By the end of 2019, more than 220 gas fields had been discovered in the South China Sea. In order to accurately determine the geological characteristics of the large- and medium-sized gas fields in the South China Sea, this study conducted a comprehensive examination of the gas fields. Based on the abundant available geologic and geochemical data, the distribution and key controlling factors of the hydrocarbon accumulation in the South China Sea were analyzed. The geological and geochemical features of the gas fields were as follows: (1) the gas fields were distributed similar to beads in the shape of a “C” along the northern, western, and southern continental margins; (2) the natural gas in the region was determined to be composed of higher amounts of alkane gas and less CO₂; (3) the majority of the alkane gas was observed to be coal-type gas; (4) the gas reservoir types included structural reservoirs, lithologic reservoirs, and stratigraphic reservoirs, respectively; (5) the reservoir ages were mainly Oligocene, Miocene, and Pliocene, while the lithology was mainly organic reef, with some sandstone deposits; and (6) the main hydrocarbon accumulation period for the region was determined to be the late Pliocene-Quaternary Period. In addition, the main controlling factors of the gas reservoirs were confirmed to have been the development of coal measures, sufficient thermal evolution, and favorable migration and accumulation conditions.

The pre-Cenozoic northern South China Sea (SCS) Basin basement was supposed to exist as a complex of heterogeneous segments, divided by dozens of N−S faulting. Unfortunately, only the Hainan Island and the northeastern SCS region were modestly dated while the extensive basement remains roughly postulated by limited geophysical data. This study presents a systematic analysis including U-Pb geochronology, elemental geochemistry and petrographic identification on granite and meta-clastic borehole samples from several key areas. Constrained from gravity-magnetic joint inversion, this interpretation will be of great significance revealing the tectono-magmatic evolution along the southeastern margin of the Eurasian Plate. Beneath the thick Cenozoic sediments, the northern SCS is composed of a uniform Mesozoic basement while the Precambrian rocks are only constricted along the Red River Fault Zone. Further eastern part of the northern SCS below the Cenozoic succession was widely intruded by granites with Jurassic-to-early Cretaceous ages. Further western part, on the other hand, is represented by meta-sedimentary rocks with relatively sporadic granite complexes. To be noted, the western areas derived higher-degree and wider metamorphic zones, which is in contrast with the lower-degree and narrower metamorphic belt developed in the eastern region. Drastic collisions between the Indochina Block and South China continent took place since at least late Triassic, resulting in large-scale suturing and deformation zones. At the westernmost part of the northern SCS, the intracontinental amalgamation with closure of the Meso-Tethys has caused fairly stronger and broader metamorphism. One metamorphic biotite granite is located on the suturing belt and yields a Precambrian U-Pb age. It likely represents the relict from the ancient Gondwana supercontinent or its fringes. Arc-continental collision between the Paleo-Pacific and the southeast China Block, on the other hand, results in a relatively narrow NE–SW trending metamorphic belt during the late Mesozoic. Within the overall geological setting, the Cenozoic SCS oceanic basin was subsequently generated from a series of rifting and faulting processes along the collisional-accretionary continental margin.

Numerous elongated mounds and channels were found at the top of the middle Miocene strata using 2D/3D seismic data in the Liwan Sag of Zhujiang River Mouth Basin (ZRMB) and the Beijiao Sag of Qiongdongnan Basin (QDNB). They occur at intervals and are rarely revealed by drilling wells in the deepwater areas. Origins of the mounds and channels are controversial and poorly understood. Based on an integrated analysis of the seismic attribute, palaeotectonics and palaeogeography, and drilling well encountering a mound, research results show that these mounds are dominantly distributed on the depression centres and/or slopes of the Liwan and Beijiao sags and developed in a bathyal sedimentary environment. In the Liwan and Beijiao sags, the mounds between channels (sub) parallel to one another are 1.0–1.5 km and 1.5–2.0 km wide, 150–300 m and 150–200 m high, and extend straightly from west to east for 5–15 km and 8–20 km, respectively. Mounds and channels in the Liwan Sag are parallel with the regional slope. Mounds and channels in the Beijiao Sag, however, are at a small angle to the regional slope. According to internal geometry, texture and external morphology of mounds, the mounds in Beijiao Sag are divided into weak amplitude parallel reflections (mound type I), blank or chaotic reflections (mound type II), and internal mounded reflections (mound type III). The mounds in Liwan Sag, however, have the sole type, i.e., mound type I. Mound type I originates from the incision of bottom currents and/or gravity flows. Mound type II results from gravity-driven sediments such as turbidite. Mound type III is a result of deposition and incision of bottom currents simultaneously. The channels with high amplitude between mounds in the Beijiao and Liwan sags are a result of gravity-flow sediments and it is suggested they are filled by sandstone. Whereas channels with low-mediate amplitudes are filled by bottom-current sediments only in the Beijiao Sag, where they are dominantly composed of mudstone. This study provides new insights into the origins of the mounds and channels worldwide.

Taiwan Island’s outcropping strata can provide important insights into the sedimentary environment and source development of the southeast China margin. This research is based on the Eocene–Miocene strata of the Tsukeng area in the central Western Foothills, northeast shoreline of Taiwan Island and two sites of the East China Sea Shelf Basin (ECSSB), using petrology and detrital zircon U-Pb age for the analysis. Results show that central and northeast Taiwan Island experienced a transformation from continental to marine facies during the Eocene–Miocene, and the sandstone maturity changed with time. Source analysis shows that sediments from the Eocene–early Oligocene strata mainly originated from near-source Mesozoic rocks, whose zircon age is consistent with the igneous rock in the surrounding area and coastal Cathaysia, showing 120 Ma and 230 Ma peaks in the age spectrum diagram. Since the late Oligocene, peaks of 900 Ma and 1 800 Ma are seen, indicating that deposition of matter from the old block began. The sediments could be a mixture of the surrounding Mesozoic volcanic and fewer pre-Cambrian rocks sourced from the coastal river and sporadic old basement in the ECSSB instead of long-distance transportation.

The synsedimentary faults and basin-marginal fans located in the central part of the deep-water area of the early Oligocene Qiongdongnan Basin have been investigated using seismic profiles, boreholes, and well-log data. Through the formations of the characterized paleogeomorphology, such as transverse anticlines, fault ditches, and step-fault belts, the synsedimentary faults are known to have controlled the development position, distribution direction, and extension scales of the basin-marginal fans. For example, at the pitching ends of two adjacent faults, transverse anticlines developed, which controlled the development positions and distributions of the fans. During the early Oligocene, the faults controlled the subsidence center, and fault ditches were formed at the roots of the faults. In the surrounding salient or low salient areas, which were exposed as provenance areas during early Oligocene, the fault ditches acted as the source channels and determined the flow paths of the clastics, where incised valley fills were obviously developed. The fault ditches which developed in the sedimentary basins were able to capture the drainage systems and influenced the distributions of the fans. The large boundary faults and the secondary faults generated two fault terraces and formed step-fault belts. The first fault terrace caused the clastics to be unloaded. As a result, fans were formed at the entrance to the basin. Then, the second fault terrace caused the fans to move forward, with the fans developing in a larger extension scale. The results obtained in this study will potentially be beneficial in the future prospecting activities for reservoirs and coal-measure source rocks in the basins located in the deep-water areas of the South China Sea.

In this study, element geochemistry and zircon chronology are used to analyze the Oligocene sediments in the Baiyun Sag, Zhujiang River Mouth Basin. The experimental results are discussed with respect to weathering conditions, parent rock lithologies, and provenances. The chemical index of alteration and the chemical index of weathering values of mudstone samples from the lower Oligocene Enping Formation indicate that clastic particles in the study area underwent moderate weathering. Mudstone samples exhibit relatively enriched light rare earth elements and depleted heavy rare earth elements, “V”-shaped negative Eu anomalies, and negligible Ce anomalies. The rare earth element distribution curves are obviously right-inclined, with shapes and contents similar to those of post-Archean Australian shale and upper continental crust, indicating that the samples originated from acid rocks in the upper crust. The Hf-La/Th and La/Sc-Co/Th diagrams show this same origin for the sediments in the study area. For the samples from the upper Enping deltas, the overall age spectrum shows four major age peaks ca. 59–68 Ma, 98–136 Ma, 153–168 Ma and 239–260 Ma. For the Zhuhai Formation samples, the overall age spectrum shows three major age peaks ca. 149 Ma, 252 Ma and 380 Ma. The detrital zircon shapes and U-Pb ages reveal that during Oligocene sedimentation, the sediments on the northwestern margin of the Baiyun Sag were supplied jointly from two provenances: Precambrian-Paleozoic metamorphic rocks in the extrabasinal South China fold zone and Mesozoic volcanic rocks in the intrabasinal Panyu Low Uplift, and the former supply became stronger through time. Thus, the provenance of the Oligocene deltas experienced a transition from an early proximal intrabasinal source to a late distal extrabasinal source.

Previous studies of gas hydrate in the Dongsha area mainly focused on the deep-seated gas hydrates that have a high energy potential, but cared little about the shallow gas hydrates occurrences. Shallow gas hydrates have been confirmed by drill cores at three sites (GMGS2 08, GMGS2 09 and GMGS2 16) during the GMGS2 cruise, which occur as veins, blocky nodules or massive layers, at 8–30 m below the seafloor. Gas chimneys and faults observed on the seismic sections are the two main fluid migration pathways. The deep-seated gas hydrate and the shallow hydrate-bearing sediments are two main seals for the migrating gas. The occurrences of shallow gas hydrates are mainly controlled by the migration of fluid along shallow faults and the presence of deep-seated gas hydrates. Active gas leakage is taking place at a relatively high-flux state through the vent structures identified on the geophysical data at the seafloor, although without resulting in gas plumes easily detectable by acoustic methods. The presence of strong reflections on the high-resolution seismic profiles and dim or chaotic layers in the sub-bottom profiles are most likely good indicators of shallow gas hydrates in the Dongsha area. Active cold seeps, indicated by either gas plume or seepage vent, can also be used as indicators for neighboring shallow gas hydrates and the gas hydrate system that is highly dynamic in the Dongsha area.

An investigation of equatorial near-inertial wave dynamics under complete Coriolis parameters is performed in this paper. Starting from the basic model equations of oceanic motions, a Korteweg de Vries equation is derived to simulate the evolution of equatorial nonlinear near-inertial waves by using methods of scaling analysis and perturbation expansions under the equatorial beta plane approximation. Theoretical dynamic analysis is finished based on the obtained Korteweg de Vries equation, and the results show that the horizontal component of Coriolis parameters is of great importance to the propagation of equatorial nonlinear near-inertial solitary waves by modifying its dispersion relation and by interacting with the basic background flow.

The present work describes the basic features of super typhoon Meranti (2016) by multiple data sources. We mainly focus on the upper ocean response to Meranti using multiplatform satellites, in situ surface drifter and Argo floats, and compare the results with the widely used idealized wind vortex model and reanalysis datasets. The pre-existing meso-scale eddy provided a favor underlying surface boundary condition and also modulated the upper ocean response to Meranti. Results show that the maximum sea surface cooling was 2.0°C after Meranti. The satellite surface wind failed to capture the core structure of Meranti as the idealized wind vortex model deduced. According to the observation of sea surface drifters, the near-inertial currents were significantly enhanced during the passage of Meranti. The temperature and salinity profiles from Argo floats revealed both the mixed-layer extension and subsurface upwelling induced by Meranti. The comparison results show that the sea surface temperature and surface wind in the reanalysis datasets differs from those in remote sensing system. Sea surface cooling is similar in both satellite and in situ observation, and sea surface salinity response has a lower correlation with the precipitation rate.