Accumulation characteristics and occurrence differences of multitype gas hydrates in the northern South China Sea

doi:10.13745/j.esf.sf.2024.11.19

Abstract

Abstract:

Recent gas hydrate drilling expeditions, combined with extensive 3D seismic data, have revealed the widespread occurrence of multiple types of gas hydrates on the northern slope of the South China Sea. However, the anomalous geophysical responses associated with variations in gas hydrate types, accumulations, and their primary geological controls remain poorly understood. Fracture-filling gas hydrates, often linked to cold seep systems, are characterized by chimney-like seismic reflections. In contrast, pore-filling gas hydrates exhibit high-amplitude reflections above the bottom simulating reflector (BSR) and are vertically superimposed with fracture-filling gas hydrates in the Tainan and Qiongdongnan basins. In the Pearl River Mouth Basin, pore-filling gas hydrates are predominantly found near the base of the gas hydrate stability zone. The occurrence of fracture-filling gas hydrates is controlled by relatively fine-grained and mass transport deposit (MTD) reservoirs with low porosity and permeability, which are matched with high-flux fluid systems. On the other hand, pore-filling gas hydrates are primarily controlled by coarse-grained reservoirs and relatively low-flux fluid systems. Recent drilling results suggest that the presence of structure II gas hydrates, the coexistence of gas hydrates and free gas, and active gas hydrate systems are closely associated with the supply of thermogenic gas and high-flux fluids from deep gas sources. The study highlights that the diverse tectonic and sedimentary settings in the northern South China Sea are the primary factors driving the occurrence of multiple types of gas hydrates. The coupling of fluid migration pathways, migration mechanisms, and reservoir lithology plays a critical role in controlling variations in gas hydrate accumulations. Key geological features such as faults, gas chimneys formed by magmatism, intrusive structures, mud volcanoes, and basement uplift significantly influence fluid migration and gas hydrate formation. A comprehensive understanding of the differences in gas hydrate occurrences and the identification of key geological controls are essential for exploring highly concentrated gas hydrate reservoirs in the region.

Key words: gas hydrate, multitype, fluid migration, reservoir, northern South China Sea

CLC Number:

P618.13

JIN Jiapeng, WANG Xiujuan, DENG Wei, LI Qingping, LI Lixia, YU Han, ZHOU Jilin, WU Nengyou. Accumulation characteristics and occurrence differences of multitype gas hydrates in the northern South China Sea[J]. Earth Science Frontiers, 2025, 32(2): 61-76.

Figures/Tables 7

Fig.1 BSR distribution in different basins in the northern South China Sea and enlarged map of typical gas hydrate drilling areas a—Map of sedimentary basins and BSR distribution in the northern South China Sea; b—Distribution of multiple types of BSR in the submarine canyon zone of the Baiyun Sag, Pearl River Mouth Basin; c—BSR distribution in the mass-transported sediment zone of the Qiongdongnan Basin; d—BSR distribution in the thrust-extrusion tectonic zone of the Taixinan Basin; e—BSR distribution in the erosional submarine canyon zone of the Jieyang Sag, eastern Pearl River Mouth Basin.

Fig.2 Comparison of stratigraphic ages, lithologies, and depositional environments in the Pearl River Mouth Basin, Qiongdongnan Basin, and Taixinan Basin, northern South China Sea (modified after [12,23-24]).

Fig.3 Seismic profiles across typical gas hydrate occurrence areas in the northern slope of the South China Sea, showing various BSR reflection characteristics associated with multitype gas hydrate development. a, b, c—Seismic profiles across site GMGS2W08, cold seep site F, and Penghu Canyon in the Taixinan Basin show continuous and discontinuous BSR, high-amplitude reflections above the BSR, pull-up reflections, and strong carbonate reflections on the seafloor or buried in sediments. d, e, f—Seismic profiles of the Jieyang Sag in the Pearl River Mouth Basin show continuous, discontinuous, and paleo-BSR on the flanks of juvenile, young, and mature canyons, as well as the downward adjustment of BSR caused by canyon erosion. g, h, i—Seismic profiles of the Baiyun Sag in the Pearl River Mouth Basin across sites 18 &19, 11 &17, and 07 show vertical migration of high-flux fluids, thermogenic gas-related IIBSR, and dynamically adjusted BSR2. j, k, l, m, h—Seismic profiles in the Qiongdongnan Basin across sites W09, W08&W07, W01, W03&W04, and the Haima cold seep site show seismic reflection characteristics near the cold seep in the MTD area, BSR crosscutting with strata, and local sand-rich turbidite layers. n—Typical seismic profiles in the Zhongjian Basin show BSR crosscutting with strata and strong reflections above it. The locations of seismic profiles are shown in Fig.1.

Fig.4 Seismic profiles across typical basins in the northern slope of the South China Sea showing the regional tectonic features controlling gas hydrate occurrence a—Seismic profile of the Taixinan Basin showing mud volcanoes, faults, and their relationship with the shallow BSR and cold seep system under an active tectonic setting; b—Seismic profile of the Jieyang Sag in the Pearl River Mouth Basin showing the downward shift of the BSR caused by seafloor erosion above the basement uplift; c—Seismic profile of the Baiyun Sag in the Pearl River Mouth Basin showing magmatic volcanoes, intrusive sills, and associated faults and gas chimneys related to the shallow BSR; d—3D seismic profile of the Qiongdongnan Basin showing the relationship between basement uplift, faults, gas chimneys, and the occurrence of shallow cold seep-related gas hydrates. The locations of seismic profiles are shown in Fig.1.

Table 1 Geological controls and occurrence characteristics of multitype gas hydrates in different basins in the northern slope of the South China Sea

构造背景	盆地	构造单元	水合物赋存类型	BSR特征	饱和度	储层条件	气源条件	流体疏导条件	特殊地质要素	典型站位
被动大陆边缘	珠江口盆地	白云凹陷	孔隙充填型、部分站位发现Ⅱ型水合物	连续、不连续、羽状、II-BSR、 BSR2	中-高	细粒泥质粉砂或粉砂质泥、局部富有孔虫砂	混合成因气	正断层、气烟囱、渗透性地层	侵入岩席、单向迁移峡谷	GMGS3-W07、 W18&W19、 W11&W17 GMGS4-SC01、 SC02、SC03
		云荔低凸起		连续、不连续、羽状BSR	中?	细粒泥质粉砂或粉砂质泥			火山复合体
		揭阳凹陷		连续、不连续、古BSR	中?	细粒泥质粉砂或粉砂质泥			海底侵蚀峡谷、基底隆起
	琼东南盆地	松南低凸起	裂隙充填型与孔隙充填型叠置、部分站位发现Ⅱ型水合物	BSR与MTD底重合、上拱、仅局部发育穿层的BSR	中-高	细粒泥质粉砂或粉砂质泥	热成因气为主	正断层、气烟囱	基底隆起	GMGS5-W07、 W08、W09
	琼东南盆地	陵南低凸起	裂隙充填型与孔隙充填型叠置、部分站位发现Ⅱ型水合物	BSR与MTD底重合、上拱、仅局部发育穿层的BSR	中-高	细粒泥质粉砂—粗粒砂	热成因气为主	正断层、气烟囱	基底隆起	GMGS6-W01、 W03、W04
主动大陆边缘	台西南盆地		裂隙充填型与孔隙充填型叠置	连续、不连续、 BSR上拱、冷泉下部缺失	中	细粒泥质粉砂或粉砂质泥、局部富有孔虫砂	生物成因气、局部热成因气	逆冲断层、正断层、气烟囱	底辟构造	GMGS2-W08、 W09、W16、 F站位

Fig.5 Plot of δ13C1 versus the C1/(C2+C3) ratio of gases from gas hydrate-bearing and oil field sites in the northern slope of the South China Sea

Fig.6 Differences in the geological controls of multitype gas hydrate occurrences under different tectonic-sedimentary settings in the northern South China Sea

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构造背景	盆地	构造单元	水合物赋存类型	BSR特征	饱和度	储层条件	气源条件	流体疏导条件	特殊地质要素	典型站位
被动大陆边缘	珠江口盆地	白云凹陷	孔隙充填型、部分站位发现Ⅱ型水合物	连续、不连续、羽状、II-BSR、 BSR2	中-高	细粒泥质粉砂或粉砂质泥、局部富有孔虫砂	混合成因气	正断层、气烟囱、渗透性地层	侵入岩席、单向迁移峡谷	GMGS3-W07、 W18&W19、 W11&W17 GMGS4-SC01、 SC02、SC03
		云荔低凸起		连续、不连续、羽状BSR	中?	细粒泥质粉砂或粉砂质泥			火山复合体
		揭阳凹陷		连续、不连续、古BSR	中?	细粒泥质粉砂或粉砂质泥			海底侵蚀峡谷、基底隆起
	琼东南盆地	松南低凸起	裂隙充填型与孔隙充填型叠置、部分站位发现Ⅱ型水合物	BSR与MTD底重合、上拱、仅局部发育穿层的BSR	中-高	细粒泥质粉砂或粉砂质泥	热成因气为主	正断层、气烟囱	基底隆起	GMGS5-W07、 W08、W09
	琼东南盆地	陵南低凸起	裂隙充填型与孔隙充填型叠置、部分站位发现Ⅱ型水合物	BSR与MTD底重合、上拱、仅局部发育穿层的BSR	中-高	细粒泥质粉砂—粗粒砂	热成因气为主	正断层、气烟囱	基底隆起	GMGS6-W01、 W03、W04
主动大陆边缘	台西南盆地		裂隙充填型与孔隙充填型叠置	连续、不连续、 BSR上拱、冷泉下部缺失	中	细粒泥质粉砂或粉砂质泥、局部富有孔虫砂	生物成因气、局部热成因气	逆冲断层、正断层、气烟囱	底辟构造	GMGS2-W08、 W09、W16、 F站位

构造背景	盆地	构造单元	水合物赋存类型	BSR特征	饱和度	储层条件	气源条件	流体疏导条件	特殊地质要素	典型站位
被动大陆边缘	珠江口盆地	白云凹陷	孔隙充填型、部分站位发现Ⅱ型水合物	连续、不连续、羽状、II-BSR、 BSR2	中-高	细粒泥质粉砂或粉砂质泥、局部富有孔虫砂	混合成因气	正断层、气烟囱、渗透性地层	侵入岩席、单向迁移峡谷	GMGS3-W07、 W18&W19、 W11&W17 GMGS4-SC01、 SC02、SC03
		云荔低凸起		连续、不连续、羽状BSR	中?	细粒泥质粉砂或粉砂质泥			火山复合体
		揭阳凹陷		连续、不连续、古BSR	中?	细粒泥质粉砂或粉砂质泥			海底侵蚀峡谷、基底隆起
	琼东南盆地	松南低凸起	裂隙充填型与孔隙充填型叠置、部分站位发现Ⅱ型水合物	BSR与MTD底重合、上拱、仅局部发育穿层的BSR	中-高	细粒泥质粉砂或粉砂质泥	热成因气为主	正断层、气烟囱	基底隆起	GMGS5-W07、 W08、W09
	琼东南盆地	陵南低凸起	裂隙充填型与孔隙充填型叠置、部分站位发现Ⅱ型水合物	BSR与MTD底重合、上拱、仅局部发育穿层的BSR	中-高	细粒泥质粉砂—粗粒砂	热成因气为主	正断层、气烟囱	基底隆起	GMGS6-W01、 W03、W04
主动大陆边缘	台西南盆地		裂隙充填型与孔隙充填型叠置	连续、不连续、 BSR上拱、冷泉下部缺失	中	细粒泥质粉砂或粉砂质泥、局部富有孔虫砂	生物成因气、局部热成因气	逆冲断层、正断层、气烟囱	底辟构造	GMGS2-W08、 W09、W16、 F站位