Marine Geological Survey Research Articles

Spatial Distribution of Physical and Chemical Properties of Deep Sea Water in Xisha, South China Sea

Deep sea water (DSW) is a globally utilized source of renewable energy and other resources. To understand the characteristics of DSW resources in the South China Sea, in July 2022, the Guangzhou Marine Geological Survey (GMGS) investigated temperature, salinity, pH, dissolved oxygen (DO), inorganic salts (DIN, PO43−-P, and SiO3-Si), heavy metals (Hg, Pb, As, and Cd), trace elements (Cu, Zn, Fe, Mn, Ni, Se, and Mo), and other related indicators. The results of this investigation elucidate the horizontal and vertical changes in the physical and chemical properties of deep sea water in the Xisha Sea. The surface seawater quality in Xisha was found to be excellent and to meet first-class seawater survey standards. However, the concentrations of various nutrient salts in the surface layer were relatively low. As the seawater depth increased, different trace elements and heavy metals exhibited variations, and the concentrations of various nutrients also gradually increased.

Representative Dynamic Accumulation of Hydrate-Bearing Sediments in Gas Chimney System since 30 Kyr BP in the QiongDongNan Area, Northern South China Sea

A stratigraphic complex composed of mass transport deposits (MTDs), where the gas occurrence allows for the formation of a gas chimney and pipe structure, is identified based on seismic interpretation in the QiongDongNan area of the northern South China Sea. During the Fifth Gas Hydrate Drilling Expedition of the Guangzhou Marine Geological Survey, this type of complex morphology that has close interaction with local gas hydrate (GH) distribution was eventually confirmed. A flow-reaction model is built to explore the spatial–temporal matching evolution process of massive GH reservoirs since 30 kyr before the present (BP). Five time snapshots, including 30, 20, 10, and 5 kyr BP, as well as the present, have been selected to exhibit key strata-evolving information. The results of in situ tensile estimation imply fracturing emergence occurs mostly at 5 kyr BP. Six other environmental scenarios and three cases of paleo-hydrate existence have been compared. The results almost coincide with field GH distribution below the bottom MTD from drilling reports, and state layer fracturing behaviors always feed and probably propagate in shallow sediments. It can be concluded that this complex system with 10% pre-existing hydrates results in the exact distribution and occurrence in local fine-grained silty clay layers adjacent to upper MTDs.

A Flat‐Lying Transitional Free Gas to Gas Hydrate System in a Sand Layer in the Qiongdongnan Basin of the South China Sea

AbstractMost marine gas hydrate systems follow a vertical pattern with hydrate overlying free gas. Here we document the discovery of a gas to hydrate system in a horizontal sand layer in the Qiongdongnan Basin of the South China Sea. Eight wells were drilled by the Guangzhou Marine Geological Survey in 2021–2022 to investigate the occurrence and mechanisms responsible for the formation of the system. We describe a free gas‐bearing sand reservoir at the center of the system sustained by advecting hot fluids and gas; away from the advecting zone, the cooler, surrounding sand reservoir is filled with hydrate. Observations at this site show that advective heat has a large control on hydrate formation in sands and may be a key mechanism which allows gas migration within the hydrate stability zone and the formation of high‐saturation hydrate in sand layers.

Analysis of the Mechanical Characteristics of Tunnels Under the Coupling Effect of Submarine Active Faults and Ground Vibrations

With the rapid development of modern transportation construction, the construction of cross-harbor tunnels has solved the problem of traffic connection between cross-straits, bays and islands. The construction of sub-sea tunnels has technical difficulties such as high difficulty of marine geological survey, close hydraulic connection between strata and seawater, and more developed adverse geology. Based on this, this paper studies the mechanical characteristics of the submarine tunnel under seismic action at the active fault. Firstly, the mechanical model of the universal fault interface is established, and the calculation model of the fault interface is theoretically derived by the method of vibration mechanics, and the influence of the change of the strength of the contact surface and the stiffness of the surrounding rock on both sides of the fault on the transfer coefficient is obtained. Secondly, based on the ground motion input method of two-dimensional homogeneous half-space field, the relevant program of viscoelastic artificial boundary ground motion input is written by MATLAB program, which lays the foundation and premise of load input for mechanical response calculation. Finally, the outcomes of the tunnel parameters and the interplay between the tunnel and the surrounding rock on the cracking of the tunnel lining shape and the mechanical response of the cross-fault sub-sea tunnel underneath seismic motion are mentioned, and it is concluded that the increase in seismic intensity for different seismic intensities under the sea floor has an essentially constant proportion to the increase in acceleration of the seismic response; the seepage effect under the sea floor for the tunnel lining structure reduces the seismic response displacement, velocity and The seabed seepage for the tunnel lining structure reduces the peak seismic response displacement, velocity and acceleration by about 20–35%.

Application of the Navigational Air-Sea Methane Exchange Flux Observation System in the Qiongdongnan Basin of the Northern South China Sea

The sources and sinks of dissolved CH4 in offshore waters are becoming diversified with the rapid increase in human activities. The concentration and air–sea exchange flux of dissolved CH4 present new characteristics of more intense spatiotemporal evolution, and the contribution to atmospheric CH4 continues to increase. Herein, a new model based on navigable air–sea exchange flux observations was proposed, which replaced the traditional station-based sampling analysis and testing method, realizing the synchronous measurement of methane in the atmosphere and surface seawater carried by ships. Based on the Marine Geological Survey project of the China Geological Survey, comprehensive environmental surveys were conducted in April 2018, September 2018, and June 2019 in the Qiongdongnan area in the northern part of the South China Sea, and the dissolved methane content in the sea surface atmosphere and surface seawaters in 2019 were simultaneously obtained. The methane exchange flux ranges of the southeastern sea area were calculated as −0.001~−0.0023 μmol·m−2·d−1 and −0.00164~−0.00395 μmol·m−2·d−1 by using the Liss and Merlivat formula (LM86), the Wanninkhof formula (W92), and the field-measured wind speed. The feasibility of the navigational air–sea methane exchange flux observation system was proven in a sea trial, and the measurement accuracy and observation efficiency of air-sea flux were improved with the designed system, providing a new technical means for further research on multiscale air–sea interactions and global climate change.

Special issue on marine geology around Tokara Islands in Kagoshima Prefecture: result of marine geological mapping survey cruse in 2020

Special issue on marine geology around Tokara Islands in Kagoshima Prefecture: result of marine geological mapping survey cruse in 2020

Effects of Depositional Processes in Submarine Canyons and Distribution of Gas Chimneys on Gas Hydrate Accumulation in the Shenhu Sea Area, Northern South China Sea

Previous gas hydrate production tests conducted by the Guangzhou Marine Geological Survey (GSGM) in 2017 and 2020 indicated the great potential of gas hydrates in the Shenhu Sea area in the Pearl River Mouth Basin (PRMB), China. In this study, the effects of deposition processes in submarine canyons and the distribution of gas chimneys on gas hydrate accumulation were investigated using high-resolution two- dimensional (2D) and three-dimensional (3D) seismic data. Four intact submarine canyons were identified in the study area. Five deepwater depositional elements are closely related to submarine canyons: lateral accretion packages (LAPs), basal lags, slides, mass transport deposits (MTDs), and turbidity lobes. MTDs and lobes with multiple stages outside the distal canyon mouth reveal that the sedimentary evolution of the canyon was accompanied by frequent sediment gravity flows. Gas chimneys originating from Eocene strata are generally up to 3 km wide and distributed in a lumpy or banded pattern. The analysis of seismic attributes confirmed fluid activity in these gas chimneys. Gas hydrates are mainly distributed in ridges among different canyons. Based on the gas sources of gas hydrates and depositional evolution of submarine canyons, depositional processes of sediment gravity flows in submarine canyons and the distribution of gas chimneys significantly affect the accumulation of gas hydrates. Based on these findings, this study establishes a conceptional model for the accumulation of gas hydrate, which can provide guidance in the prediction for favorable gas hydrates zones in the area and nearby.

Underwater image restoration through regularization of coherent structures

Restoration of underwater images plays a vital role in underwater target detection and recognition, underwater robots, underwater rescue, sea organism monitoring, marine geological survey, and real-time navigation. Mostly, physics-based optimization methods do not incorporate structural differences between the guidance and transmission maps (TMs) which affect the performance. In this paper, we propose a method for underwater image restoration by utilizing a robust regularization of coherent structures. The proposed method incorporates the potential structural differences between TM and the guidance map. The optimization of TM is modeled through a nonconvex energy function which consists of data and smoothness terms. The initial TM is taken as a data term whereas the smoothness term contains static and dynamic structural priors. Finally, the optimization problem is solved using majorize-minimize (MM) algorithm. The proposed method is tested on benchmark dataset and its performance is compared with the state-of-the-art methods. The results from the experiments indicate that the proposed regularization scheme adequately improves the TM, which results in high-quality restored images.

Mechanical properties of methane hydrate-bearing sandy sediments under various temperatures and pore pressures

According to the marine geological survey, marine hydrate reservoir's temperature (T) and pore pressure (P.P) conditions change with water depth and burial depth. Besides, the change in T and P.P conditions would also occur during the exploitation of hydrate. It is meaningful to study the combined effects of T and P.P on mechanical behaviors of methane hydrate-bearing sediments (MHBSs) to evaluate the hydrate reservoir's stability. In this work, a set of mechanical experiments were carried out to study the effect of T and P.P on the mechanical properties of MHBSs. With decreasing T and increasing P.P, the samples' strength increased, and the stress-strain behaviors also changed. Moreover, with decreasing T and increasing P.P, the samples' dilatant volumetric strain increased. The mechanical properties of MHBSs were affected by the ‘distance’ between the test condition point and the phase equilibrium curve. With increasing ‘distance’, the sample s' strength increased; simultaneously, the stress-strain behaviors also changed. A shift line was introduced in temperature-pressure space, which was the boundary between the hardening and softening behaviors of the MHBSs. Furthermore, we established the numerical relation between strength, T and P.P in this paper. The results of this study can be used for the studies of reservoir stability during methane hydrate productions.

Geomechanical properties of gas hydrate-bearing sediments in Shenhu Area of the South China Sea

It is essential to understand the in-situ geomechanical properties of natural gas hydrate-bearing reservoirs for hydrate drilling, the design of reservoir stimulation and production schemes, geologic disaster prevention, and environmental impact assessment. In this way, it serves as the basis for the development and utilization of gas hydrate resources. In 2016, the Guangzhou Marine Geological Survey conducted the fourth natural gas hydrate drilling (Expedition GMGS4) in the north of the South China Sea, during which the effective stress of gas hydrate-bearing reservoirs in the study area was ascertained and gas hydrate-bearing pressure cores were collected. The effective stress was determined to be 700–800 kPa by in-situ piezocone penetration tests. The saturation, permeability, and shear strength of gas hydrates in the pressure core samples were measured under the in-situ stress state using the shipboard multi-sensor core logger (MSCL), pressure core analysis and transfer system (PCATS), and PCATS Triaxial system. Based on this, the distribution characteristics and the relationship of the permeability and gas hydrate saturation of the gas hydrate-bearing reservoirs were explored. Furthermore, the effects of effective stress and gas hydrate saturation on shear strength of gas hydrate-bearing clayey-silt sediments were mainly analyzed, as well as the stress–strain characteristics of gas hydrate-bearing sediments. The results are as follows: (i) the gas hydrate saturation and reservoir permeability in the study area are 0.7%–55% and 10−3–10−1mD, respectively, indicating low-permeability reservoirs; (ii) the gas hydrate saturation and reservoir permeability have obvious anisotropy in horizontal and vertical directions; (iii) the undrained shear strength of gas hydrate-bearing sediments is 0.24–0.52 MPa, and the shear strength of fine-grained sediments is more sensitive to the changes of in-situ stress. In the case that deviatoric stress continues increasing, micro-fractures may occur inside gas hydrate-bearing sediments, and this can lead to drastic changes in the strength of gas hydrate-bearing reservoirs. Therefore, it is necessary to take into account the effects of micro-fractures in gas hydrate-bearing reservoirs on engineering safety during long-term hydrate production.

RETRACTED ARTICLE: Application of GIS real-time monitoring system in the ecological impact of marine tourism industry development and marine resource protection

With the development of society and the continuous improvement of people’s living standards, people are more eager to explore the mysterious ocean zone, which also makes the focus of tourism development in the world gradually shift to marine tourism. This phenomenon undoubtedly promotes people’s economic development, but at the same time, because of human’s too frequent activities and neglect of the protection of the marine environment, marine pollution has become more and more serious, especially the pollution caused by oil spill from ships. This situation not only destroys the ecological environment in the ocean, leads to the reduction of marine biological species, and affects the ornamental value of the marine environment but also has a great impact on the future development prospects of marine tourism. During the research of GIS marine geological survey information model, we should start from the overview of information model, unified modeling language and information modeling, research of marine geological survey information model, and analysis of system overall design to better complete the development of resources. If marine tourism wants to get a long-term development, it cannot be separated from marine tourism resources. Therefore, only by mobilizing them to strengthen the implementation of policies on marine resources, countries should increase support for the prevention and control technology organizations of marine pollution, so as to maintain the marine ecological environment and so as to realize the long-term development of marine tourism.

Mixed gas sources induced co-existence of sI and sII gas hydrates in the Qiongdongnan Basin, South China Sea

Crystal structure and gas source are two important aspects in studying natural gas hydrate systems. In 2018, Guangzhou Marine Geological Survey carried out its fifth natural gas hydrate drilling expedition (GMGS5) in the Qiongdongnan basin of the South China Sea. In this paper, analyses of gas composition, stable carbon isotopes and Raman spectroscopy were conducted on twelve gas hydrate samples recovered at drill site W08, at depths of 17–170 m below seafloor. The results show that the main component of the hydrate-bound gas is methane (72.5–97.5%), mixed with other hydrocarbon gases (ethane: 1.9–12.1%, propane: 0.1–11.3%, i-butane: 0.0–3.5%, n-butane: 0.0–0.2% and C5+: 0.0–0.2%). The δ13C of methane, ethane and propane in the hydrate-bound gases are −63.6 ~ −51.9‰, −29.2 ~ −26.6‰ and −25.6 ~ −23.7‰, respectively. The hydrate-bound gas is inferred to be a mixture of microbial and thermogenic gases. Raman spectroscopic analysis indicates that structure I and structure II gas hydrates co-exist in the samples. Structure II gas hydrate is increasingly being discovered in the South China Sea, and it is suggested that natural gas hydrate resources might be underestimated and need to be re-evaluated.

Numerical Analyses on Controlling Factors for Hydrate Differential Accumulation in the Shenhu Drilling Area, South China Sea

In 2007, the first marine gas hydrate drilling expedition of the Guangzhou Marine Geological Survey (GMGS-1) was carried out in the Shenhu area on the northern continental slope of the South China Sea, collecting geophysical and drilling data from eight different sites. No hydrate sample was recovered at site SH5, even though it locates close to site SH2 where the hydrate was drilled. The reason for the different distribution of hydrate is not thoroughly understood. On the basis of the actual exploration data, the key factors controlling hydrate formation in the Shenhu area were analyzed by employing the numerical modeling. To evaluate the key controlling factors among gas flux, water depth, temperature gradient, and cap rock location, sensitive studies were conducted. The results show that the gas flux is the prerequisite factor determining the hydrate quantity. The seawater depth does not affect the hydrate formation significantly. The temperature gradient plays an important role in hydrate quantity and saturation. The cap rock location controls the gas migration to the gas hydrate stability zone (GHSZ). According to the sensitivity analyses and drilling data, we proposed a mechanism of hydrate differential accumulation in the Shenhu area. The relatively high geothermal gradient and the cap rock located below the GHSZ cause the absence of hydrate at site SH5, leading to the differential accumulation.

Characteristics of gravity and magnetic fields and deep structural responses in the southern part of the Kyushu-Palau Ridge

The southern part of the Kyushu-Palau Ridge (KPR) is located at the conjunction of the West Philippine Basin, the Parece Vela Basin, the Palau Basin, and the Caroline Basin. This area has extremely complex structures and is critical for the research on the tectonic evolution of marginal seas in the Western Pacific Ocean. However, only few studies have been completed on the southern part, and the geophysical fields and deep structures in this part are not well understood. Given this, this study finely depicts the characteristics of the gravity and magnetic anomalies and extracts information on deep structures in the southern part of the KPR based on the gravity and magnetic data obtained from the 11th expedition of the deep-sea geological survey of the Western Pacific Ocean conducted by the Guangzhou Marine Geological Survey, China Geological Survey using the R/V Haiyangdizhi 6. Furthermore, with the data collected on the water depth, sediment thickness, and multichannel seismic transects as constraints, a 3D density model and Moho depths of the study area were obtained using 3D density inversion. The results are as follows. (1) The gravity and magnetic anomalies in the study area show distinct zoning and segmentation. In detail, the gravity and magnetic anomalies to the south of 11°N of the KPR transition from high-amplitude continuous linear positive anomalies into low-amplitude intermittent linear positive anomalies. In contrast, the gravity and magnetic anomalies to the north of 11°N of the KPR are discontinuous and show alternating positive and negative anomalies. These anomalies can be divided into four sections, of which the separation points correspond well to the locations of deep faults, thus, revealing different field-source attributes and tectonic genesis of the KPR. (2) The Moho depth in the basins in the study area is 6–12 km. The Moho depth in the southern part of KPR show segmentation. Specifically, the depth is 10–12 km to the north of 11°N, 12–14 km from 9.5°N to 11°N, 14–16 km from 8.5°N to 9.5°N, and 16–25 km in the Palau Islands. (3) The KPR is a remnant intra-oceanic arc with the oceanic-crust basement.which shows noticeably discontinuous from north to south in geological structure and is intersected by NEE-trending lithospheric-scale deep faults. With large and deep faults F3 and F1 (the Mindanao fault) as boundaries overall, the southern part of the KPR can be divided into three zones. In detail, the portion to the south of 8.5°N (F3) is a tectonically active zone, the KPR portion between 8.5°N and 11°N is a tectonically active transition zone, and the portion to the north of 11°N is a tectonically inactive zone. (4) The oceanic crust in the KPR is slightly thicker than that in the basins on both sides of the ridge, and it is inferred that the KPR formed from the thickening of the oceanic crust induced by the upwelling of deep magma in the process of rifting of remnant arcs during the Middle Oligocene. In addition, it is inferred that the thick oceanic crust under the Palau Islands is related to the constant upwelling of deep magma induced by the continuous northwestward subduction of the Caroline Plate toward the Palau Trench since the Late Oligocene. This study provides a scientific basis for systematically understanding the crustal attributes, deep structures, and evolution of the KPR.©2021 China Geology Editorial Office.

Grain-size characteristics of fine-grained sediments and association with gas hydrate saturation in Shenhu Area, northern South China Sea

Fine-grained gas hydrate (GH) reservoirs are extensively studied worldwide, among which the Shenhu Area (located on the northern slope of the South China Sea) is a world-class GH exploration area. However, the lithology, physical properties, and depositional origins of the fine-grained GH reservoirs are not well known. Understanding how sediment grain-size parameters affect the fine-grained GH reservoir quality could provide and important breakthrough for reservoir evaluation. Eight cores, recovered from various expeditions of the Guangzhou Marine Geological Survey, can be combined with 2D/3D seismic data to provide a rare opportunity to systematically investigate the grain-size characteristics of the GH reservoir, as well as the surrounding sediments.A combination of lithology, grain size characteristics (mean size, sorting, skewness, kurtosis), high-resolution seismic features, and associated bivariate and cluster analysis results support the identification of two distinct intervals of fine-grained sediments that were deposited by different sedimentary processes. There is a relatively higher content of coarser silt in the lower interval than in the upper interval, and their boundary depths are highly consistent with those of the GH-bearing layer and the overlying non-GH-bearing layer. With respect to the unconsolidated GH-bearing sediments from Well G, both the porosity (52%–64%) and sorting coefficient (1.68–2.2) have limited variation, while high GH saturation (>30%) occurs at the top layer. The positive correlation between saturation and the coarsest one-percentile grain size (R = 0.55) reveals that an increase in the coarse fraction/particle size favours the development of a larger pore throat diameter and improves the initial permeability and reservoir properties.The seismic features and cross-plots of the coarsest one-percentile and median values indicate that the lower thin-bedded fine-grained sediments with hydrate may be fine-grained turbidite complexes, including channels/levees/lobes and mass transport deposits. If this is the case, then it may be inferred that turbidite sediments provide good reservoir physical properties, favourable for GH formation and accumulation. These insights into the relationship between the Quaternary fine-grained turbidites and GH saturation may promote a clearer understanding of the characteristics and development of fine-grained GH reservoirs globally, including in the Shenhu Area of the South China Sea.

Monitoring and Early Warning Technology of Hydrate-induced Submarine Disasters

With the depletion of fossil energy sources, such as coal and petroleum, natural gas hydrate has been proposed as an alternative energy source. However, the synthesis of natural gas hydrate may cause marine engineering disasters, such as submarine cable breaks and the collapse of sub-sea oil rigs, and even submarine earthquakes, triggering tsunamis and seriously endangering human safety. Hydrate test production monitoring in various countries focuses on the temperature, pressure, and methane concentration near test production wells. Effective observation equipment and methods for the monitoring and early warning of submarine geological hazards are lacking. This paper reviewed the possible geological hazards caused by deep seawater hydrate mining and the monitoring and early warning technologies for deep seawater hydrate mining in various countries and introduced the in-situ Surveying Equipment of Engineering Geology in Complex Deep Sea (SEEGeo). The equipment uses 3D resistivity measurement and data inversion technology, seabed sediment acoustic measurement and data inversion technology, seabed sediment pore water pressure measurement and data inversion technology, and renewable energy seawater-dissolved oxygen battery power supply technology. SEEGeo can realize the in situ, real-time, long-term, effective monitoring and early warning of geological disasters induced by hydrate development. At present, the equipment has conducted two sea trial experiments on the continental slope of the South China Sea, equipped with the “Marine Geology No. 6” scientific research vessel of the Guangzhou Marine Geological Survey and the “Dongfanghong No.3” scientific research vessel of the Ocean University of China, and obtained in situ engineering geological data of the South China Sea.

Characteristics of gas chimneys and their implications on gas hydrate accumulation in the Shenhu area, northern south China sea

Gas hydrates have captivated extensive concentration from scholars all over the world as potential alternative energy sources. In 2017, the Guangzhou Marine Geological Survey (GMGS) carried out a successful gas hydrate production test in the Shenhu area, which was of considerable significance to the study of gas hydrates in China. Several studies describing gas chimneys as the primary vertical gas migration pathways in the Shenhu area were mainly based on two-dimensional (2D) seismic data. In this research, on the basis of high-resolution three-dimensional (3D) seismic data newly acquired in 2018, combined with well logging data acquired during the 2007 and 2015 expeditions, the contribution of gas chimneys to gas hydrate accumulation was determined. As a result, four types of gas chimneys with various top shapes were identified, and their internal seismic reflection characteristics were predominantly chaotic, acoustic blanking, and high-amplitude reflection from bottom to top. Moreover, thick hydrate layers were discovered above some of the gas chimneys. Further analyses on the hydrocarbon generation and tectono-sedimentary evolution in the Baiyun Sag indicated the evolution of gas chimneys could be divided into five stages. Furthermore, as the primary vertical gas migration pathways in the Shenhu area, gas chimneys could be sourced from the Wenchang formation and Enping formation and could pierce into the Quaternary gas hydrate reservoirs. This resulted in deep thermogenic gas directly migrating upward into the gas hydrate stability zone (GHSZ) and accumulating into thick hydrate layers on or above the top of gas chimneys.

Fine-grained gas hydrate reservoir properties estimated from well logs and lab measurements at the Shenhu gas hydrate production test site, the northern slope of the South China sea

Gas hydrate reservoir properties provide critical information on the controlling mechanisms of gas hydrate formation and accumulation in natural depositional environments. They also are essential to understanding and accurately predicting gas production characteristics. However, the properties of fine-grained reservoirs that host hydrates are poorly studied.Guangzhou Marine Geological Survey acquired a comprehensive set of logging-while-drilling (LWD) logs, in situ tests, and lab measurements from 2015 to 2018 in the W11-17 fine-grained, gas hydrate reservoir of the Shenhu on the South China Sea. Porosity, hydrate saturation, and gas saturation were derived from neutron porosity, nuclear magnetic resonance (NMR), and electrical resistivity logs of SH-W17 and SHSC-4J1 wells and compared with lab measurements of pressurized cores. Units A to C contain three hydrate-concentrated intervals, with a maximum thickness of 32 m and an average hydrate saturation of 0.32 ±0.05. The hydrate saturation was estimated using resistivity and NMR models. Units D and E were inferred to be approximately 20 m thick, free-gas layers containing gas hydrates. Compared to hydrate saturation, the estimations of gas saturation from resistivity and NMR models are less reliable with a range of 0.05–0.4 due to the coexistence of gas hydrates and free gas.Permeabilities estimated from the NMR log agree well with those from neuron porosity in non-hydrate bearing intervals but are slightly higher in hydrate-bearing intervals. By incorporating the lab measurements and in situ tests into the NMR pore-size analysis, the permeability of water-bearing sediments in the hydrate intervals in the SHSC-4J1 well can be constrained to the range of 0.002–0.1 md, with 0.015 md being our best estimate. The NMR pore-size geometries indicate gas hydrates appear to preferentially fill bigger pores within fine-grained reservoirs, which exhibit a similar behavior to coarse-grained reservoirs. Our resistivity and relative permeability modeling indicate that the growth of gas hydrates within pore spaces is characterized by pore-filling and cementation behaviors.

Sill-related seafloor domes in the Zhongjiannan Basin, western South China Sea

Magmatism can exert significant impact on sedimentary basins such as the Zhongjiannan Basin (ZJNB), western South China Sea. We have evaluated multibeam bathymetric and multichannel seismic reflection data acquired by the Guangzhou Marine Geological Survey in recent years, in order to investigate the distribution, the characteristics and the subsurface structures related to seafloor domes found in the northeastern ZJNB. Our data revealed forty-two domes at water depths between 2312 m and 2870 m, which are clustered around volcanic mounds, large seamounts and along the edge of the central depression in the study area. These domes are generally circular to elongate or irregular in plan view with large basal areas, and they also have gentler flanks (dips of 1.46°–7.73°) with vertical reliefs ranging from tens to hundreds of meters. In seismic sections, majority of the domes are underlain by variably shaped and complex magmatic sills, which provide a cause-effect relationship between domes formation and igneous intrusions. These intrusions heat surrounding organic-rich sediments, release hydrocarbons, fluidize sediment pore waters and form gas-rich fluids, which fill in sediment and uplift overlying strata immediately above the sills to form forced folds, which are manifested as seafloor domes. These sill-folds-dome structures have important implications for understanding geomorphologic features caused by sills emplaced at depth.

New sedimentological and coastal and marine geological data on the Quaternary marine deposits of the Ischia Island (Gulf of Naples, Southern Tyrrhenian Sea, Italy)

Quaternary marine deposits of the Ischia Island have been analyzed based on marine geological survey and sedimentological data. The Ischia Island represents the emerged part of a large volcanic field, extending from the Procida island to the submerged volcanoes of the western Ischia offshore. The volcanic field develops over an area of about 42 km2, E-W trending, composed of calderas, such as the Ischia caldera, linked to the great explosive eruptions, plinian and ignimbritic (“Sintema del Rifugio di S. Nicola”). The grain-size analyses of sea bottom samples have shown that gravelly sands, sands, silty sands, muddy sands, sandy silts, and silts all occur. The comparison of the obtained results with previous sedimentological data collected in the Ischia offshore, coupled with the construction of marine geological maps at the 1:10,000 scale, has shown that sands seem to prevail in the eastern Ischia offshore, while gravelly sands, mainly bioclastic, are located at the top of relict volcanic edifices, such as the Forio Bank (western Ischia offshore) and the Ischia Bank (south-eastern Ischia offshore). Moreover, the obtained data show that rhodolith beds occur on the relict volcanic edifice of the Ischia Bank, on the related parasitic vent, and in the Ischia Channel, a morphological saddle located between the Ischia and Procida islands.