Source and accumulation of gas hydrate in the northern margin of the South China Sea

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A comprehensive analysis on high‐resolution seismic data and the geological characteristics of the Qiongdongnan Basin (QDNB) in the northern South China Sea reveals multiple seismic evidence indicative of hydrocarbon migration and gas hydrate accumulation, including mud diapirs, gas chimneys, bottom simulating reflectors (BSRs), acoustic blanking, enhanced reflections, V‐AMP structures, elevated stacking velocity of sediments, and seafloor expressions. Mud diapirs and gas chimneys, which are characterized by acoustic turbidity, fuzziness, and blanking reflections on seismic profiles, with a diversity of structures and scales, are widespread and concentrated in the centres of sags and the transitional zones between sags and uplifts. Five BSR regions, with a total area of approximately 11,000 km2, have been delineated in the QDNB. They have high or medium‐high continuity amplitudes with overlying acoustic blanking on seismic profiles, indicating the occurrence of gas hydrate. Distinct fluid seepage phenomena are common below the BSRs, demonstrating that abundant free gas has migrated and accumulated beneath the gas hydrate. The distribution of BSRs and the developed zones of mud diapirs and gas chimneys presents a spatial superposition feature, and the BSRs are usually located on top of or within the structural highs of the mud diapirs and gas chimneys, indicating a close relationship between them. Mud diapirs, gas chimneys, and associated faults are efficient pathways for migrating hydrocarbons, through which the mature to highly mature thermogenic gas of the Palaeogene strata and the biogenic gas of the Neogene sediments migrate to the gas hydrate stability zones and form gas hydrate. Gas hydrate samples were recovered from the gas chimney area during the remote operated vehicle (ROV) investigation in deepwater QDNB in 2015. Based on the seismic interpretation and geochemical results, we propose two models of gas hydrate accumulation in the deepwater area of the QDNB: (a) near source biogenic gas hydrate with self‐generation and self‐accumulation, and (b) distal source thermogenic gas hydrate with lower generation and upper accumulation. We believe that the areas with a fine spatial coupling configuration between the mud diapirs, gas chimneys, and BSRs are favourable exploration targets for gas hydrate in the QDNB.

Section: Controls On the Spatial Coupling Between Mud Diapirs Gas mentioning

confidence: 92%

Section: Introductionmentioning

confidence: 99%

Distribution and characteristics of mud diapirs, gas chimneys, and bottom simulating reflectors associated with hydrocarbon migration and gas hydrate accumulation in the Qiongdongnan Basin, northern slope of the South China Sea

Zhang

Gong

et al. 2018

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“…However, the NSCSM had been convergent through the Mesozoic era under multi‐source stress fields due to closures of the Tethys and Paleo‐Pacific oceans among the Indo‐Australian, Eurasian, and Paleo‐Pacific plates. The NE‐trending dextral strike‐slip faults inherited and propagated from the Late Mesozoic faults with the same strike direction in South China, which mainly consist of a series of horse‐tail‐like secondary splay faults in plan view (Hui et al, , ; Li et al, ; Wang et al, ; Zhang et al, ). The Meso‐Cenozoic basins in the northern SCS continental margin are partially exposed to reveal a chessboard pattern in a NE‐oriented distribution due to the NE‐trending dextral strike‐slip faulting which dominate over the E–W‐ and NW‐trending faults (Li et al, ; Figure ).…”

Section: Geological Settingmentioning

confidence: 99%

Mechanisms of submarine canyon formation on the northern continental slope of the South China Sea

Hui

Guo

et al. 2018

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Submarine canyons are common features on the northern South China Sea (SCS) continental slope. Three main submarine canyon systems were observed by multi‐beam investigation and evaluated by a pre‐existing sedimentary dataset. They include the Xisha Canyon Zone, Shenhu Canyon Zone, and Taiwan Canyon Zone on the northern SCS continental slope which developed from the middle Miocene to the present‐day. Previous analysis implied that an enriched oil‐gas accumulation formed under this region with distinct gravity anomalies or active tectonic activity. Previous interpretation and geodynamics studies have revealed the detailed sedimentary records and the coupled tectonic‐sedimentary processes. However, the timing and mechanism of formation of the canyons on the northern SCS slope are still poorly constrained and vigorously debated, especially the differences among three submarine canyons from west to east that requires allowances for a variety of conflicting datasets. Here, we adopt the high‐resolution multi‐beam relief map and seismic profiles from these three canyon systems to estimate if the SCS slope controlled geometric segmentation and differentiation of canyon formation mechanisms and times. For this purpose, we ascertained that these canyons were initially controlled by the NE‐ to NW‐trending faults as a better constraint on the sediment flow direction. All the submarine canyons in the northern SCS slope extend in a range from NE‐/N–S‐ to NW‐ and E–W‐trending directions associated with the SCS oceanic spreading and the subsequent subduction‐related deformation along the Manila Trench. Furthermore, the present‐day morphology of these canyons implies a remarkable tectonic jump from west to east in the SCS continental slope and a north‐westward recession of the continental slope. Apart from the neo‐tectonic faults, the sea‐level fluctuation, gas hydrate disassociation, sediment source, and transportation channel also yielded dynamic processes and preservation for the submarine canyons. We finally propose a coupling model of the sedimentary and tectonic processes and confirm the spatial and temporal evolution of mechanisms of these canyons.

“…Because the SCS ceased spreading at 15 Ma, the northern continental margin of the SCS became a typical passive continental margin Zhang et al, 2013;Hui et al, 2016). Moreover, the gradual indentation of the Philippines Sea Plate to the Eurasian Plate along the Luzon Arc and the Taiwan Orogenic Belt during the 10-5 Ma interval Y.F.…”

Section: Collision Indentation Escaping Tectonics and Related Volcamentioning

confidence: 99%

Temporal and spatial distribution of Cenozoic igneous rocks in the South China Sea and its adjacent regions: implications for tectono‐magmatic evolution

Hui

et al. 2016

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The ocean–continent transition/conjunction zone is a mysterious and significant region in global tectonics and geodynamic evolution. The South China Sea (SCS) is one of the largest marginal basins in the Western Pacific with significant Cenozoic magmatic activities. The relationship between the tectono‐magmatic evolution and the tectonic setting of these Cenozoic igneous rocks is still controversial. In this study, the interpretation of gravity data, magnetic observation and seismic profiles is combined to identify the Cenozoic igneous rocks in the SCS and adjacent regions, and more than 70 magmatic activities have been discerned. We also show how the tectonic movements control the lithology and distribution of these magmatic activities. These Cenozoic magmatic rocks are related to the pre‐spreading, syn‐spreading and post‐spreading of the SCS. We suggest a geodynamic evolution of the SCS according to the temporal and spatial distribution of the igneous rocks. (1) An early period of bimodal volcanism (>32 Ma ago) mainly happened in the Paleogene basins at the northern margin of the SCS. This earlier volcanism happened under a trans‐extensional regime, which resulted from the collision between the Indian and the Eurasian plates since the early Cenozoic. The magmatic rocks may be related to the NNE‐trending right‐stepping dextral strike‐slip faults and the syn‐kinematic pull‐apart basins. These igneous rocks are mostly NNE trending. It is consistent with the Cenozoic dextral trans‐extensional tectonic setting in the East Asian Continental Margin. (2) Syn‐spreading igneous rocks first formed in the Northwest Sub‐basin of the SCS, and then later in the Central and Southwest sub‐basins. The mechanism of the magmatic emplacement results from the NNE‐trending right‐stepping dextral strike‐slip pull‐apart tectonic activity. (3) The spatial alignment of the Quaternary post‐spreading basalts is nearly E–W‐trending, and the different magmatic zones are segmented by NNE‐trending strike‐slip faulting. These volcanics are under an extrusion regime, which resulted from the arc–continent collision between the Luzon–Taiwan Arc and the Eurasian Plate due to the continuously WNW‐directed indentation of the Philippine Sea Plate during the period 9–6 Ma ago. In addition, the nascent NNE trending and dextral Changle–Nanao Fault may have played a significant role in the southward extrusion of the Taiwan Island Arc and the southeast SCS, resulting in an E–W‐trending distribution or strike of the volcanics and normal faults. In Indochina and the Malay Peninsula, widespread Cenozoic extensional post‐spreading granitic plutons are possibly derived from the mantle plume. Magnetic anomaly interpretation and tectonic analysis on these mafic rocks suggest that they were controlled by the Ailaoshan Red River left‐lateral ductile strike‐slip fault zone. Copyright © 2016 John Wiley & Sons, Ltd.