Tm) and C 29 20S/(20S + 20R) sterane ratios cannot be acquired in this study.…”

“…It harbors numerous natural gas hydrates and conventional gas/oil reservoirs [2][3][4]. In light of the great gas hydrate and petroleum/gas reservoir potential, as well as the significant environmental effects (e.g., global warming and petroleum/gas exploration), the SCS has attracted the increasing attention of marine geologists and geochemists [5][6][7].…”

“…The prospecting areas of gas hydrates, such as Dongsha, Xisha, Qiongdongnan and Shenhu, were firstly determined via the extension of bottom-simulating reflectors (BSRs) [9,10]. After several hydrate survey expeditions, some hydrate-bearing areas, e.g., Dongsha and Shenhu, have further been confirmed [7,10,11]. In addition, numerous cold seeps have also been discovered in the Dongsha and Shenhu areas, indicating the hydrocarbon seepage and occurrence of underlying gas hydrates [5].…”

“…In addition, numerous cold seeps have also been discovered in the Dongsha and Shenhu areas, indicating the hydrocarbon seepage and occurrence of underlying gas hydrates [5]. In these areas, many studies have been conducted, mainly including geochemistry, geophysics, numerical simulation, sediment fabrics, microbial diversity, and biomarkers [7,[11][12][13][14][15][16][17][18][19].…”

“…Notably, the contribution of petroleum hydrocarbons from deep oil reservoirs in these areas was revealed by the analyses of 16S rRNA and biomarkers [7,11,19].…”