Shengxiong Yang

Widespread cold seeps along continental margins are significant sources of dissolved carbon to the ocean water. However, little is known about the methane turnovers and possible impact of seepage on the bottom seawater at the cold seeps in the South China Sea (SCS). We present seafloor observation and porewater data of six push cores, one piston core and three boreholes as well as fifteen bottom-water samples collected from four cold seep areas in the northwestern SCS. The depths of the sulfate–methane transition zone (SMTZ) are generally shallow, ranging from ~7 to −2 yr⁻¹. The δ¹³C and Δ¹⁴C values of bottom-water dissolved inorganic carbon (DIC) suggest discharge of ¹³C- and ¹⁴C-depleted fossil carbon to the bottom water at the cold seep areas. Based on a two-endmember estimate, cold seeps fluids likely contribute 16–26% of the bottom seawater DIC and may have an impact on the long-term deep-sea carbon cycle. Our results reveal the methane-related carbon inventories are highly heterogeneous in the cold seep systems, which are probably dependent on the distances of the sampling sites to the seepage center. To our knowledge, this is the first quantitative study on the contribution of cold seep fluids to the bottom-water carbon reservoir of the SCS, and might help to understand the dynamics and the environmental impact of hydrocarbon seep in the SCS.

The continuous bottom-simulating reflection (BSR) is commonly considered to mark the base of gas hydrate stability zone. Below this depth, gas hydrate gives away to free gas or water filling with pore spaces of sediments. We integrated and analyzed seismic data collected in 2008, and logging-while-drilling (LWD) data and coring results acquired by the Fugro Voyager in 2015 in the Shenhu area on the northern slope of the South China Sea. Based on seismic and well-log correlation, a BSR with typical characteristics of gas hydrates and free gas was identified at 237 m, below the mudline (BML). However, LWD data reveal a 63 m thick hydrate layer from 205 to 268 m BML. Increases in resistivity and velocity at 262 m BML indicate that gas hydrate is likely presented below the BSR. The observed pore-water freshening with depth and infrared image of core samples are consistent with geophysical interpretation. Seismic and well interpretations reveal continuous, discontinuous, and pluming BSRs in the Shenhu area. The continuous BSR indicates the base of the methane gas hydrate stability zone, and structure II gas hydrate is likely presented below the BSR. Deep thermogenic fluid locally entrapped within shallow-buried sediments may reinforce gas-hydrate accumulations near the discontinuous and pluming BSRs. We conclude that seismic responses of structure II gas hydrate can be distinct from structure I gas hydrate. Understanding the seismic characterizations of structures I and II will help in the evaluation of gas-hydrate reservoirs and inferring the presence of deep thermogenic reservoirs.

The characteristics of the hydrate-bearing layer are important parameters for gas hydrate exploration and exploitation, which can be provided by laboratory analysis. We have carried out a systematic analysis of modern instruments, including the laser grain size analyzer, X-ray computed tomography, scanning electron microscopy, X-ray diffraction (XRD), Raman spectroscopy, gas chromatography, and isotope mass spectrometry, on the hydrate-bearing sediment samples recovered from the Shenhu area in the South China Sea in 2016. We focused on the structure, hydration number, and gas composition of the gas hydrate samples. We also evaluated the effects of sediments on the occurrence and distribution of gas hydrate. Our results showed that the sediments are much finer than those recovered from the Shenhu area in 2007. Some samples are found to contain calcareous microfossils and foraminifera, which are beneficial for dispersed gas hydrate formation. Whereas the only visible hydrate sample (FC4) has not found any bioclast in the sediments. Based on the Raman data and the XRD analysis, the gas hydrates demonstrate typical structure I (sI) hydrates, with cage occupancies of methane molecules higher than 99.3% in large cages and 81.5%–91.4% in small cages, respectively. The crystal lattice parameter is 11.89 Å with the hydration number being approximately 6.0. Methane is obviously the dominant component, ranging from 97.6% to 99.95% in these samples. Comprehensive study of carbon and hydrogen isotopes suggests that the hydrate-bound gases are from biogenic and thermogenic gases in the study area.