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

Yan Li,Jin-Zhong Liu,Xiang-Po Xu,Jiang-Hai Wang,Yun-Xin Fang,Qian-Zhi Zhou

doi:10.3390/molecules26226808

Abstract

Although various geochemical and geophysical investigations have already indicated a great resource potential in the Xisha area of the South China Sea, the origin of organic matter and molecular evidence for tracing the migration of hydrocarbons from deep petroleum reservoirs are still lacking. In this study, systematic organic geochemical analyses, including bulk organic matter parameters and lipid biomarkers were performed for deep sediments from two cores. The C/N ratios and δ13C and δ15N values of organic matter in most of the samples, together with the maxima of short-chain n-alkanoic acids and mid-chain n-alkanols, high abundances of monounsaturated fatty acids C18:1ω9 and C22:1ω13, jointly indicate the dominance of marine organic matter. n-Alkanes in sediments from core GMGS4-XH-W06B are characterized by small unresolved complex mixture (UCMs) humps, high odd/even predominance (OEP) and carbon preference index (CPI) values, clearly exhibiting characteristics of modern sediments. However, the sediments for core GMGS4-XH-W03B are featured with big UCMs, associated with OEP and CPI values around 1.0, showing signatures of petroleum hydrocarbons from high maturity sources. Considering the geologic background, the biomarker signatures are solid evidence for indicating the existence of underlying petroleum reservoirs, and may provide the valuable information for assessing the hydrocarbon resources in the Xisha area.

Highlights

The South China Sea (SCS) is located at the confluence of the Pacific, Eurasian, and Indian-Australian plates, and was formed during the late Jurassic-early Cretaceous [1]
The atomic C/N ratios in the samples from core GMGS4-XH-W03B are at the interval of 3.46 to 4.65; while those for core GMGS4-XH-W06B range from 5.40 to 6.00
The difference between the C/N ratios of both core samples is mainly due to their total organic carbon (TOC) contents rather than total nitrogen (TN) contents

Summary

Introduction

The South China Sea (SCS) is located at the confluence of the Pacific, Eurasian, and Indian-Australian plates, and was formed during the late Jurassic-early Cretaceous [1]. It harbors numerous natural gas hydrates and conventional gas/oil reservoirs [2,3,4]. Numerous cold seeps have been discovered in the Dongsha and Shenhu areas, indicating the hydrocarbon seepage and occurrence of underlying gas hydrates [5]. 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]

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