Organic geochemical characteristics of sediments in the natural gas hydrate geo-system in Shenhu area, South China Sea

Abstract

The Shenhu area of the South China Sea is one of the leading areas for natural gas hydrate investigation and exploitation in China. In this study, total organic carbon (TOC) content, total nitrogen (TN) content, and gas chromatographic-mass spectrometric (GC–MS) analysis of the soluble organic matter of sediments from sites W01B, W02B, and W03B were completed to understand the characteristics of organic matter in the sedimentary system with gas hydrates and provide theoretical support for further exploration in this area. Source indicators such as δ¹³C_org, TAR, and C_21-/C₂₁₊ revealed that sites W01B and W02B have similar provenance characteristics in vertical profiles, with an increase of terrestrial source in the hydrate occurrence layers, while site W03B shows a gradual increase in marine source contribution as depth decreases. The asymmetric erosion and lateral deposition of the channel-levee depositional system are considered the controlling factors for the discrepancies in provenance indexes among the three sites. Analysis of the different source indicators revealed that the different kinds of soluble organic matter were subjected to varying degrees of biodegradation, and the distribution characteristics of n-alkanes were preferentially altered. Furthermore, it is found that biodegradations at sites W01B and W02B were more pronounced, particularly in hydrate occurrence layers. All of the microbial degradation indexes increase simultaneously and are consistent with the vertical distribution of hydrates, showing a similar triangular distribution trend. The increase in squalane, a characteristic compound representing methanogens, indicates that microbial degradation and methanogenesis are synchronized in the hydrate occurrence horizon, providing sufficient microbial gases for gas hydrates formation.