Hydrocarbon charging stages and natural gas hydrate accumulation models were established in the Songnan Low Uplift, Qiongdongnan Basin (QDNB), China. Detailed geochemical analysis, paleotemperature and paleopressure analyses, seismic interpretation, and hydrocarbon charging characterization were conducted to investigate the controlling factors of natural gas hydrate accumulation. The Yacheng and Lingshui formations in the Lingshui Sag were identified as effective source rocks. The δ13C1 values of the gas hydrates vary from −35.97‰ to −59.50‰, following a direction from the Sag Center to the Low Uplift, indicating that δ13C1 values became relatively lighter during gas migration. Seismic data evidence revealed that large-scale faults, laterally distributed sandstones, and gas chimneys were comprehensive, however efficient hydrocarbon migration pathways. Systematic overpressure was developed in the Yacheng and Huangliu formations, which offer sufficient hydrocarbon migration impetus. Hydrocarbon-bearing aqueous inclusions and their coexisting aqueous inclusions were observed in the Huangliu Formation in the Lingshui Sag, indicating four stages of hydrocarbon charging in the QDNB. Based on the comprehensive analysis in this research, two gas hydrate accumulation models were proposed in the Songnan Low Uplift, QDNB, as follows: 1) a mixture of gas migrated by large-scale faults and a thermogenic-biogenic mixed gas model and 2) gas hydrates from laterally distributed sandstones sealed by MTDs.

Lipid biomarkers of seep carbonates and sediments retrieved from the Dongsha area, Shenhu, Site F and Haima in the South China Sea (SCS) over the last two decades were studied. Biomarker inventories, microbial consortia, seepage dynamics, and biogeochemical processes of anaerobic oxidation of methane (AOM), aerobic oxidation of methane (AeOM), and oxidation of non-methane hydrocarbons, were reconstructed. Authigenic carbonates contained varying contents of 13C-depleted archaeal and bacterial biomarkers, reflecting their formation as a result of AOM under varying conditions. Except for the typical isoprenoids found in various cold seeps worldwide, 3,7,11,15-tetramethyl hexadecan-1,3-diol and two novel sn2-/sn3-O-hydroxyphytanyl glycerol monoethers with notable 13C-depletion were observed in authigenic carbonates obtained from Haima, which are most likely hydrolysis products of archaea-specific diethers. Furthermore, molecular fossils, compound-specific δ13C values, and mineralogies, were used to trace dominant microbial consortia, seepage activities, and environmental conditions in the cold seep ecosystems of the SCS. In this chapter, the archaeal and bacterial lipid biomarker geochemistry of methane seeps is systematically introduced. AOM, AeOM, oxidation of non-methane hydrocarbons, oil degradation, and the diagenetic fate of glycerol ethers, are further summarized.