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Based on an analysis of the limitations of conventional production component methods for natural gas development planning, this study proposes a new one that uses life cycle models for the trend fitting and prediction of production. In this new method, the annual production of old and new wells is predicted by year first and then is summed up to yield the production for the planning period. It shows that the changes in the production of old wells in old blocks can be fitted and predicted using the vapor pressure model (VPM), with precision of 80%e95%, which is 6.6%e13.2% higher than that of other life cycle models. Furthermore, a new production prediction process and method for new wells have been established based on this life cycle model to predict the production of medium-to-shallow gas reservoirs in western Sichuan Basin, with predication error of production rate in 2021 and 2022 being 6% and 3% respectively. The new method can be used to guide the medium- and long-term planning or annual scheme preparation for gas development. It is also applicable to planning for large single gas blocks that require continuous infill drilling and adjustment to improve gas recovery. [ABSTRACT FROM AUTHOR]

Based on the drilling, logging, experimental and testing data of Well PD1, a shallow normal-pressure shale gas well in the Laochangping anticline in southeastern Sichuan Basin, the shallow shale gas reservoirs of the Ordovician Wufeng Formation to Silurian Longmaxi Formation (Wufeng–Longmaxi) were investigated in terms of geological characteristics, occurrence mechanism, and adsorption-desorption characteristics, to reveal the enrichment laws and high-yield mechanism of shallow normal-pressure shale gas in complex structure areas. First, the shallow shale gas reservoirs are similar to the medium–deep shale gas reservoirs in static indicators such as high-quality shale thickness, geochemistry, physical properties and mineral composition, but the former is geologically characterized by low formation pressure coefficient, low gas content, high proportion of adsorbed gas, low in-situ stress, and big difference between principal stresses. Second, shallow shales in the complex structure areas have the gas occurrence characteristics including low total gas content (1.1–4.8 m3/t), high adsorbed gas content (2.5–2.8 m3/t), low sensitive desorption pressure (1.7–2.5 MPa), and good self-sealing. Third, the adsorbed gas enrichment of shales is mainly controlled by organic matter abundance, formation temperature and formation pressure: the higher the organic matter abundance and formation pressure, the lower the formation temperature and the higher the adsorption capacity, which is more beneficial for the adsorbed gas occurrence. Fourth, the shallow normal-pressure shale gas corresponds to low sensitive desorption pressure. The adsorbed gas can be rapidly desorbed and recovered when the flowing pressure is reduced below the sensitive desorption pressure. Fifth, the exploration breakthrough of Well PD1 demonstrates that the shallow complex structure areas with adsorbed gas in dominance can form large-scale shale reservoirs, and confirms the good exploration potential of shallow normal-pressure shale gas in the margin of the Sichuan Basin. [ABSTRACT FROM AUTHOR]

Restricted by detective equipment and tidal influence seriously, coastal tidal flats are often treated as the transition belt of paralic zone in terms of seismic detection, causing low exploration degree and accuracy in these areas. What is worse, conventional shallow seismic method has a complex system and low acquisition efficiency, which cannot meet the requirements for high-efficiency acquisition in tidal flats. To solve these problems, a towed shallow seismic detection system is specially established for coastal tidal areas, mainly including electric spark source and towable ship-type geophone strings. After comparisons, some advantages of this method are obtained, such as high efficiency, low cost and reliability. Furthermore, its detective accuracy and depth can reach the meter level and more than 800 m, respectively. Then, a series of shallow seismic processing methods are improved for coastal tidal flats. Deconvolution and velocity analysis are emphatically introduced to multiple-wave attenuation. A short array of this towable detection can keep a far more effective wave from being cut off because of its small NMO stretch. Last but not least, the towable method in coastal tidal-flat zones can successfully identify the stratigraphic structure, interface, palaeo-channel, concealed active fault and submarine shallow gas in Jiangsu Coast, Eastern China, which has great practical results and significance for geological and environmental surveys, as well as scientific geo-hazard prevention and mitigation in these areas. [ABSTRACT FROM AUTHOR]

High-resolution shallow seismic investigations on inner continental shelf of Goa have been carried out to map gas-charged sediment zones and investigate origin of trapped gas. Analyses of data reveal that the gas is trapped by different stratigraphic layers and is distributed in two zones. Zone-1 is restricted up to ~30 m water depth and in the southern part of the study area. In this zone, gas is confined within the Holocene layers above the Holocene maximum Hooding surface (MFS). Whereas, zone-2 is comprised of several isolated gas-charged patches which are distributed in the entire study area mostly between ~20 m and ~45 m water depth. In this zone, gas is confined below Holocene- MFS in incised river bed or a layer just below MFS or both. Gas-charged sediment in zone-1 is formed due to degradation of organic material which is predominantly early Holocene mangrove derived, whereas, zone-2 it is formed due to degradation of organic material which were deposited by ancestral rivers during the post-glacial sea level rise of the Late Pleistocene. The distribution pattern of zone-1 is controlled by coastline configuration, long-term longshore current pattern, Holocene rapid sea level rise and lithological character of exposed inter-tidal zone. Whereas, distribution of zone-2 is controlled by palaeo-rivers of different glacial periods and their hydraulic energy condition during the base level rise. [ABSTRACT FROM AUTHOR]

Abstract Understanding the shallow gas system is critical for assessing its potential as an energy source, for evaluating the possible hazard of shallow gas for drilling and wind farm locations and for evaluating the effect of gas emissions at the seabed on marine ecosystems and climate. This paper presents the key elements and processes of the microbial shallow gas system in the Plio-Pleistocene Dutch Southern North Sea Delta based on recent findings from different projects. Geochemical and carbon isotopic composition of shallow gas occurrences in the delta are indicative of a microbial origin of the gas. Shallow gas mainly occurs in stratigraphic traps and stacked anticlinal structures above salt structures, as indicated by identified direct hydrocarbon indicators on seismic, such as bright spots. Organic matter in the delta deposits is of predominantly land plant origin with TOC values varying between <1% and 5%, and mostly between 1 and 2%. Simulations of temperature and burial history in combination with dedicated 1D simulations of microbial gas generation reveal that gas generation in the delta started in Early Pleistocene Calabrian times and is still ongoing. Simulated volumes of gas generation are more than enough to fill published estimated volumes of shallow gas prospects in the delta. The geometry of the E-W prograding delta sequences and the close interbedding of interglacial silty/sandy and glacial clayey/silty sediments focus gas migration updip through the foresets towards the topsets of the delta sequences, and ultimately into the anticlinal stacked traps. Grain-size based calculations of the permeability and capillary seal capacity of clayey/silty seals of the stratigraphic and anticlinal traps provided first estimates of permeability values ranging from 2.8E-20 m2 to 1.1E-18 m2 and capillary seal capacity values between 10m and 24m. Comparison of gas column heights derived from grain-size based calculations, cross plots of neutron and density logs and pressure measurements, with trap heights derived from seismic bright spots suggests that many traps in stacked bright spots are not filled to structural spill point. This suggests that filling of the stacked reservoirs is not related to fill-spill migration, but rather to leakage through the top seal. In absence of fault and fracture zones crossing the seal, this leakage is related to the capillary seal capacity and permeability of the top seal. The leakage extends, locally, to the seabed. The microbial gas system in the delta today is a highly dynamic system driven by ongoing burial of the delta sediments and microbial gas generation. Highlights • Integrated evaluation of all key elements and processes of the microbial system. • Modelling results of microbial gas generation. • Evaluation of permeability and capillary seal capacity of intra-delta clay/silty layers. [ABSTRACT FROM AUTHOR]

Abstract This paper presents exponential model created by the author to estimate gas flow/time relationships for shallow methane-bearing Donetsk sandstones in Donetsk (Ukraine). Such estimation is indispensable in case of passing subway tunnels through these sandstones. The proposed model involves not only time, but also a ratio between average widths of shallow fractures in sandstones and average horizontal distances between these fractures. It is revealed that an increase in this ratio leads to an increase in gas volumetric flow rate, acceleration of gas emission, and an increase in the volume of methane emitted from borehole. It is shown that gas flow vs. time curves obtained for 17 boreholes drilled from the ground surface through sandstones are similar to those estimated by the proposed model. Observations show that the volume of methane extracted from each borehole is between 161 m3 and 673 m3, and maximum value of volumetric flow rate is ranged from 0.00265 m3/min to 0.0455 m3/min. Highlights • Methane emission from shallow sandstones was measured in-situ. • The model of gas emission in terms of time and fracture intensity is proposed. • Estimated and measured gas flow-time curves are very close to each other. [ABSTRACT FROM AUTHOR]

Drilling into offshore oil and gas fields often meets many challenges and uncertainties, such as a narrow window of drilling fluid density and shallow gas zones. Managed pressure drilling (MPD) techniques are increasingly used as alternatives to conventional drilling operations to manage such extreme conditions and reduce drilling costs and risks. Many safety and operational issues related to MPD process need to be investigated more thoroughly. Well kick is considered a typical hazardous event that may occur at different drilling phases, and such an event is prone to develop into a blowout. During offshore drilling phases, the risk of accidents may change with time, and such a dynamic characteristic should be recorded in risk assessment. This study presents a method for the application of dynamic Bayesian networks (DBNs) in conducting accident scenario analysis and dynamic quantitative risk assessment for MPD safety. This method can model the influence of uncertain risk factors, which have been ignored in existing research, by introducing an additional probability parameter. The effects of degradation are also taken into account. DBN inference is adopted to perform quantitative risk analysis and dynamic risk evolution. Then, the vulnerable root causes are identified by sensitivity analysis for accident prevention and mitigation measures. Well kick for four drilling cases is analyzed as a case study to demonstrate the feasibility of the proposed method. Three-step analysis partially validates the correctness and rationality of the proposed DBN model. [ABSTRACT FROM AUTHOR]

There has been a growing interest in the study of shallow gas due its importance in relation to the marine environment, climate change and human activities. In Bahía Blanca estuary, Argentina, shallow gas has a wide distribution. Acoustic turbidity and blanking are the main seismic evidence for the presence of shallow gas in the estuary. The former prevails in the inner sector of the estuary where gas is either near or in contact with the seabed. Gas deposits are generally associated with paleochannels corresponding to the Holocene paleodeltaic environment. Distribution studies of shallow gas in this estuary are necessary because its presence implies not only a geological risk for harbor activities but also because it may have noxious effects on the marine ecosystem, mainly on benthic communities. The comparison of benthic communities at a gas site (GS) with those at a control site (CS) indicated that gas could generate impoverishment in terms of individuals' abundance (GS: N = 357; CS: N = 724). Also, diversity indices showed great differences in the community structure at each site. This indicates that methane gas may act as a natural disturbance agent in estuarine ecosystems. The presence of gas in seabed sediments must therefore be taken into account when distribution studies are conducted of estuarine benthic communities. [ABSTRACT FROM AUTHOR]

Shallow gas, here defined as free gas that is trapped in unconsolidated, deltaic and shallow marine siliciclastic sediments of Plio-Pleistocene age, is found within the topmost 1000 m of sediment in the southern North Sea. Shallow amplitude anomalies in seismic reflection data are likely due to the presence of gas. The most prominent and easy-to-recognize indicators are high-amplitude anomalies, or “bright spots”, that are widespread within the southern North Sea. Gas from shallow reservoirs is currently produced offshore The Netherlands. In this study, we determine whether there are analogous shallow gas accumulations within the German North Sea. Therefore, we screened 2D and 3D multichannel seismic data for shallow amplitude anomalies. Several clusters of bright spots are identified above salt domes that closely resemble the economic deposits known in the Dutch sector in both size and their characteristic multilayered shape. Three of these potential gas accumulations, occurring in combination with additional hydrocarbon indicators such as seismic attenuation and velocity push-downs, are investigated in detail and compared to shallow gas fields from offshore The Netherlands. Amplitude anomalies indicate gas seepage on the seafloor that may contribute to the atmospheric methane budget and may have an impact on offshore infrastructure. [ABSTRACT FROM AUTHOR]

Shallow gas migration along hydrocarbon wells constitutes a potential methane emission pathway that currently is not recognized in any regulatory framework or greenhouse gas inventory. Recently, the first methane emission measurements at three abandoned offshore wells in the Central North Sea (CNS) were conducted showing that considerable amounts of biogenic methane originating from shallow gas accumulations in the overburden of deep reservoirs were released by the boreholes. Here, we identify numerous wells poking through shallow gas pockets in 3-D seismic data of the CNS indicating that about one-third of the wells may leak, potentially releasing a total of 3-17 kt of methane per year into the North Sea. This poses a significant contribution to the North Sea methane budget. A large fraction of this gas (∼42%) may reach the atmosphere via direct bubble transport (0-2 kt yr-1) and via diffusive exchange of methane dissolving in the surface mixed layer (1-5 kt yr-1), as indicated by numerical modeling. In the North Sea and in other hydrocarbon-prolific provinces of the world shallow gas pockets are frequently observed in the sedimentary overburden and aggregate leakages along the numerous wells drilled in those areas may be significant. [ABSTRACT FROM AUTHOR]

The Changjiang (Yangtze) is one of the largest rivers in theworld. It formed a huge incised valley at its mouth during the Last Glacial Maximum; the incised-valley fill, approximately 80-110 m thick, supplies an important foundation for the generation of shallow biogenic-gas reservoirs. Twocores and 13 cone penetration testswere used to elaborate the characteristics, formation mechanism, and distribution of the shallow biogenic-gas reservoirs in the study area. The natural gas ismainly composed ofCH4 (generally[95%)with a δ13CCH4 and δ13CCO2 of -75.8 to -67.7% and -34.5 to -6.6%, respectively, and a δDCH4 of -215 to -185%, indicating a biogenic origin by the carbon dioxide reduction pathway. Commercial biogenic gas occurs primarily in the sand bodies of fluvial-channel, floodplain, and paleo-estuary facies with a burial depth of 50-80 m. Gas sources as well as cap beds are gray to yellowish-gray mud of floodplain, paleoestuary, and offshore shallow marine facies. The organic matter in gas sources is dominated by immature type III kerogen (gas prone). The difference in permeability (about 4-6 orders of magnitude) between cap beds and reservoirs makes the cap beds effectively prevent the upward escape of gas in the reservoirs. This formation mechanism is consistent with that for the shallow biogenic gas in the late Quaternary Qiantang River incised valley to the south. Therefore, this study should provide further insight into understanding the formation and distribution of shallow biogenic gas in other similar postglacial incised-valley systems. [ABSTRACT FROM AUTHOR]

Detailed reviews of multichannel seismic reflection, sparker, chirp and multibeam data that were collected on the southern Marmara Sea shelf revealed various shallow gas indicators and related sedimentary structures, including enhanced reflections, seismic chimneys, acoustic blanking, bright spots, pockmarks, mound-like features and seeps. Seismic attribute analyses were applied to characterise the existence of gas-bearing sediments. The distribution of shallow gas indicators provides important insights into their origin and the geological factors that control them. Prominent gas accumulations and seeps are observed along the profiles that cross the branches of the central segment of the North Anatolian Fault Zone, which indicates that the gas seeps are controlled by active faulting. This indicates that the faults act as conduits through the sedimentary column. The dense occurrences of gas directly off the river mouths along the shallow bays provide clues about the organic-rich carbon content of the sediments and biogenic methane generation. In some areas, the gas-related acoustic anomalies are mostly located in the upper sediments below the marine unit, which indicates that the gas emissions in these areas were terminated as a result of the increased overburden pressure after the Holocene sea level rise and the deposition of the marine unit. [ABSTRACT FROM PUBLISHER]

This study integrates high-resolution surface and sub-surface geophysical data to investigate possible fluid (gas and/or liquids) migration pathways to the seafloor in Grønfjorden, west Spitsbergen. Grønfjorden is an N-S aligned tributary fjord, located in the western part of the Isfjorden fjord system. Pockmarks were identified on the bathymetric data in the southern and northern parts of the fjord. Three types of shallow seep-related acoustic features have been identified in the marine sediments: i) enhanced reflections, ii) acoustic turbid zones, and iii) acoustic blankings. They occur beneath some of the pockmarks and circular patches of high-backscatter on the seafloor in the central part of the fjord, which are potential seep-related seafloor features. 6–8 km long NNW-SSE striking ridges have been identified in the southern part of the study area. Thrust faults were interpreted on 2D multichannel seismic data truncating the Triassic, Jurassic and Cretaceous successions of western Isfjorden, located north of Grønfjorden. A possible link has been found between these fault systems and the seafloor ridges which strike in a similar direction. Further, a normal fault has been identified within the shallow marine sediments which drapes the bedrock of the fjord. The fractured bedrock of the fjord and faults associated with the West Spitsbergen fold-and-thrust belt could possibly represent major conduits for fluid migration from deeper stratigraphy to shallow marine sediments and the seafloor. [ABSTRACT FROM AUTHOR]

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Shallow gas accumulates in coastal marine sediments when the burial rate of reactive organic matter beneath the sulfate zone is sufficiently high and the methanogenic zone is sufficiently deep. We investigated the controls on methane production and free methane gas accumulation along a 400 m seismo-acoustic transect across a sharp transition from gas-free into gas-bearing sediment in Aarhus Bay (Denmark). Twelve gravity cores were taken, in which the pore water was analyzed for inorganic solutes while rates of organic carbon mineralization were measured experimentally by 35 SO 4 2− radiotracer method. The thickness of organic-rich Holocene mud increased from 5 to 10 m along the transect concomitant with a shallowing of the depth of the sulfate-methane transition from >4 m to 2.5 m. In spite of drastic differences in the distribution of methane and sulfate in the sediment along the transect, there were only small differences in total mineralization, and methanogenesis was only equivalent to about 1% of sulfate reduction. Shallow gas appeared where the mud thickness exceeded 8–9 m. Rates of methanogenesis increased along the transect as did the upward diffusive flux of methane. Interestingly, the increase in the sedimentation rate and Holocene mud thickness had only a modest direct effect on methanogenesis rates in deep sediments. This increase in methane flux, however, triggered a shallowing of the sulfate-methane transition which resulted in a large increase in methanogenesis at the top of the methanogenic zone. Thus, our results demonstrate a positive feedback mechanism that causes a strong enhancement of methanogenesis and explains the apparently abrupt appearance of gas when a threshold thickness of organic-rich mud is exceeded. [ABSTRACT FROM AUTHOR]

Methane gas explosions and spontaneous combustion of coal severely threaten mining production safety and efficiency. Methane drainage can reduce the risk of a methane explosion while it may intensify the self-ignition of coal. To understand the relationship between these two issues, a working face of the Xinji No.2 Coal Mine in China was studied. A model of high drill holes extraction (HDHE) is established and the methane concentration distribution and air leakage under four different drainage pressures (below atmospheric) were determined. The simulation results show that drainage pressures can control the methane concentration in the air, so as not to exceed a threshold value. However, these drainage pressures enlarged the size of the zone where spontaneous combustion could occur due to the increase in air leakage. The possibility of the coexistence of a methane explosion and spontaneous coal combustion was proposed. This is an important index for the selection of a reasonable methane drainage pressure. The monitoring data from the mine were used to validate the model. [ABSTRACT FROM AUTHOR]

High-resolution seismic survey was conducted to investigate acoustic characteristics of gassy sediments along the southern shelf of the Sea of Marmara. The acoustic turbidity zones outlined within the study area are generally below 2-9 m (2-10 ms TWT) the seafloor whilst this vertical distance varies between 9 and 21 m (10-25 ms TWT) for acoustic blanket type reflections. The gassy sediments cover an area of sea floor of about 45, 110, and 75 km2 in front of Gönen River, Kocasu River, and Gemlik Bay, respectively. The gassy sediments in the center of Gemlik Bay exhibited an elliptical geometry similar to its basin while the others have deltaic forms in front of the rivers. The sea bottom and near surface sedimentary units are made-up of organic-rich sediments, mostly transported by the southern rivers. The gas observed in sediments is thought to be of biogenic origin, which may be caused by degradation of organic matter in the sediment. [ABSTRACT FROM AUTHOR]

We present new data on the study of subsurface gas accumulations in the sediments of Amurskiy Bay (the Sea of Japan). Geophysical research (single-channel continuous seismic profiling with airguns and high-resolution acoustic investigations with a high-frequency GeoPulse profiler) shows that the bay depression is filled by Pre-Quaternary sediments that are at least 0.8 s (TWT) thick and overlaying Late Pleistocene–Holocene sediments that are up to 30 m thick. The Pre-Quaternary deposits are possibly represented by coal-containing Mesozoic and Cenozoic rocks. Gas-related acoustic anomalies were found in the Late Pleistocene–Holocene silty sediments in the northern part of the bay. These gas-related anomalies have been divided into blanket column, turbidity and pillar anomalies. A single pockmark was also observed in the bottom relief of the bay. The gas accumulation area in the sediments is 19 km long and 12 km wide. The degassing of the Pre-Quaternary coal-containing deposits is assumed to be the major cause of gas. The formation of most of the gas anomalies in the Late Pleistocene–Holocene sediments and the trigger mechanism of gas migration may be associated with sea-level changes, paleoceanographic conditions, sedimentation features and the hydrodynamic regime in the bay. Increases in the number of diatoms and the presence of aberrant forms of some silicoflagellates have been observed in areas with high gas contents. Areas with numerous small underwater elevations or mounds were distinguished in the bay. In the northern part of the bay, the mounds correspond to ostraceous reefs or other local shellfish colonies. In the southern part of the bay, the elevations could be related to methane-derived authigenic carbonates, which formed pavements for the fixation of shellfish colonies and construction of reefs. [ABSTRACT FROM AUTHOR]

The Ría de Vigo (NW Spain) has a high organic matter content and high rates of sedimentation. The microbial degradation of this organic matter has led to shallow gas accumulations of methane, currently distributed all along the ría. These peculiar characteristics favor the development of anoxic conditions that can determine the dynamics of iron and manganese. In order to study the role played by iron and manganese in the processes that take place in anoxic sediments with shallow gas, four gravity cores were retrieved in anoxic sediments of the Ría de Vigo in November 2012. Methane was present in two of them, below 90 cm in the inner zone and below 200 cm, in the outer zone. Pore water was collected and analyzed for vertical profiles of pH, sulfide, sulfate, iron and manganese concentrations. Sulfate concentrations decreased with depth but never reached the minimum detection limit. High sulfide concentrations were measured in all cores. The highest sulfide concentrations were found in the inner zone with methane and the lowest were in the outer zone without methane. Concentrations of iron and manganese reached maximum values in the upper layers of the sediment, decreasing with depth, except in the outer zone without gas, where iron and manganese concentration increased strongly toward the bottom of the sediment. In areas with shallow gas iron reduction, sulfate reduction and methane production processes coexist, showing that the traditional redox cascade is highly simplified and suggesting that iron may be involved in a cryptic sulfur cycle and in the oxidation of methane. [ABSTRACT FROM AUTHOR]

The principal objective of this paper is to understand the gas migration process and the structures of mud volcanoes in the central Black Sea. Multichannel seismic lines were acquired across 6 mud volcanoes in the central Black Sea: MSU, Yuzhmorgeologiya, Malyshev, Kornev, Goncharov and Vassoevitch. Based on a high resolution seismic processing, we analyzed acoustic anomalies and studied near-surface sediment structures of these mud volcanoes. Four types of pathways for gas and fluid migration and three types of gas reservoirs were recognized. A regional “Bottom Simulating Reflection” (BSR) seems to be absent in most parts of the study area, however a clear BSR was observed in one of the seismic profiles. The free gas migrating upwards along these pathways were sealed by gas hydrates or fine-grained sediments in the gas hydrate stability zone. Four seismic units were separated according to a suggested age model and identified seismic facies. Combining with the possible sedimentary processes of the central Black Sea, we try to reveal the active mechanism of these mud volcanoes. These mud volcanoes reveal two to three major active stages. These active stages might be related to distinct sea level falls, which seem to be one of the main trigger factors of the mud volcano eruptions in the central Black Sea. A structural model for the MSU mud volcano was presented with respect to mechanisms of gas migration and the origin of the gas. [ABSTRACT FROM AUTHOR]

Semi-enclosed areas, such as rías , offer low-energy sedimentary conditions, which allow the necessary environment for the deposition of clays and organic-rich deposits that generate gas through methanogenic bacteria. In Saco do Mamanguá (Rio de Janeiro, Brazil), acoustic data from low frequency echo sounder and high-resolution seismic-reflection data from chirp sonar and boomer from three surveys – February 2009 and January and June 2014 – enabled for the first time the identification of acoustic disturbances related to gas occurrences. Saco do Mamanguá hosts shallow gas accumulation throughout its entire area, expressed as acoustic turbidity, enhanced reflections or acoustic blanking, intra-sedimentary plumes and acoustic plumes. However, gas occurrence evidence decreases towards the open ocean. A lithological control occurs on the sealing layer in the inner area, whereas the smaller gas occurrence in the outer area can be due to (1) absence of a sealing layer and/or (2) the re-oxidizing of methane as more oxygenated waters are available. We propose a sequence stratigraphic model in which the gas formation is Holocene or either pre-Holocene age. [ABSTRACT FROM AUTHOR]

Chemical reactions of molecular gases like methane are so complicated that a chart of decomposed and/or synthesized species originating from molecules in plasma resembles a weblike network in which we write down species and reactions among them. Here we consider properties of the network structures of chemical reactions in methane plasmas. In the network, atoms/molecules/radical species are assumed to form nodes and chemical reactions correspond to directed edges in the terminology of graph theory. Investigation of the centrality index reveals importance of CH3 in the global chemical reaction, and difference of an index for each radical species between cases with and without electrons clarifies that the electrons are at an influential position to tighten the network structure. [ABSTRACT FROM AUTHOR]

SO 2 camera measurements at Kīlauea Volcano, Hawaii, in May of 2010 captured two occurrences of lava lake rise and fall within the Halema'um'au Crater summit vent. During high lava stands we observed diminished SO 2 emission rates and decreased seismic tremor. Similar events at Kīlauea have been described as the result of sporadic degassing following gas accumulation beneath a mostly impermeable lava lake surface. Incorporation of SO 2 camera data into a multi-parameter dataset gives credence to the likelihood of shallow gas accumulation as the cause of these high stand events, with accumulated gas release upon lake-level drop compensating for the gas deficit reached during accumulation. [ABSTRACT FROM AUTHOR]

Shallow gas accumulations and gas seepage may indicate deeper hydrocarbon reservoirs. Sonar and high resolution seismic recordings were carried out to obtain gas migration and accumulation below seabed in Northeast Persian Gulf. Results revealed some structures and reflectivity effects, for example few pockmarks, gas seepage, and other surface structures in sonar and seismic data. Most of the features observed on seismic data were located at depths between 5 and 10 m (sub-sea bed) that include acoustic blanking and turbidity, gas packets, gas chimneys, and enhanced reflection. Most of the gas is accumulated on young and thin parallel layers, but the presence of acoustic effect in Aghajari Formation indicates that the origin of the gas accumulation in shallow strata may be located in a deeper reservoir. [ABSTRACT FROM AUTHOR]

Conventional 3D seismic data provides direct evidence for glacial influence during the early Pleistocene sedimentation in the Central North Sea. We identify iceberg ploughmarks as dim linear to curve-linear features in three early Pleistocene horizons that have high reflection amplitude compared to adjacent horizons. The anomalous horizons are interpreted to be reflections from thin sandy layers saturated with gas. The gas acts as a contrast liquid illuminating the thin sand layers. The reason for this is the difference in acoustic properties between water and gas saturated sand layers. The combination of thin bed effects and shallow gas makes the iceberg ploughmarks easily detectable as dim features in seismic reflection amplitude maps. Our interpretation is based on analysis of real seismic data, well logs and modeled seismic response. The methods we use include interpretation of horizons followed by extraction of reflection amplitudes, well log analysis, pre-stack amplitude versus offset analysis of high resolution 2D seismic data and time-lapse analysis of seismic. Seismic modeling is performed to study interactions between thin sand beds, shallow gas and iceberg ploughmarks. A new trapping mechanism for shallow gas is presented and seismic modeling of this trap strengthens our interpretation. The trap is created by iceberg ploughmarks in sandy layers that are covered by finer and less permeable sediments. For this area we find that conventional seismic interpretation is superior to the much used method of studying seismic time-slices for detection of iceberg ploughmarks, both with respect to time and detectability. This study shows that the interpreter should look for high amplitude horizons with amplitude variations laterally when trying to detect iceberg ploughmarks. [ABSTRACT FROM AUTHOR]

Short and sparse vegetation near shallow gas wells has generally been attributed to residual effects from well construction, but other mechanisms might also explain these trends. We evaluated effects of distance to shallow gas wells on vegetation and bare ground in mixed-grass prairies in southern Alberta, Canada, from 2010 to 2011. We then tested three hypotheses to explain why we found shorter vegetation and more bare ground near wells, using cattle fecal pat transects from 2012, and our vegetation quadrats. We evaluated whether empirical evidence suggested that observed patterns were driven by (1) higher abundance of crested wheatgrass ( Agropyron cristatum) near wells, (2) residual effects of well construction, or (3) attraction of livestock to wells. Crested wheatgrass occurrence was higher near wells, but this did not explain effects of wells on vegetation structure. Correlations between distance to wells and litter depth were the highest near newer wells, providing support for the construction hypothesis. However, effects of distance to wells on other vegetation metrics did not decline as time since well construction increased, suggesting that other mechanisms explained observed edge effects. Cattle abundance was substantially higher near wells, and this effect corresponded with changes in habitat structure. Our results suggest that both residual effects of well construction and cattle behavior may explain effects of shallow gas wells on habitat structure in mixed-grass prairies, and thus, to be effective, mitigation strategies must address both mechanisms. [ABSTRACT FROM AUTHOR]

Research on gas-charged sediments is generally based on identification and mapping with hydroacoustic methods, and seismic sections are typically verbally described. The terminology, however, does often not properly account for different instrumentation, as the seismic response of gas-charged sediments is frequency-dependent, and seismic appearance may vary with the instrumentation used. We therefore combine and compare the expressions of shallow gas in high frequency Parasound data and lower frequency multichannel seismic data for the Bornholm Basin, Baltic Sea, to generalize the descriptional approach and to indicate distinct imaging differences. Gas bubbles strongly scatter the acoustic energy of a high frequency signal, shallow gas appears therefore in Parasound data as a cloud of point scatterers and acoustic blanking. The seismic waves undergo reflection from the gassy sediment, but with sufficient transmission, and the top of the gassy sediment appears as a reversed polarity reflector. In seismic data, free gas can modify the seafloor reflection in different ways depending on the proximity of the gas front to the seafloor. High reflectivity at this interface causes reverberation of the water bottom reflector. Gas signatures also reveal relative measures about the amount and concentration of gas bubbles, distribution of bubble sizes, and the depth distribution of gas. [ABSTRACT FROM AUTHOR]

Highlights: [•] Submarine canyons can be formed by pockmark activities. [•] Sea level falls can trigger dissociation of gas hydrate. [•] Pockmarks and canyons are active during times of lowered sea-level. [•] Shallow gas hydrate history can be inferred based on submarine canyon development. [Copyright &y& Elsevier]

Abstract: We report on the application of seismic noise investigations, including H/V (horizontal to vertical) spectral ratio and array techniques, to a shallow gas-rich geothermal reservoir in Heybeli, southwestern Turkey. Fundamental resonant frequencies were determined to estimate the sediment thickness. Using small-scale seismic arrays, phase velocity dispersion curves were derived by correlating noise recordings according to the extended spatial autocorrelation method. Improved shear wave velocity profiles were estimated by combining Rayleigh wave dispersion curves and horizontal to vertical spectral ratios in a joint inversion. We found that the velocities obtained for the reservoir site are higher than those for a location outside the reservoir. In addition to the fundamental resonant peaks in the spectra, a clear 6-Hz-signal could be identified originating from the center of the geothermal field, repeatedly observed in 2010 and 2011. It had been claimed that low frequency (1–10Hz) seismic signal anomalies were correlated with the occurrence of hydrocarbons. One of the physical mechanisms under consideration to explain these tremor-like signals above such reservoirs is resonant amplification due to the oscillation of bubbles. Based on the signal similarity with volcanic tremors, it is not a priori given that the liquid phase must be oil for resonance effects to occur. We therefore applied array techniques to identify potential noise originating from the Heybeli reservoir. In fact, the frequency–wavenumber (f–k) method clearly indicated a noise source coming from the main production well of the reservoir. In 2011, as part of our assessment, the operators of the spa facility stopped the extraction of thermal water for 2h: the 6-Hz-signal disappeared after the pump had been stopped and reappeared after the pump began operating again. Thus, the 6-Hz-signal is likely of artificial origin. In addition, no natural noise source inside the reservoir could be identified. [Copyright &y& Elsevier]

The crest of the Absheron anticline in the South Caspian Basin at a few hundred meters below the present seafloor shows a subcircular depression about 8 km in diameter and 200 m deep, bounded by steep edges dipping 15° to 45° into it. The depression and the surrounding series are respectively filled and overlain by a regional mass-transport deposit (MTD) 150 m thick outside the depression and 300 m thick inside, composed mostly of extensional blocks. Geometric and stratigraphic analyses indicate that 150 m of initially deposited sediment were removed from a closed area after burial. Seismic evidence of shallow gas accumulations below the crater-like feature suggests that gas likely played a significant role in its development. The model proposed for the emplacement of the crater is that the gas-bearing cover of a shallow gas reservoir underwent exsolution when its overburden thinned during an episode of extensional slope failure. This resulted in loss of resistance to shear and evacuation of the gas-bearing sediment, likely at the shearing base of the failed mass. This evacuation feature is considered an example where the presence of gas locally governs the morphology of an MTD. The interpreted process shows a positive feedback between slope failure and loss of strength at the base of the resulting MTD. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

Abstract: Analysis of multi-channel seismic reflection and chirp data from Lake Van (eastern Turkey) reveals various shallow gas indicators including seismic chimneys, enhanced reflections, bright spots, mud volcanoes, pockmarks, and acoustic blanking. The enhanced reflections, suggesting the presence of free gas, are most dominant and observed at more than 200 locations. They are characterized by very-high amplitude reflections and occur in both deep and shallow sedimentary sections. Some enhanced reflections are accompanied by very subtle seafloor expressions such as mounds, which may suggest active venting activity. Seismic chimneys or columnar zones of amplitude blanking have been observed in much of the surveyed area. Seismic chimneys in the study area cannot be associated with any known faults that would act as migration pathways for deep fluids. This suggests that the observed structures in Lake Van sediments allow the preferential emission of gases which might be for a large share of biogenic origin. The acoustic blanking, characterized by transparent or chaotic seismic facies, is seen in the eastern part of the lake. The lakeward edge of the acoustic blanking largely coincides with the 100 m water depth contour, indicating that (past) changes of the hydrostatic pressure may be responsible for the distribution of these anomalies. Mound-like features, interpreted as mud volcanoes, occur in a few locations. The presence of these features may suggest active gas emission. Very strong amplitude anomalies or bright spots with negative polarity, indicating gas-charged zones, are also seen in a number of locations. Pockmarks are observed only in the northeastern part of the study area. The scarce occurrence of pockmarks in the study area might be ascribed to a higher permeability of the lake sediments or to the absence of the substrate/reservoir providing the critical mass of gases necessary to produce such features. Turbidites, tephra layers, and deltaic deposits have the potential to provide ideal conditions to allow the sediments to act as a gas reservoir. [Copyright &y& Elsevier]

Abstract: Late Quaternary shallow-biogenic gas reservoirs are well-developed in the Qiantang River incised valley, eastern China. They present as sand bodies in the floodplain–estuary facies. Silty clay beds of floodplain–estuary facies and mud beds of estuarine–shallow marine facies serve as the direct and indirect cap beds, respectively, and the former has much better sealing ability. Capillary pressure sealing, pore water pressure sealing, and hydrocarbon concentration sealing contribute to the conservation of shallow-biogenic gas in the reservoirs, in which the pore water pressure sealing may be the most important factor. Here the capillary pressure sealing mechanism is clarified by an indirect method, i.e., the difference of permeability between the cap beds and reservoirs, as the displacement pressure of late Quaternary sediments which is unconsolidated is difficult to be tested. The cap beds have higher capillary pressure than the reservoirs for the far lower permeability. Pore water pressure of cap beds exceeds the total of pore water pressure and original gas pressure of underlying reservoirs. Disequilibrium compaction, swelling of clay minerals and gas generation are responsible for the generation of high pore water pressure. In addition, the hydrocarbon content of the cap beds is higher than that of the reservoirs which could restrain the flow up of gas in the reservoirs. [Copyright &y& Elsevier]

Abstract: This study highlights the potential of using a low frequency multibeam echosounder for detection and visualization of shallow gas occurring several meters beneath the seafloor. The presence of shallow gas was verified in the Bornholm Basin, Baltic Sea, at 80 m water depth with standard geochemical core analysis and hydroacoustic subbottom profiling. Successively, this area was surveyed with a 95 kHz and a 12 kHz multibeam echosounder (MBES). The bathymetric measurements with 12 kHz provided depth values systematically deeper by several meters compared to 95 kHz data. This observation was attributed to enhanced penetration of the low frequency signal energy into soft sediments. Consequently, the subbottom geoacoustic properties contributed highly to the measured backscattered signals. Those appeared up to 17 dB higher inside the shallow gas area compared to reference measurements outside and could be clearly linked to the shallow gas front depth down to 5 m below seafloor. No elevated backscatter was visible in 95 kHz MBES data, which in turn highlights the superior potential of low frequency MBES to image shallow sub-seafloor features. Small gas pockets could be resolved even on the outer swath (up to 65°). Strongly elevated backscattering from gassy areas occurred at large incidence angles and a high gas sensitivity of the MBES is further supported by an angular response analysis presented in this study. We conclude that the MBES together with subbottom profiling can be used as an efficient tool for spatial subbottom mapping in soft sediment environments. [Copyright &y& Elsevier]

Abstract: We have studied the growth and properties of SiC films on Si wafers, under ultrahigh vacuum background conditions, using a remote-, microwave excited, methane plasma as a source of active carbon and hydrogen, while the Si substrates were held at a temperature of near 700°C. The reaction is diffusional and should thus be self-limiting, but it actually reaches up to 20–25nm thickness due to direct transport of Si from the substrate to the top surface through initially open defects in the growing film. These defects gradually fill up during the growth, and the resulting films have a relatively low density of these, and are otherwise very uniform and polycrystalline. They are characterized with scanning electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and hardness measurements. [Copyright &y& Elsevier]

The presence of shallow gas constitutes a drilling hazard (several gas blow outs occurred recently in shallow wells in Lower Austria), but provides also a valuable tool for hydrocarbon exploration. 65 gas shows, 12 oil and gas shows, and 18 oil shows are known in Lower Austria at the surface or at shallow depth (< 300 m below surface; b.s.). Most gas shows are located in the Vienna Basin (39) followed by the Molasse Basin (28) and the Flysch Zone (13). Most oil (and gas) shows occurred in the Vienna Basin (11), the Northern Calcareous Alps (8) and the Flysch Zone (7). In addition five gas fields are present in Lower Austria, where at least one reservoir horizon is at a depth of less than 300 m b.s. Shallow hydrocarbon shows in the Vienna Basin are connected to the foot- and hangingwall of the Steinberg, Pirawarth and Leopoldsdorf faults and hydrocarbon fields in the central part (Matzen, Fischamend) of the basin. There are only a few oil and gas shows without obvious connection to known deposits. Some hydrocarbon shows contain hydrogen sulfide, which is probably linked to the presence of Permo-Triassic gypsum/anhydrite in underlying units. The gas shows in the Molasse Basin east of the Bohemian Massif coincide with a hydrocarbon field and coal-bearing Permo-Carboniferous graben sediments. Numerous oil and gas shows are located along a north-south trend in the southern Waschberg Zone and the northeasternmost Alps (Flysch Zone, Northern Calcareous Alps). The regional distribution suggests an origin from Gresten Formation of Doggerian age. A hydrogen-rich gas show near the southern margin of the Northern Calcareous Alps is linked to evaporitic Permo-Triassic rocks near the base of the Northern Calcareous Alps. Most of the gas shows in the Molasse Basin west of the Bohemian spur are located in an area close to the river Enns. With the current data it is impossible to decide whether the gas is of biogenic origin or formed by degradation of preexisting oil generated from Oligocene source rocks (Schöneck Formation). Whereas oil (and gas) shows occur in the Klippen Belt and the Inneralpine Molasse, pure gas shows, partly with high pressure, have been recorded in the Flysch Zone. Oil shows in the Gresten Klippen Belt are probably related to coals of the Jurassic Gresten Formation. An oil seep initiated drilling of the well Urmannsau 1 in the Northern Calcareous Alps, which encountered non-commercial oil and gas. Biomarker data suggests a source rock with relatively low maturity deposited in an anoxic environment and a significant contribution of terrestrial plants. The knowledge of the areal distribution of shallow gas accumulations is essential to define zones with a very high risk to encounter gas at shallow depths. [ABSTRACT FROM AUTHOR]

We demonstrate an on-chip sensor for room-temperature detection of methane gas using a broadband spiral chalcogenide glass waveguide coupled with off-chip laser and detector. The waveguide is fabricated using UV lithography patterning and lift-off after thermal evaporation. We measure the intensity change due to the presence and concentration of methane gas in the mid-infrared (MIR) range. This work provides an approach for broadband planar MIR gas sensing. [ABSTRACT FROM AUTHOR]