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In the Central Sichuan region, oil-gas fields are widely distributed, and due to the toxicity and explosiveness of natural gas, tunnel construction poses safety hazards. Existing prediction methods for oil-gas tunnels have deficiencies in accuracy and applicability. Therefore, developing a new method for the classification prediction of harmful gas is crucial for the design and construction of projects in the Central Sichuan region. This research proposed a prediction method based on the Support Vector Machine (SVM) optimized using the Energy Valley Optimizer (EVO) algorithm. Initially, 114 sets of harmful gas tunnel cases were selected based on existing engineering data. Parameters such as tunnel depth, length, oil-gas field location score, structural score, and lithology score were used as inputs, while the actual gas classification served as the output for model validation. The results show that the method achieved a high accuracy of 96.67%, along with higher convergence speed and higher predictive accuracy. The method was applied to the Sichuan Basin region to predict the hazard levels of 89 tunnels within the area, obtaining the hazard levels and distribution map of harmful gas in the area, providing valuable guidance for tunnel construction projects in the area. [ABSTRACT FROM AUTHOR]

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Investigating the primary influencing factors that regulate the enrichment of shallow gas not only deepens our understanding of the rules governing shallow gas enrichment in deep-ocean environments but also has the potential to enhance the success rate of locating shallow gas reservoirs. Recent drilling activities in the LS36 gas field located in the central Qiongdongnan Basin have revealed a substantial shallow gas reserve within the sediments of the Quaternary Ledong Formation, marking it as the first shallow gas reservoir discovered in the offshore region of China with confirmed natural gas geological reserves surpassing 100 billion cubic meters. However, the formation mechanism and influencing factors of shallow gas enrichment remain elusive due to the limited availability of 3D seismic and well data. This study employs seismic interpretation and digital simulation to decipher the dynamics of shallow gas accumulation and utilizes the carbon isotope composition of methane to ascertain the origin of the shallow gas. Our results show that the shallow gas is primarily concentrated within a large-scale submarine fan, covering a distribution region of up to 2800 km2, situated in the deep-sea plain. The δ13 C1 methane carbon isotope data ranges from −69.7‰ to −45.2‰ and all δ13 C2 values are above −28‰, suggesting that the shallow gas within the Ledong Formation is derived from a mix of biogenic gas produced in shallow strata and thermogenic gas generated in deeper source rocks. The results of gas sources, seismic profiles, and digital simulations suggest that thermogenic gas originating from the Lingshui and Beijiao sags was transported to the Quaternary submarine fan via a complex system that includes faults, gas chimneys, and channel sands. The mass-transported deposits (MTDs) in the upper reaches of the submarine fan have effectively acted as a seal, preventing the escape of shallow gas from the fan. Therefore, the factors contributing to the enrichment of shallow gas in the Qiongdongnan Basin include the presence of favorable submarine fan reservoirs, the availability of two distinct gas sources, the effective sealing of MTDs, and the presence of two efficient transport pathways. A conceptual model for the accumulation of shallow gas is developed, illuminating the complex formation–migration–accumulation process. This study underscores the importance of aligning multiple influencing factors in the process of shallow gas accumulation, and the suggested accumulation model may be pertinent to shallow gas exploration in other marginal sea basins. [ABSTRACT FROM AUTHOR]

Due to the poor cementation and low strength of shallow strata in deep water, reservoir instability and collapse are easy to occur in the process of shallow gas exploitation, which seriously restricts the production capacity of shallow gas. In order to understand the reservoir variation law during shallow gas production, the simulation experiments of different clay content, main control mining parameters and porosity were carried out to analyze the influencing factors of gas and water production. The results show that pressure sensitive effect and particle transport exist simultaneously. The permeability damage caused by the pressure sensitive effect near the wellbore is more serious. Under the premise of ensuring formation stability, a small degree of particle migration can increase pore connectivity over time, which has a positive effect on stable gas production and increased permeability. With the gas production, the water in the reservoir will also be carried and produced, and part of the irreducible water will be transformed into movable water. The conclusion can provide reference for sand control and stimulation of shallow gas wells in sea area. [ABSTRACT FROM AUTHOR]

Seismic attribute analysis, a method of highlighting important information in seismic data, has been used in various fields. Recently, several studies have used seismic attributes to interpret seismic anomalies associated with subsurface gas. In this study, we applied seismic attribute analysis to seismic data acquired in Yeongil Bay to identify the subsurface gas distribution. Before applying seismic attributes to the Yeongil Bay seismic data, we selected promising seismic attributes and applied them to F3 Block seismic data to analyze the relation between the seismic attributes and seismic anomalies caused by subsurface gas. Since there are previous studies on the subsurface gas in the F3 Block, we could identify the characteristics of the seismic attributes based on the F3 Block seismic data. Then, we applied the selected seismic attributes to the Yeongil Bay seismic data to define the subsurface gas distribution. Through seismic attribute analysis, a three-dimensional gas distribution model of Yeongil Bay was generated, and this model can provide quantitative information on the subsurface geology of Yeongil Bay. [ABSTRACT FROM AUTHOR]

Seismic data from the North Sea commonly show vertical acoustic blanking (VAB) often interpreted as fluid conduits with implications for Quaternary development. The robustness of this interpretation has long been controversial as the infill of tunnel valleys can also cause vertical blanking. Using 2D and 3D seismic data and sediment echosounder data from the German North Sea, we investigate VAB to determine a geological or imaging origin of these anomalies. We detected multiple VAB occurrences throughout the North Sea. 3D data from the Ducks Beak ('Entenschnabel') reveal a correlation of VAB with bright spots in incised channels directly below the seafloor. Large source–receiver distances allow imaging the subsurface below the channel without signal penetrating through it (undershooting). This method removes the blanking. Energy absorption by shallow biogenic gas trapped within the channels explains the observed VAB. Hence, the blanking represents an imaging artifact, highlighting the need for careful seismic processing with sufficient offset before interpreting such anomalies as fluid pathways. The channels belong to a postglacial channel system related to the now submerged lowlands of Doggerland. This work demonstrates the usability of mapping VAB to detect shallow features for paleo‐landscape reconstruction and identification of shallow gas for hazard assessments, for example. [ABSTRACT FROM AUTHOR]

To address the deformation and instability characteristics of a formation after an offshore shallow gas well blowout, a theoretical model of formation deformation caused by shallow gas blowouts was constructed, based on porous elastic medium theory and incorporating the sand-out erosion criterion. The spatiotemporal dynamics of formation subsidence were then investigated, and deformation patterns during a blowout were analyzed under various factors. The results indicate that, following a blowout, a shallow gas formation near a borehole experiences significant subsidence and uplift at the upper and lower ends, with the maximum subsidence values at 12 h, 24 h, 36 h, and 48 h post blowout being 0.072 m, 0.132 m, 0.164 m, and 0.193 m, respectively. The overlying rock layer forms a distinctive "funnel" shape, exhibiting maximum subsidence at the borehole, while more distant strata show uniform subsidence. The effective stress within the shallow gas stratum and surrounding rock layers increases gradually during the blowout, with lesser impact in distant areas. The ejection rate and sand blast volume demonstrate an exponential change pattern, with a rapid decline initially and later stabilization. Formation deformation correlates positively with factors like burial depth; shallow gas layer extent; pressure coefficient; sand blast volume; gas blowout rate; and bottomhole difference pressure. Formation pressure, ejection rate, and bottomhole difference pressure have the most significant impact, followed by sand blast volume and burial depth, while the extent of the shallow gas layer has a less pronounced effect. These simulation results offer valuable theoretical insights for assessing the destabilization of formations due to blowouts. [ABSTRACT FROM AUTHOR]

To facilitate the recovery of natural gas hydrate (NGH) deposits in the South China Sea, we have designed and developed the world's largest publicly reported experimental simulator for NGH recovery. This system can also be used to perform CO2 capture and sequestration experiments and to simulate NGH recovery using CH4/CO2 replacement. This system was used to prepare a shallow gas and hydrate reservoir, to simulate NGH recovery via depressurization with a horizontal well. A set of experimental procedures and data analysis methods were prepared for this system. By analyzing the measurements taken by each probe, we determined the temperature, pressure, and acoustic parameter trends that accompany NGH recovery. The results demonstrate that the temperature fields, pressure fields, acoustic characteristics, and electrical impedances of an NGH recovery experiment can be precisely monitored in real time using the aforementioned experimental system. Furthermore, fluid production rates can be calculated at a high level of precision. It was concluded that (1) the optimal production pressure differential ranges from 0.8 to 1.0 MPa, and the wellbore will clog if the pressure differential reaches 1.2 MPa; and (2) during NGH decomposition, strong heterogeneities will arise in the surrounding temperature and pressure fields, which will affect the shallow gas stratum.

Investigating the primary influencing factors that regulate the enrichment of shallow gas not only deepens our understanding of the rules governing shallow gas enrichment in deep-ocean environments but also has the potential to enhance the success rate of locating shallow gas reservoirs. Recent drilling activities in the LS36 gas field located in the central Qiongdongnan Basin have revealed a substantial shallow gas reserve within the sediments of the Quaternary Ledong Formation, marking it as the first shallow gas reservoir discovered in the offshore region of China with confirmed natural gas geological reserves surpassing 100 billion cubic meters. However, the formation mechanism and influencing factors of shallow gas enrichment remain elusive due to the limited availability of 3D seismic and well data. This study employs seismic interpretation and digital simulation to decipher the dynamics of shallow gas accumulation and utilizes the carbon isotope composition of methane to ascertain the origin of the shallow gas. Our results show that the shallow gas is primarily concentrated within a large-scale submarine fan, covering a distribution region of up to 2800 km2, situated in the deep-sea plain. The δ13 C1 methane carbon isotope data ranges from −69.7‰ to −45.2‰ and all δ13 C2 values are above −28‰, suggesting that the shallow gas within the Ledong Formation is derived from a mix of biogenic gas produced in shallow strata and thermogenic gas generated in deeper source rocks. The results of gas sources, seismic profiles, and digital simulations suggest that thermogenic gas originating from the Lingshui and Beijiao sags was transported to the Quaternary submarine fan via a complex system that includes faults, gas chimneys, and channel sands. The mass-transported deposits (MTDs) in the upper reaches of the submarine fan have effectively acted as a seal, preventing the escape of shallow gas from the fan. Therefore, the factors contributing to the enrichment of shallow gas in the Qiongdongnan Basin include the presence of favorable submarine fan reservoirs, the availability of two distinct gas sources, the effective sealing of MTDs, and the presence of two efficient transport pathways. A conceptual model for the accumulation of shallow gas is developed, illuminating the complex formation–migration–accumulation process. This study underscores the importance of aligning multiple influencing factors in the process of shallow gas accumulation, and the suggested accumulation model may be pertinent to shallow gas exploration in other marginal sea basins.

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Heat flow measurements are a standard technique in Geophysics both onshore and offshore. Recently, such measurements became increasingly important in shallow waters. The increasing amount of offshore power installations makes it necessary to have a good knowledge about the subsurface heat flow and the thermal properties of the sediments to optimize the construction of the necessary powerlines. While the thermal properties are well studied for deep ocean sediments, only few published data exist for nearshore sediments. In this study, we investigate the sediment temperatures and thermal conductivities of nearshore sediments in the German part of the Baltic Sea. The shallow sediment temperatures reflect the interplay of the response to the seasonal cycle in connection with the sediments’ thermal conductivity. We find thermal conductivity values ranging from 0.67 to 3.34 W/(m*K) for the sediments down to ~ 4.2 m below seafloor. This variability exceeds that of conservative estimates widely used for coastal sediments and is also much higher than the variability found in the deep oceans. Sandy sediments show thermal conductivities larger than 1 W/(m*K) whereas organic-rich muds have lower values (< 1 W/(m*K)). Furthermore, the thermal conductivities seem to decrease with increasing free gas content in the sediment. The latter needs to be confirmed by further investigations. [ABSTRACT FROM AUTHOR]

Offshore oil development is gradually advancing to deep water, and the complex seabed topography and shallow geological conditions greatly affect the difficulty of subsea well construction and threaten the safety of drilling operations. To reduce the risk during drilling and optimize the drilling plan, the cause mechanism of complex seabed topography and shallow geological conditions is outlined, and the countermeasures taken by China National Oil Corporation (CNOOC) to deal with different seabed topography and shallow geological conditions are collected. The research results are of great significance in guiding the construction of subsea wells in the South China Sea. [ABSTRACT FROM AUTHOR]

Gas hydrates possess significant potential as an energy resource and exert a notable influence on global climate change. The Shenhu Area is one of the globally recognized focal points for gas hydrate research, and additional investigation is required to fully comprehend its gas migration mechanism. By utilizing the most recent core-log-seismic data and gas geochemical data, a comprehensive analysis was conducted to determine the influence of gas migration pathways on gas hydrate accumulation in the study area. This study investigated the various types of gas migration pathways, employing integrated geological models that incorporate faults and gas chimneys to understand their respective contributions to the accumulation of gas hydrates. Based on these findings and drilling constraints, a three-gas combined production model was subsequently proposed. Thermogenic gas, secondary microbial gas, and in situ microbial gas are all potential sources of the gas responsible for hydrate formation. Thermogenic gas plays a significant role in the gas hydrate system, as evidenced by distinct features of late-mature thermogenic gas observed in gas samples extracted from hydrates in Well W18. In the study area, the primary conduits for gas migration encompass deep faults, branch faults, and gas chimneys. Among these, deep faults act as the most crucial pathways of thermogenic gas migration. The integration of geological models that incorporating deep faults and gas chimneys has profoundly impacted the accumulation of gas hydrates in the Shenhu Area, consequently influencing the distribution of shallow gas and gas hydrate. Furthermore, the proposed three-gas combined production model, which involves the simultaneous extraction of deep gas reservoirs, shallow gas reservoirs, and gas hydrates, holds significant implications for exploring and developing deep-water natural gas resources. However, its successful implementation necessitates interdisciplinary collaboration among scientists.

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Introduction: As our understanding of the ocean and its uses continues to advance, countries worldwide with ocean access are implementing new marine strategies. For example, understanding marine geological disasters can help develop the use of ocean resources and oceanic engineering.Methods: Therefore, this study used recent data from the Oujiang Estuary, a nearshore area in China, to determine its geological hazards.Results: The geological hazards were classified as active or restricted based on topography, geomorphology, shallow seismic profile, single-channel seismic profile, and geological drilling data. Active geological hazards primarily include shallow gas and active sand waves, whereas restrictive geological hazards include irregularly buried bedrock, erosion channels, steep submarine slopes, and buried paleochannels. We also evaluated the distribution characteristics and scope, such as the vertical distribution of shallow gas based on the seismic profiles, drilling rock facies, methane, and carbon dioxide contents in the top air, and the isotope values. We found that shallow gas was vertically distributed among multiple layers. The main gas-bearing layers were the clayey silt and sandy (silt) clay layers of the early Holocene and late Pleistocene strata. The shallow gas content was relatively low in the coarse sediment layer at the bottom of the late Pleistocene succession. Generally, the Holocene and late Pleistocene deposits do not contain gas, and the gas content in the middle and late Pleistocene strata (at greater depths) is relatively low.Discussion: The combined effects of the regional geological structure, sea level changes, modern hydrodynamics, and human activities have formed the geological environment of the Oujiang Estuary.

Marine sediment is an important channel for methane leakage from the earth interior to the atmosphere. The investigation of gas invasion in fine-grained marine sediments is of great theoretical and practical significance in marine science and engineering. To study the mechanical mechanisms of fine-grained marine sediments subjected to shallow gas invasion, a gas injection test with a self-developed experimental apparatus was performed, and the gas invasion behavior was investigated. The results showed that the behavior of gas invasion in fine-grained sediments can be divided into different phases; the fracturing direction β gradually changes from vertical to horizontal, and finally fractures along the roof. Based on the 2D undrained elliptical cavity model and the tensile strength of sediments, considering both tensile and shear failure modes, a discrimination criteria of gas invasion was proposed. It revealed that gas invasion gradually changes from shear failure to tensile failure, and the fracturing angle θ predicted by the criteria is consistent with the experimental phenomenon. [ABSTRACT FROM AUTHOR]

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Methane is a pivotal component of the global carbon cycle. It acts both as a potent greenhouse gas and a vital energy source. While the microbial cycling of methane in subsurface environments is crucial, its impact on geological settings and related engineering projects is often underestimated. This review uniquely integrates the latest findings on methane production, oxidation, and migration processes in strata, revealing novel microbial mechanisms and their implications for environmental sustainability. We address critical issues of methane leakage and engineering safety during resource extraction, underscoring the urgent need for effective methane management strategies. This work clarifies geological factors affecting methane budgets and emissions, deepening our understanding of methane dynamics. It offers practical insights for geological engineering and sustainable natural gas hydrate exploration, paving the way for future research and applications. [Display omitted] [ABSTRACT FROM AUTHOR]

Methane is a powerful greenhouse gas and the second most important when considering global warming due to anthropogenic added gasses. Global inventories of greenhouse gasses currently do not take into consideration methane emitted from the ocean and seas. The North Sea is an intensely exploited seas for oil and gas and it was recently suggested to be a major source for manmade methane emissions. All wells drilled through shallow gas (methane) were found to be leaking and one-third of all abandoned wells was found to be drilled through shallow gas. Here we present the results from a research expedition to investigate methane leakage at abandoned wells drilled through shallow gas in the Dutch North Sea. We surveyed 57 abandoned wells of which 33 were drilled through shallow gas. Nine locations showed bubble plumes (acoustic flares). We noted a distinct difference between gas leakage of abandoned wells and locations with natural gas seepage. Whereas well leakage consists of one or two bubble plumes at the wellhead itself and no bubble plumes in the surrounding area, natural plume fields are characterized by tens to hundreds of plumes and none at the wellhead. At six wells, we conclude that the plumes are caused by the well leaking shallow gas, whereas three observed plume fields classify as natural seepage. We found that 18% of wells drilled through shallow gas were leaking, with 11% of all abandoned wells being drilled through shallow gas. When we compensated for over-representation of shallow gas wells in our sample (58% of our sample is drilled through shallow gas), we find that less than 2% of all abandoned wells in the Dutch North Sea is likely leaking. Well leakage seems to occur when large quantities of shallow gas are present and the abandoned well apparently suffers from an integrity issue. [Display omitted] • 57 wells in the Dutch North Sea were surveyed for well leakage. • Six abandoned wells, drilled through shallow gas were found to be leaking. • 18% (6 of 33) of the wells drilled through shallow gas leak. • Less than 2% of all abandoned wells in the Netherlands are expected to leak shallow gas. [ABSTRACT FROM AUTHOR]

Seabed fluid flow is a widespread and important natural phenomenon in marine environments, which involves complex multi-physics, multi-process and multi-scale processes. The developments in offshore geophysical technology have facilitated the discovery of the widespread emissions of seabed fluids. For an overview on the state-of-the-art seabed fluid flow research and for obtaining a perspective on future research in the China Seas, we reviewed the data, reports, and publications particularly that associated with cold seeps such as pockmarks, seeps, domes, mud volcanoes, and gas hydrates in the Bohai Sea, the Yellow Sea, the East China Sea, and the South China Sea. This study presents the first report for seabed fluid flow on all China Seas with the basic information required to undertake additional analytical studies of these features. Furthermore, we explore processes responsible for them and their implications. Although the seabed fluid flow is widespread, dynamic, and influential, it is still poorly examined and understood. To understand seabed fluid flow in both time and space, it is important to investigate how and why these seabed fluids form and migrate.

Shallow gas is generally extensively distributed in the Holocene muddy sediments and gas seepage has been increasingly reported to induce geohazards in coastal seas, but controls on gas distribution and migration remain elusive. This study explores gas distribution and migration in the Yangtze subaqueous delta and the Hangzhou Bay using high-resolution acoustic profiles and core data. Shallow gas is widely detected by the common presence of acoustic anomalous reflections including enhanced reflection, gas chimney, bright spot, acoustic blanking, and acoustic turbidity. The gas front depth is generally less than 17.5 m, and is meanly shallower in the Hangzhou Bay than in the Yangtze subaqueous delta because of relatively shallower water depth and coarser Holocene sediments in the Hangzhou Bay. Shallow gas is inferred to be a biogenic product, and its distribution is highly contingent on the Holocene stratal thickness and water depth. Active gas migration and seepages are evident, and recently increasing occurrences of gas seepage can be ascribed to global warming and seabed erosion due to sediment deficit. The findings warn us to pay more attention to the positive feedback loops of gas seepages with global warming and seabed erosion for the associated geohazard prediction and reduction, typically in the highly developed coastal regions.

River deltas have long been considered important carbon sinks. However, the presence of shallow gas and the processes of delta erosion caused by diminished sediment supplies could reduce the strength of the carbon sink. In this study, based on historical bathymetric data and a data set obtained from a seismic survey, we investigate the response of gas‐charged deposits in the Yangtze subaqueous delta to seabed erosion. A conservative estimate of the total methane reserves of 0.55–4.35 × 1011 mol was obtained in the delineated gassy area of ∼3,800 km2. The seismic and bathymetric data reveal a prominent erosional belt at water depths ranging from 5 to 20 m and extending from the southwestern to northeastern nearshore areas of the Yangtze subaqueous delta. Erosion is severe in the south and slight in the north due to differences in the hydrodynamic conditions, sediment erodibility, and sensitivity to sediment reduction. Seabed erosion reduces the thickness of the cap bed and the overburden pressure at the gas front, making it easier for gas to seep through the sediment column and bypass the anaerobic oxidation of methane. The good agreement between the elevated methane concentrations and the erosional belts and the spatial coincidence between the shallower gas front and pockmarks indicate that seabed erosion accelerates gas seepage activities. In the context of global deltaic degradation, the increasing greenhouse gas emissions from deltaic deposits are worthy of further attention. Plain Language Summary: River deltas have played an important role in regulating atmospheric carbon dioxide concentrations throughout geological history. High sediment accumulation rates and organic carbon contents in delta areas can lead to the formation of abundant biogenic methane in deltaic sediments, termed "shallow gas," when the methane concentration exceeds the solubility level and free gas is present. In recent decades, the erosion of river deltas induced by diminished riverine sediment supplies has been a worldwide problem. The impact of seabed erosion caused by sediment starvation on gas‐charged deposits is unclear. To understand the influence of seabed erosion on gas‐charged deposits, we conducted seismic surveys in the Yangtze subaqueous delta area. The data reveal the distribution of shallow gas and its seepage activities. An analysis of our data and historical bathymetric data showed that where severe erosion occurs, gas flow can easily seep through the overlying sediment to the water column due to a decrease in the sediment thickness and overburden pressure. Since river deltas worldwide share the same fate of sediment starvation and erosion, there is a need to evaluate the magnitude and significance of increasing methane emissions associated with delta erosion in the context of global warming. Key Points: A seismic study identified extensive gas accumulation and active seepages in Holocene subaqueous deltaic depositsWhere severe erosion occurs, gas chimneys extend close to the seabed and potentially develop into pockmarksSeabed erosion aggravates gas seepage in the subaqueous delta [ABSTRACT FROM AUTHOR]

High-resolution seismoacoustic data represent a useful tool for the investigations of gas-charged sediments occurring beneath the seabed through the identification of the diagnostic intrasedimentary features associated with them. Acoustic blanking revealed shallow gas pockets in the seismostratigraphic units of the inner shelf off the Northern Cilento promontory. Six main seismostratigraphic units were recognized based on the geological interpretation of the seismic profiles. Large shallow gas pockets, reaching a lateral extension of 1 km, are concentrated at the depocenter of Late Pleistocene–Holocene marine sediments that are limited northwards by the Solofrone River mouth and southwards by the Licosa Cape promontory. A morphobathymetric interpretation, reported in a GIS environment, was constructed in order to show the main morphological lineaments and to link them with the acoustic anomalies interpreted through the Sub-bottom chirp profiles. A newly constructed workflow was assessed to perform data elaboration with Seismic Unix software by comparing and improving the seismic data of the previously processed profiles that used Seisprho software. The identification of these anomalies and the corresponding units from the offshore Cilento promontory represent a useful basis for an assessment of marine geohazards and could help to plan for the mitigation of geohazards in the Cilento region. [ABSTRACT FROM AUTHOR]

Shallow gas is a potential risk in deep-water drilling that must not be ignored, as it may cause major safety problems, such as well kicks and blowouts. Thus, the pre-drilling prediction of shallow gas is important. For this reason, this paper conducted deep-water shallow gas acoustic simulation experiments based on the characteristics of deep-water shallow soil properties and the theory of sound wave speed propagation. The results indicate that the propagation speed of sound waves in shallow gas increases with an increase in pressure and decreases with increasing porosity. Pressure and sound wave speed are basically functions of the power exponent. Combined with the theory of sound wave propagation in a saturated medium, this paper establishes a multivariate functional relationship between sound wave speed and formation pressure and porosity. The numerical simulation method is adopted to simulate shallow gas eruptions under different pressure conditions. Shallow gas pressure coefficients that fall within the ranges of 1.0–1.1, 1.1–1.2, and exceeding 1.2 are defined as low-, medium-, and high-risk, respectively, based on actual operations. This risk assessment method has been successfully applied to more than 20 deep-water wells in the South China Sea, with a prediction accuracy of over 90%. [ABSTRACT FROM AUTHOR]

Low gas production rates and insufficient gas yield hinder the large-scale production from natural gas hydrates; a joint production of gas hydrate and its underlying shallow gas is expected to address this limitation. This study focuses on the interaction between heterogeneous hydrate reservoirs and the upward migration of the shallow gas. The results indicated a 38.4 % increase in production efficiency with the assistance of the shallow gas. Specially, the melt water generated from extensive hydrate decomposition could potentially induce a lateral migration of the shallow gas at the interfacial zones of the heterogeneous reservoirs. Further investigation revealed that a lower production pressure would help the release of the shallow gas as well as the hydrate decomposition, thereby contributing to a 64.55 % shorter t 90. A faster depressurization rate could facilitate the temperature recovery by intensifying the lateral movement of the shallow gas in the junction layer. Consequently, a proper control of the upward channeling of the shallow gas was suggested in the field test for a successive and secure gas production. Our results could be of help in elucidating the interlayer interference mechanisms and the selection of the depressurization strategy for a better recovery efficiency from the multi-gas source reservoirs. [Display omitted] • An upward channeling of the shallow gas occurred during the gas production. • The production efficiency increased by 38.4 % with the assistance of shallow gas. • An enhanced release of the shallow gas contributed to a 64.55 % shorter t 90. [ABSTRACT FROM AUTHOR]

• Efficiency of shallow gas-assisted production was 10.71 % higher than single one. • Decrease in interlayer permeability increased possibility of interlayer blockage. • High-speed gas flow under 2 MPa provided possibility of unblocking interlayer. • Shallow gas showed a significant temperature bounce in constant-pressure stage. Natural gas is crucial for the global energy transition to low-carbon and clean energy utilization. Natural gas hydrate, rich in high energy–density natural gas, has excellent production potential. At present, even with the most commercially viable depressurization method, the production efficiency is still limited by the weakening of the hydrate decomposition driving force. That is expected to be enhanced by optimizing the upward migration of the shallow gas. However, the effect of production elements of the depressurization method on the shallow-gas upward migration remains unclear. In this study, several production elements were systematically analyzed. Results showed that the efficiency of shallow gas-assisted hydrate production was 10.71 % higher than that of a single one, with a 79.16 % increase in the hydrate stage decomposition rate. Reducing the production pressure from 3 MPa to 2 MPa boosted the production efficiency by 366.85 %. The resulting high-speed shallow gas provided the possibility of unblocking the interlayer. In addition, the shallow gas layer exhibited a more significant temperature rise effect during the constant-pressure stage when employing a stepwise depressurization. The injection of hotter gas was able to further promote the hydrate decomposition. Finally, the crucial role of the shallow-gas sweep behaviors in optimizing production performance was verified. The results could guide the efficient use of the shallow gas to strengthen the hydrate decomposition in the field joint production of multi-gas source reservoirs. [ABSTRACT FROM AUTHOR]

Shallow gas is considered one of the most serious geological hazards in deepwater drilling because it has the characteristics of suddenness and is difficult to deal with. To perform a quantitative evaluation of shallow gas risk during deepwater drilling, a numerical model for calculating gas invasion volume is established based on gas-water two-phase flow theory. The model considers the effect of the dynamic drilling process, and the influencing factors which affect the gas invasion volume are analyzed. Results indicate that the gas invasion rate and accumulated gas invasion volume increase with increasing bottom-hole pressure difference. A linear relationship exists between gas invasion volume and bottom-hole pressure difference. The duration of gas invasion increases as the shallow gas zone thickness increases, and the accumulated gas invasion volume grows as shallow gas zone thickness increases. The increase in formation permeability, water depth, and rate of penetration will enhance the gas invasion rate. However, these three factors can hardly affect the accumulated gas invasion volume. The gas flow rate increases significantly with increasing burial depth of shallow gas. On the basis of influencing factor analysis, a series of methods that consider different risk levels is proposed to control shallow gas, which can provide a reference for the prevention of shallow gas disasters during deepwater drilling. [ABSTRACT FROM AUTHOR]

In hydrocarbon exploration, information carried by diving waves and post-critical reflections that are used to reconstruct the long-to-intermediate wavelength of the subsurface is an integral part of successful velocity model building. Diving wave tomography (DWT) is one of the tools for shallow velocity assessment particularly when seismic data has poor signal-to-noise ratio (SNR) with complex geologic settings where no clear reflector is present. Considering the relationship between velocity with time and space, the output from tomography plays a crucial role to align data between time and depth domain and produce a reliable image of the deeper structure where hydrocarbon reservoir is typically located. In geophysics, tomography is primarily used to correct seismic trace alignment to produce a reliable stack section. In advanced imaging it is used as an initial model for waveform inversion in an integrated workflow. In the post-processing stage, it is used to correct the misfit between well logs and seismic data and is crucial for the quantitative analysis of rock physics. In this paper, we focus on tomography and its working principle on near-surface velocity modelling. We restricted our workflow to 2D synthetic data simulating the shallow gas occurrence that is prominent in the offshore Malay Basin to demonstrate how tomography works in velocity reconstruction. Results from synthetic and real data example shows that DWT can recover local large-scale structure and improved stacked data, considering no other seismic data and constraint from well data is included in the iterative process. [ABSTRACT FROM AUTHOR]

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Copyright of East China Geology / Huadong dizhi is the property of Editorial Department of East China Geology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

A 10-in (1 in = 2.54 cm) subsea oil pipeline and a 14-in gas pipeline of an oilfield crosses a shallow gassubsidence area at about 53 km (KP53) away from the landing point of Hangzhou Gulf. These two pipelines have beenin service for nearly 20 years, but each damaged happened after typhoon. Cause analysis was carried out based on theinvestigation data of damage accidents of oil pipeline crossing the shallow gas subsidence area, and it was found that thepipeline fracture was caused by the fatigue load due to soil liquefaction and subsidence during typhoon. Thereby, a repairplan was designed according to the engineering characteristics for guidance of pipeline repair construction. The repairedpipeline has been subject to several times of typhoon and it is still in good conditions. The research results are of guidingsignificance to the construction of submarine pipeline crossing shallow gas geological hazard area or the seabed with soilliquefaction trend.

Marine sediment is an important channel for methane leakage from the earth interior to the atmosphere. The investigation of gas invasion in fine-grained marine sediments is of great theoretical and practical significance in marine science and engineering. To study the mechanical mechanisms of fine-grained marine sediments subjected to shallow gas invasion, a gas injection test with a self-developed experimental apparatus was performed, and the gas invasion behavior was investigated. The results showed that the behavior of gas invasion in fine-grained sediments can be divided into different phases; the fracturing direction β gradually changes from vertical to horizontal, and finally fractures along the roof. Based on the 2D undrained elliptical cavity model and the tensile strength of sediments, considering both tensile and shear failure modes, a discrimination criteria of gas invasion was proposed. It revealed that gas invasion gradually changes from shear failure to tensile failure, and the fracturing angle θ predicted by the criteria is consistent with the experimental phenomenon.

High-resolution seismoacoustic data represent a useful tool for the investigations of gas-charged sediments occurring beneath the seabed through the identification of the diagnostic intrasedimentary features associated with them. Acoustic blanking revealed shallow gas pockets in the seismostratigraphic units of the inner shelf off the Northern Cilento promontory. Six main seismostratigraphic units were recognized based on the geological interpretation of the seismic profiles. Large shallow gas pockets, reaching a lateral extension of 1 km, are concentrated at the depocenter of Late Pleistocene–Holocene marine sediments that are limited northwards by the Solofrone River mouth and southwards by the Licosa Cape promontory. A morphobathymetric interpretation, reported in a GIS environment, was constructed in order to show the main morphological lineaments and to link them with the acoustic anomalies interpreted through the Sub-bottom chirp profiles. A newly constructed workflow was assessed to perform data elaboration with Seismic Unix software by comparing and improving the seismic data of the previously processed profiles that used Seisprho software. The identification of these anomalies and the corresponding units from the offshore Cilento promontory represent a useful basis for an assessment of marine geohazards and could help to plan for the mitigation of geohazards in the Cilento region.

Hydrocarbon plumbing systems have been extensively documented in the past two decades using high‐resolution 3D seismic data, exploiting the ability of seismic imaging techniques to reveal the subsurface geometry of gas charged sediments. In this paper, we present a detailed study of a hydrocarbon plumbing system from the South China Sea, that involves both vertical and lateral (stratal) hydrocarbon migration in Miocene to Recent clastic sediments that comprise multilayer stacking of thinly layered clays, silts and sands. We show that a transtensive fault system that provides lateral seal for fault‐dip traps of deep Miocene reservoirs, and offers a vertical pathway for migration to shallower silty units. These silty units in turn form a 'spillway' in a regional, northward migration path. This path involves filling each shallow fault‐dip trap to spill point towards the fault tips, with stratal migration forced around the outer flanks of the fault‐related folds. Successive fill‐to‐spill leads to a continuous trail of amplitude anomalies that merge into a continuous, larger, gas‐charged anomaly pattern. The migrating gas finally accumulates within a zone bounded by a large boundary fault with full juxtaposition seal. The pattern of anomaly distribution suggests that the hydrocarbon migration has been active in the Late Pleistocene and is probably continuing at the present day. Hence this plumbing system may be one of very few examples described to date in which dynamic hydrocarbon migration pathways have been directly imaged by seismic data. [ABSTRACT FROM AUTHOR]

Soil layers including shallow gas threaten underground structures. Considering the special geological conditions of the gas-bearing layers under the Yangtze River for the Sutong River-crossing unity tunnel project, a solid-liquid-gas coupled 3-D numerical model was established to consider two different gas pocket conditions (a single gas pocket and multiple random gas pockets), and the effects of shallow gas on the tunnel segment and the adjacent soils in the long-term process of gas leakage were analyzed and discussed. A brief discussion of the displacement results and a comparison of the stress results determined by simulation and the designed data indicated that as the maximum displacement is on the order of millimeters for both gas pocket conditions and the simulated maximum bending moment is less than the design value for the bending moment, the structure of the tunnel segment has remained in a safe state during the leakage of the gas pocket, and the normal operation of the tunnel segment is basically not affected by the leakage of the gas pocket. [ABSTRACT FROM AUTHOR]

Sedimentary rock deposition occur in very fast rates in offshore basin and might cause shallow subsurface geohazards that will incur high risk and increase cost of drilling operations. In general, offshore geohazards consist of a variety of geological features that contribute potential risks to the labour force, offshore amenities including the environment and surrounding areas due to the consequences of long or short period of geological processes. Therefore, further study need to be done properly in terms of geohazards classification that is significant to the offshore oil and gas developments in the Malay Basin (Bujang Field, refer Figure 1); such as shallow gas, gas hydrate, shallow water flow, slumping, landslides, faulting, pockmarks and liquefaction. To mitigate the point of costly drilling and safety risks, several techniques are needed during data gathering to visualize, interpret and identify the potential shallow drilling hazards. Besides, to a geoscientist, data integration and modelling techniques can be used to analyse the structural and physical circumstances of shallow subsurface. At the same time, gas models and geohazards map can be established based on seabed hazard analysis from seismic data to plan secure wells. Several seismic attributes such as instantaneous phase, instantaneous frequency, remove bias and envelope (reflection strength) had been used for channel detection. For gas cloud identification, seismic attributes such as remove bias, instantaneous phase, Chaos and RMS (Root Mean Square) amplitude are used. Besides that, spectral decomposition technique are used to display channel systems and other stratigraphic features in the field. Generally, this paper will explain about the meaning of geohazards in the oil and gas industry, the types of geohazards, general geohazards analysis, and will focuss on the identification of gas cloud through channel structure by applying several seismic attributes on specific parameters. All of this will be related to geohazards perspective and consequently, precautions can be undertaken systematically. [ABSTRACT FROM AUTHOR]

Sub-Bottom Profiling (SBP) records and results of geochemical analysis of 12 surficial sediment cores from various water depths collected from the offshore Waropen Basin-Papua are presented. Presence of gas is clearly observed on sub-bottom profiler records. Shallow gas was identified through acoustic response due to gas accumulation and gas escape on sub-bottom profiles. Acoustic evidences of gas accumulations within near surface geology consist of high amplitude reflections and associated acoustic blanking, gas plumes and morphological features like pockmarks. Total organic carbon analysis of 12 surface sediment cores varies between 0.5% to 1.3% which indicate that the sediments have an abundance of organic matters. Gas chromatographic analysis of hydrocarbon composition detected only methane, a biogenic origin of shallow gas. Acoustic and geochemical evidence in the Waropen Basin indicates extensive shallow gas accumulations in the Late Quaternary sediments, some trapped within these deposits and some escape from seabed into the water column which then created a high distribution of pockmarks. [ABSTRACT FROM AUTHOR]

Abstract Submarine groundwater discharge into coastal areas is a common global phenomenon and is rapidly gaining scientific interest due to its influence on marine ecology, the coastal sedimentary environment, and its potential as a future freshwater resource. We conducted an integrated study of hydroacoustic surveys combined with geochemical pore water and water column investigations at a well‐known groundwater seep site in Eckernförde Bay (Germany). We aim to better constrain the effects of shallow gas and submarine groundwater discharge on high‐frequency multibeam backscatter data and to present acoustic indications for submarine groundwater discharge. Our high‐quality hydroacoustic data reveal hitherto unknown internal structures within the pockmarks in Eckernförde Bay. Using precisely positioned sediment core samples, our hydroacoustic‐geochemical approach can differentiate intrapockmark regimes that were formerly assigned to pockmarks of a different nature. We demonstrate that high‐frequency multibeam data, in particular the backscatter signals, can be used to detect shallow free gas in areas of enhanced groundwater advection in muddy sediments. Intriguingly, our data reveal relatively small (typically

The Croker Carbonate Slabs, in the UK sector of the Irish Sea, has shallow (70 to 100 m) water, strong (> 2 knot) tidal currents, coarse mobile surficial sediments and the most extensive methane-derived authigenic carbonate (MDAC) known in European waters. Multi-disciplinary studies (2004 to 2015) were commissioned specifically to document the benthic habitat, and have resulted in the designation of this site as a Marine Protected Area (MPA) under the European Commission's Habitats Directive as an example of "Submarine structures formed by leaking gases". However, this paper is focussed on the geoscience aspects of the site: the mineralogy and isotopic composition of the MDAC, its formation and age. It considers the implications of these findings with respect to the timing of the deglaciation of the area since the Last Glacial Maximum (LGM), and the environmental implications of the seepage of methane from the site over a period of at least 17,000 years. Carbon isotope ratios (δ13 C − 34 to − 54‰) confirm that the carbonate minerals (high-Mg calcite and aragonite) result from the anaerobic oxidation of methane. Widespread shallow gas within post-glacial sediments is sourced from underlying coal-bearing Carboniferous strata. Geophysical (side-scan sonar and multi-beam echo sounder) and visual surveys show that the MDAC occurs as isolated lumps, continuous pavements, and cliffs < 6 m tall, which post-date the post-glacial sediments, but are in places covered by a veneer of coarse mobile surficial sediments. U-Th dates (17,000 ± 5500 to 4000 ± 200 BP) suggest continual MDAC formation since the last glacial maximum, and constrain the postglacial sea level rise in this part of the Irish Sea; the site must have been submarine before MDAC formation started, whether or not methane was escaping. Visual and acoustic evidence of gas seepage is limited, but methane concentrations in the water are high (< 21.4 nmol l−1) and suggest present-day export to the atmosphere. It is also implied that significant methane release to the atmosphere occurred immediately after the retreat of the ice that covered the site during the LGM until 21.9 to 20.7 ka BP. [ABSTRACT FROM AUTHOR]

Submarine groundwater discharge into coastal areas is a common global phenomenon and is rapidly gaining scientific interest due to its influence on marine ecology, the coastal sedimentary environment, and its potential as a future freshwater resource. We conducted an integrated study of hydroacoustic surveys combined with geochemical pore water and water column investigations at a well‐known groundwater seep site in Eckernförde Bay (Germany). We aim to better constrain the effects of shallow gas and submarine groundwater discharge on high‐frequency multibeam backscatter data and to present acoustic indications for submarine groundwater discharge. Our high‐quality hydroacoustic data reveal hitherto unknown internal structures within the pockmarks in Eckernförde Bay. Using precisely positioned sediment core samples, our hydroacoustic‐geochemical approach can differentiate intrapockmark regimes that were formerly assigned to pockmarks of a different nature. We demonstrate that high‐frequency multibeam data, in particular the backscatter signals, can be used to detect shallow free gas in areas of enhanced groundwater advection in muddy sediments. Intriguingly, our data reveal relatively small (typically <15 m across) pockmarks within the much larger, previously mapped pockmarks. The small pockmarks, which we refer to as "intrapockmarks," have formed due to the localized ascent of gas and groundwater; they manifest themselves as a new type of "eyed" pockmarks, revealed by their acoustic backscatter pattern. Our data suggest that, in organic‐rich muddy sediments, morphological lows combined with a strong multibeam backscatter signal can be indicative of free shallow gas and subsequent advective groundwater flow. Plain Language Summary: Groundwater that seeps out of the ocean floor is a common global phenomenon. Marine ecosystems are highly dependent on nutrient supply from land, and recent studies have suggested that groundwater seeping out of the seafloor supplies even more nutrients to the world's oceans than rivers do. Also, nearshore freshwater springs have been used as a source of drinking water for decades and large offshore groundwater reserves in continental shelves can potentially prevent future freshwater shortages. We use echo sounding methods to search for indications for submarine groundwater. The methods enable us to carry out detailed investigations of intriguing seafloor depressions (i.e., "pockmarks"). Our accurate measurements give new insights into the morphology and characterization of the Eckernförde Bay pockmarks and reveal a new type of pockmark that is related to seeping groundwater. In the muddy sediment of Eckernförde Bay, methane gas forms in the sediments due to microbial decomposition of biomass. In areas of enhanced groundwater advection, this methane gas is brought closer to the seafloor, where pockmarks form and where we can detect the gas with our sonar system. Given the abundant global distribution of muddy gaseous sediments, our findings have important implications for the future detection of offshore groundwater systems. Key Points: We present a new form of eyed pockmarks caused by shallow gasWe establish acoustic indicators for submarine groundwater discharge associated with gas [ABSTRACT FROM AUTHOR]

We really appreciate the interest and comments regarding our manuscript. We hope we address all the lingering issues in this reply. This also gives us the opportunity of publishing an update on our dataset that will complete the original manuscript (see Table 1). We have followed the author pattern in our answers: I-Gas sampling procedure, 2-Reported gas concentrations results, 3-General remarks, 4-Conclusions. (c) 2007 Elsevier B.V. All rights reserved.

New coring data have been acquired along the western Gulf of Lions showing anomalous concentrations of methane (up to 95,700 ppm) off the Rhone prodelta and the head of the southern canyons Lacaze-Duthiers and Cap de Creus. Sediment cores were acquired with box and kasten cores during 2004-2005 on several EuroSTRATAFORM cruises. Anomalous methane concentrations are discussed and integrated with organic carbon data. Sampled sites include locations where previous surveys identified acoustic anomalies in high-resolution seismic profiles, which may be related to the presence of gas. Interpretation of the collected data has enabled us to discuss the nature of shallow gas along the Gulf of Lions, and its association with recent sedimentary dynamics. The Rhone prodelta flood deposits deliver significant amounts of terrigenous organic matter that can be rapidly buried, effectively removing this organic matter from aerobic oxidation and biological uptake, and leading to the potential for methanogenesis with burial. Away from the flood-related sediments off the Rhone delta, the organic matter is being reworked and remineralized on its way along the western coast of the Gulf of Lions, with the result that the recent deposits in the canyon contain little reactive carbon. In the southernmost canyons, Lacaze-Duthiers and Cap de Creus, the gas analyses show relatively little shallow gas in the core samples. Samples with anomalous gas (up to 5,000 ppm methane) are limited to local areas where the samples also show higher amounts of organic matter. The anomalous samples at the head of the southern canyons may be related to methanogenesis of recent drape or of older sidewall canyon infills.

Sediment cores acquired in 2004 off the Rhone prodelta show consistent anomalous methane concentrations of up to 87,440 ppm. Methane compositional and isotopic data support a biogenic origin, although there are a few sites that show strongly depleted delta C-13 values (-53%o PDB) suggesting a mixed source for the gas (biogenic and thermogenic). Anomalous methane concentrations (samples with more than 90 ppm) are discussed and integrated with organic carbon data, sedimentary rates and ADCP profiles. Highest gas concentrations were found directly off the river mouth (20-40 m water depth) and where the IFREMER models point to the thickest accumulation (> 2 m) in response to the Rhone flood event. In areas unaffected by the high flux of organic matter and rapid/thick flood deposition, or in between flood events, the conditions for methanogenesis and gas accumulation have not been met; in these areas, the physical and biological reworking of the surficial sediment may effectively oxidize and mineralize organic matter and limit bacterial methanogenesis in the sub-surface. We propose that in the Rhone prodelta flood deposits deliver significant amounts of terrigenous organic matter that can be rapidly buried, effectively removing this organic matter from aerobic oxidation and biological uptake and leading to the potential for methanogenesis with burial. (c) 2006 Elsevier B.V. All rights reserved.