GEOCHEMICAL CHARACTERISTICS OF HYDROCARBON GASES IN SEDIMENTS IN SHENHU AREA, THE NORTHERN SOUTH CHINA SEA

The anomalous distribution of the gases in headspace of shallow sediments could be applied as an indicator to explore whether there are gas hydrate accumulations underneath. Comprehensive analyses of hydrocarbon gas composition and isotopic composition of methane in headspace gas in shallow sediments and related drilling cores in the Shenhu area are studied in this paper. Results show that CH₄ is the major component of gases from the total 291 samples 1 mbsf (bsf, below sea floor) with trace C₂H₆ and C₃H₈, and the normalized concentrations of CH₄ range from 2.49 to 77.23 μL/kg with an average value of 15.22 μL/kg. CH₄ concentration from 12 anomalies were determined using tendency analysis method, and they extend in a NE direction as a whole, that is obviously controlled by NE trend fractures. The δ¹³C values of CH₄ from 14 headspace gas samples from Sites 4PC and 23PC range from -74.3‰ to -46.2‰ with an average value of -60.9‰(PDB), and the ratios of C₁/(C₂+C₃) range from 5.78 to 49.45 with an average of 16.23. This indicates that hydrocarbon gases of those samples are mainly microbial gas with small fraction of thermogenic gas. Isotopic anaylses were applied to 4 headspace gas samples collected from the drilling cores. It is found that δ¹³C and δD values of CH₄ are -62.2‰ to -54.1‰ (PDB) and -225‰ to -180‰ (SMOW), respectively, and the ratios of C₁/(C₂+C₃) was ranging from 575 to 1 668, suggesting that CH₄ seems to be microbial gas or mixed gases with microbial gas dominating, and CH₄ was mainly generated from microbial CO₂ reduction and there is a little contribution from thermogenic gas. Our study indicates that the headspace hydrocarbon gases in shallow sediments were from gas hydrates, hydrocarbon source rocks or gas accumulations underneath. The hydrocarbon gases migrated upward along faults and diapir structures. Hydrocarbon gases originated from the dissociation of gas hydrates are mainly microbial gas, and those from hydrocarbon source rocks or gas accumulations are thermogenic gases in origin.