Temporal Variation in Natural Gas Seep Rate and Influence Factors in the Lingtou Promontory Seep Field of the Northern South China Sea

Abstract

Natural hydrocarbon seeps in marine environment are important sources of methane and other greenhouse gases into the ocean and the atmosphere. This greenhouse gas seepage influences the global methane budget and global climate change. Hydrocarbon seeps on the shallow seabed produce a near-shore gas bubble zone along the western coast of Hainan Island, in the northern South China Sea. However, few studies on the quantitative value of the methane flux and on temporal variation and influence factors of hydrocarbon seeps have been conducted until now. This study describes the results of continuous gas vent measurements for 420 hours on the seabed of the Lingtou promontory shore. The amount of gas released from a single gas vent was 30.5 m 3 during the measurement period. The gas flow rate ranged from 22 -72 L h -1 , with an average rate of 53.4 L h -1 . The time series analyses of the 420-hour record clearly show three principal tidal components with periods of 5.4, 4.6, and 2.4 hours, which are the main factors controlling the gas flow rate. Low flow rates were associated with high tide and high flow rates associated with low tide. A 1-m increase in seawater height results in a decrease of 20 -30 L h -1 or 35 -56% of the hourly flow rate. Therefore, the changes in gas volume escape from the pore could be attributed to the hydrostatic pressure effect induced by water depth. This dominant mechanism controlled pore activation as well as the gas flow rate, suggesting that in the marine environment, especially the shallow-water shelf area, sea level changes may result in great variations in methane release into the ocean and atmosphere.

The Distribution of Dissolved Methane and Its Air-Sea Flux in the Plume of a Seep Field, Lingtou Promontory, South China Sea

“…At these seeps, the gas rises to the sea surface to form a nearshore bubble zone [28]. The total gas flux emitted from these hydrocarbon seeps to the water surface has been estimated as 294-956 m 3 yr −1 based on the displacement of water estimated using an inverted funnel [28], whereas a total methane flux of 4 84 × 10 4 to 6 84 × 10 4 m 3 yr −1 has been estimated for these seeps based on extrapolation of data obtained over 420 h using a gas flux measuring device on one hydrocarbon seep [29,30,34].…”

“…The Lingtou Promontory seep field is one of the most active areas in the Yinggehai Basin for natural hydrocarbon seepage [29,30]. Over 20 perennial hydrocarbon seeps are emitting gases within an area measuring approximately 580 m × 160 m located~300 m offshore Lingtou Promontory in Hainan Province, at water depths of 3-20 m (Figure 1(a)).…”

“…Over 20 perennial hydrocarbon seeps are emitting gases within an area measuring approximately 580 m × 160 m located~300 m offshore Lingtou Promontory in Hainan Province, at water depths of 3-20 m (Figure 1(a)). It has been observed that gas bubbles (each up to 1-2 cm in diameter) are being emitted continuously from the hydrocarbon seeps on the seafloor (Figures 1(b) and 1(c)) and that these bubbles rise to the sea surface at about 10 cm s -1 and release their gases into the atmosphere [29,30,35]. Methane is released at rates ranging from 0 807 × 10 4 to 1 14 × 10 4 m 3 yr −1 from 20 hydrocarbon seeps in the Lingtou Promontory seep area, assuming that the hydrocarbon seeps have a similar gas flux [29,30].…”

“…Coastal regions are important sources of methane (13 Tg CH 4 yr −1 ) to the atmosphere [5,26,27]. Hundreds of hydrocarbon seeps are discovered in the coastal regions of Yinggehai Basin [28][29][30]. However, the distribution of dissolved methane and its air-sea fluxes has not been quantified.…”

About the Methane Flux Into the Atmosphere in the Bien Dong (South China) Sea

“…The regional profile of the methane flux and the geo-structural map of the Bien Dong with a flux pattern. Legend: 1-gas deposits; 2-oil deposits; 3-faults: 1)-the first order, 2)-the second order, 3)-the third order; 4-the main sedimentary basins (Paleocene, synphrite sedimentary strata with a high carbon content) (I-Pearl river deltas, II-Sean Khon, III-Kyongdongnan, IV-Fu Han, V-Ku Long, VI-Nam Con Son, VII-Natunsky, VIII-Malay); 5-schematic contour of the boundaries of the oceanic crust block; 6-direction of surface currents in winter; 7-oil and gas exploration drilling areas of the PRC; 8-gas hydrate section; 9-cut-in; 10-active hydrocarbon gas outlet to the bottom of the shallow shelf of Hainan island[2], 11-an anomalous field of methane in the bottom layer (70-120 nl/l), discovered in 1992[3] RESULTS AND DISCUSSIONSAs a result of the analysis of the obtained data on the profile, five main zones of methane emission into the atmosphere were identified (fig. 2).…”