Developmental characteristics and evolution of seepage gas hydrate accumulation system in the northern South China Sea

doi:10.13745/j.esf.sf.2024.6.55

Abstract

Abstract:

Seepage gas hydrate accumulation systems tend to develop medium-to-high saturation gas hydrates and their associated underlying gases, which are widely distributed gas hydrate resources in the ocean. These systems are also important research subjects for submarine disasters, cold seep systems, and climate change. Therefore, understanding their distribution patterns and formation mechanisms is of great significance. Based on seafloor observations, core samples, well logging, seismic data, and test analysis from the northern South China Sea, this study systematically summarizes the geomorphological, geological, geophysical, and geochemical indicators for identifying seepage gas hydrate accumulation systems. The findings reveal that seepage gas hydrates are primarily distributed within escape pipes. Escape pipes can be categorized into two types based on their mechanical failure mechanisms: (1) hydraulic fracturing escape pipes controlled by structural traps, which exhibit clustered distributions, and (2) shear failure escape pipes controlled by high-angle faults, which show aligned distributions. In seepage gas hydrate accumulation systems, deep gas migrates and accumulates near the bottom of the methane gas hydrate stability zone along gas chimneys and faults. When fluid pressure exceeds the fracturing strength of the overlying fine-grained sedimentary layer, free gas breaks through upward to form escape pipes. This process results in medium-to-high saturation fracture-filling gas hydrates within high-angle fracture networks in the escape pipes and high-saturation pore-filling gas hydrates in thin sand layers intersected by the pipes. The long-range migration of free gas within escape pipes is influenced by three mechanisms: (1) thermodynamic three-phase equilibrium maintained by salt exclusion, (2) methane diffusion limited by the formation of gas hydrate shells, and (3) gas hydrate kinetic formation rates constrained by high fluid fluxes. The formation process of seepage gas hydrates associated with active cold seeps can be divided into three stages. First, during the cap fracture stage, gas hydrate formation is influenced by kinetic rates under over pressure. Second, in the upward breakthrough stage, locally elevated salinity and temperature maintain thermodynamic three-phase equilibrium. Finally, when the fluid reaches the seafloor, the ground temperature approaches the background value, entering a stable leakage stage. Salt exclusion and methane diffusion limitations caused by gas hydrate shells control the generation of gas hydrates. After cold seep activity ceases, salinity decreases due to continuous diffusion, and new gas hydrates gradually form again at the bottom of the escape pipe.

Key words: seepage gas hydrates, escape pipe, mechanical failure, long-range free gas flow, northern South China Sea

CLC Number:

P618.13

CHEN Yuhe, REN Jinfeng, LI Tingwei, XU Mengjie, WANG Xiaoxue, LIAO Yuantao. Developmental characteristics and evolution of seepage gas hydrate accumulation system in the northern South China Sea[J]. Earth Science Frontiers, 2025, 32(2): 77-93.

Figures/Tables 13

Table 1 Distribution of representative seepage gas hydrates at home and abroad

典型地区		深部构造背景	水深/ m	稳定带底界	渗漏标志	储层沉积特征	水合物赋存特征	优势运移通道	参考文献
卡斯卡迪亚大陆边缘		主动大陆边缘俯冲增生楔	950~ 2 200	126~ 240 mbsf	碳酸盐岩、生物群落	黏土、细砂	薄层状、结核状、分散状	断层	[10]
印度K-G盆地		被动大陆边缘盆内局部背斜	1 519~ 2 815	2 200~ 2 500 m	碳酸盐结核、贝壳碎片	泥质碎屑流沉积	块状、脉状、透镜状	断层	[11]
韩国郁陵盆地		边缘海弧后盆地局部背斜	1 800~ 2 100	141 mbsf	海底丘状体、水合物帽	黏土	脉状、结核状、块状	气烟囱	[12]
南部水合物脊		主动大陆边缘挤压背斜	780~ 1 200	124~ 134 mbsf	碳酸盐岩、羽状流	黏土、粉砂质黏土	薄层状、结核状、脉状	高渗透层	[20]
墨西哥湾 Terrebonne盆地		被动大陆边缘盐构造	1 700~ 2 250	420~ 670 mbsf	海底丘状体	黏土	裂隙充填	高渗透层	[21-22]
鄂霍次克海		边缘海弧后盆地坳陷	300~ 1 200	665~ 960 m	碳酸盐岩、羽状流、生物群落	粉砂质黏土	薄层状、脉状、块状	泥底辟	[13]
台西南盆地		边缘海盆地挤压背斜	600~ 1 600	98~ 219 mbsf	海底丘状体、碳酸盐岩	粉砂质黏土	薄层状、块状、结核状、分散状	断层	[14]
神狐海域		边缘海盆地坳陷	667~ 1 747	194~ 225 mbsf	碳酸盐岩、双壳类生物	含有孔虫的黏土质粉砂	分散状	断层与气烟囱	[15-16]
琼东南盆地	海马冷泉	边缘海盆地低凸起	1 250~ 1 500	130 mbsf	海底羽状流、麻坑、丘状体	黏土	块状		[17]
	A区		1 500~ 1 700	160~ 170 mbsf	羽状流、冷泉生物、碳酸盐岩	黏土质粉砂夹薄层粉砂	脉状、块状结核状、厚层状		[18]
	B区		1 250~ 1 780	140 mbsf	海底麻坑、丘状体、生物群落	黏土质粉砂	脉状、片状、块状、结核状		[19]

Table 1 Distribution of representative seepage gas hydrates at home and abroad

典型地区		深部构造背景	水深/ m	稳定带底界	渗漏标志	储层沉积特征	水合物赋存特征	优势运移通道	参考文献
卡斯卡迪亚大陆边缘		主动大陆边缘俯冲增生楔	950~ 2 200	126~ 240 mbsf	碳酸盐岩、生物群落	黏土、细砂	薄层状、结核状、分散状	断层	[10]
印度K-G盆地		被动大陆边缘盆内局部背斜	1 519~ 2 815	2 200~ 2 500 m	碳酸盐结核、贝壳碎片	泥质碎屑流沉积	块状、脉状、透镜状	断层	[11]
韩国郁陵盆地		边缘海弧后盆地局部背斜	1 800~ 2 100	141 mbsf	海底丘状体、水合物帽	黏土	脉状、结核状、块状	气烟囱	[12]
南部水合物脊		主动大陆边缘挤压背斜	780~ 1 200	124~ 134 mbsf	碳酸盐岩、羽状流	黏土、粉砂质黏土	薄层状、结核状、脉状	高渗透层	[20]
墨西哥湾 Terrebonne盆地		被动大陆边缘盐构造	1 700~ 2 250	420~ 670 mbsf	海底丘状体	黏土	裂隙充填	高渗透层	[21-22]
鄂霍次克海		边缘海弧后盆地坳陷	300~ 1 200	665~ 960 m	碳酸盐岩、羽状流、生物群落	粉砂质黏土	薄层状、脉状、块状	泥底辟	[13]
台西南盆地		边缘海盆地挤压背斜	600~ 1 600	98~ 219 mbsf	海底丘状体、碳酸盐岩	粉砂质黏土	薄层状、块状、结核状、分散状	断层	[14]
神狐海域		边缘海盆地坳陷	667~ 1 747	194~ 225 mbsf	碳酸盐岩、双壳类生物	含有孔虫的黏土质粉砂	分散状	断层与气烟囱	[15-16]
琼东南盆地	海马冷泉	边缘海盆地低凸起	1 250~ 1 500	130 mbsf	海底羽状流、麻坑、丘状体	黏土	块状		[17]
	A区		1 500~ 1 700	160~ 170 mbsf	羽状流、冷泉生物、碳酸盐岩	黏土质粉砂夹薄层粉砂	脉状、块状结核状、厚层状		[18]
	B区		1 250~ 1 780	140 mbsf	海底麻坑、丘状体、生物群落	黏土质粉砂	脉状、片状、块状、结核状		[19]

Fig.1 Cold seep characteristics observed by ROV:(a) Plume (modified after [18]); (b) Cold seep organisms; (c) Massive gas hydrates; (d) Carbonate rocks (modified after [18]).

Fig.2 Seafloor seismic amplitude attribute map (a) and water depth map (b) of site W08 in the Songnan Low Uplift

Fig.3 Pictures of core samples in the Qiongdongnan Basin and Zhujiangkou Basin. (b modified after [18]; c modified after [18]).

Fig.4 X-ray images and their corresponding logging curves of the core of Well W01 in the Qiongdongnan Basin (core from Fig.5)

Fig.5 Comprehensive logging curves of Well W01 in the Qiongdongnan Basin

Fig.6 Interpreted seismic sections crossing Well W01

Fig.7 Variation in typical geochemical indicators of Well W01

Fig.8 (a) Variance attribute along the 2480 ms time window; (b) Time-domain contour map near site W09;(c) Interpreted seismic sections crossing Well W09.

Fig.9 (a) Variance attribute along the 2480 ms time window; (b) Time-domain contour map near site W08;(c) Interpreted seismic sections crossing Well W08.

Fig.10 (a) Interpreted seismic sections crossing Well W09 and W08; (b) The geographic location of the interpreted seismic profile crossing Well W09 and W08;(c) A stress model for the formation of escape pipes by hydraulic fracturing of the cap layer in Well W09;(d) A stress model for the formation of escape pipes by shear failure of the fault in Well W08.

Fig.11 (a) Thermodynamic three-phase equilibrium model; (b) Methane diffusion limited model;(c) Gas hydrate kinetic formation rate limited model (Modified after [2,25]).

Fig.12 Three stages of forming a seepage gas hydrate accumulation system (a) Cap fracture stage; (b) Upward breakthrough stage; (c) Stable leakage stage (Modified after [42]).

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