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Natural gas hydrates are a clean and efficient new energy source with the potential to replace conventional energy, holding significant production value. Studying hydrate accumulation systems is fundamental for hydrate resource evaluation and target reservoir selection. The Shenhu area of the South China Sea has abundant hydrate resources, and drilling data show that the hydrate distribution in this area exhibits noticeable heterogeneity. Aiming at this phenomenon, we used a novel thermo-flow/chemical coupled simulator to simulate the hydrate accumulation system based on the actual geological strata and provide a detailed depiction of the evolution of components in the process. The results indicate that favorable migration conditions can accelerate hydrate formation. However, to achieve the rapid formation of thick, high-saturation hydrate layers, the fluid migration conditions must be complemented by effective fluid aggregation conditions. Furthermore, a sensitivity analysis of the fault morphology was conducted, revealing that larger-scale faults are more conducive to rapid hydrate accumulation. In summary, this study provides a quantitative analysis of the hydrate accumulation process and its key influence factors using a novel simulator, offering theoretical support for resource evaluation and an exploration of hydrate distribution. [ABSTRACT FROM AUTHOR]

Dynamic triggering refers to the phenomenon of teleseismic waves propagating to some areas in a near-critical stress state, thus triggering earthquakes. Shallow earthquakes are the type of earthquakes that are most commonly triggered. Researching the dynamic triggering of shallow earthquakes can enhance understanding of the seismogenic mechanism and produce additional information about the state of shallow faults, which has great importance for earthquake prevention and disaster reduction. This article reviews possible mechanisms and detection methods of dynamic triggering, focusing on several case studies of triggered shallow earthquakes in different tectonic settings. Furthermore, by using continuous recordings from a dense seismic array in the Weiyuan area of Sichuan Province from November 2015 to November 2016, we investigated whether seismic activities in the shale gas extraction areas were influenced by the dynamic triggering of teleseismic waves. The results revealed that the seismicity in the area increased significantly within 12 hours of the MW7.4 Tajikistan earthquake on December 7, 2015, and exhibited characteristics of dynamic triggering. However, after carefully analyzing the temporal and spatial distribution of the earthquakes in the Weiyuan area following the Tajikistan earthquake, we found that the migration of the enhanced seismic activities exhibited characteristics of fluid diffusion, suggesting that these shallow earthquakes were more likely induced by industrial injection.

The paper presents geochemical study of bottom sediments from the MSU structure located on the large Gydratny Fault in the Central Basin of Lake Baikal at a depth of 1380 m. The first detailed data on the spatial variations in the qualitative and quantitative composition of the pore waters are presented. Pioneering data were obtained on Li, B, and Sr contents in the pore water of the sediments. It has been established that fluids are actively discharged within the MSU structure, and the main pathways of their near-surface migration are confined to the tops of hills of this structure on the downthrown fault block. The fluids are highly mineralized (up to 2900 mg/L), showing the highest mineralization ever found in Lake Baikal sediments. The waters are significantly enriched in Mg, Li, B, and Sr but depleted in K. The waters are thought to be generated by the processes of authigenic formation and illitization of smectite at depths of 1 to 2.5 km in the sedimentary sequence. The maximum values of concentration gradients are recorded in the pore waters of the sediments of the western hill, which may indicate a gradual westward shift of the center of the fluid seepage activity along the fault. [ABSTRACT FROM AUTHOR]

Coseismic rupture and aftershock development on a fault plane are complex and heterogeneous processes. The Mw 6.1 L'Aquila 2009 normal faulting earthquake is a perfect case to explore how fault geometry and rheology influence the rupture process and aftershocks distribution. In this study, we use for the first time a dense set of earthquake data to obtain enhanced images of the causative normal fault structure to the kilometer scale. The hypocenter of the emergent onset of the mainshock took place within a low Vp/Vs volume, while the large coseismic slip occurred a few kilometers above, as the rupture propagated through a high Vp and high Vp/Vs fluid‐filled rock volume. The increase of Vp/Vs in the fault hanging wall during the sequence suggests a strong dehydration in the earthquake asperity, with an upward fluid pressure migration along the fault toward the host rock volume. We propose that the localization of deformation on the fault plane is favored by high fluid pressure, while the spreading of aftershocks on a wide volume around the fault is driven by the depletion of fluids from the slipped portion of the fault plane and migration to small segments within the fault host rocks. Plain Language Summary: The relationship between coseismic slip during mainshocks, aftershock distribution and fault geometry and rheology is fundamental to understanding the earthquake's nucleation process. Here, we compute high‐resolution Vp and Vp/Vs models for the L'Aquila fault ruptured in 2009 using a vast data set of aftershocks and a double‐difference tomographic method. We compare aftershock distribution with velocity anomalies on the fault plane and surrounding volume, observing that the localization of deformation on the fault plane and the dispersion of aftershocks within the host volume is favored by high fluid pressure. Transient changes of Vp and Vp/Vs during the first month of aftershocks reveal an increase of pore pressure in the upper tip of the fault and in the surrounding host rock, generated by fluid depletion in the earthquake asperity. Key Points: We used about 14,000 aftershocks to compute high‐resolution Vp and Vp/Vs models of the Mw 6.1 L'Aquila 2009 causative faultThe localization of deformation on the fault plane is favored by high fluid pressureThe depletion of fluids from the slipped portion of the fault plane drives the spreading of the aftershocks on a wide volume around the fault [ABSTRACT FROM AUTHOR]

Monitoring underground fluid migration caused by injection/production processes is crucial for guiding petroleum exploitation in mature oilfields and ultimately enhancing petroleum production. In this paper, we propose a time-lapse reverse time imaging (RTI) to dynamically monitor the injection/production processes within oilfield. By using RTI to track microseimicities at different time periods, we can analyze the relationship between injection/production activities and the spatiotemporal changes in microseismic distribution. The inferred relationship enables the time-lapse RTI to infer fluid migration patterns within oil reservoirs. To assess the accuracy and spatiotemporal resolution of the time-lapse RTI, we conducted numerical experiments to evaluate the imaging quality under different microseismic distribution scenarios. In addition, we assessed the method's stability under low signal-to-noise ratio conditions. Numerical results indicate that the time-lapse RTI can effectively distinguish the spatiotemporal variations of seismic swarms at depths of 0.5 kilometers within the target layer, even in the presence of strong noise. Practical applications show a significant correlation between changes in swarm distribution surrounding reservoirs and fluctuations in oil production. Using time-lapse RTI enables real-time monitoring of oilfield injection/production processes, thereby offering valuable insights for optimizing oilfield development and fostering future increases in petroleum production. [ABSTRACT FROM AUTHOR]

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Ambient noise correlation analyses are largely used to monitor temporal medium changes generally associated with stress field variations and/or fluid movement. Here, we analyze the 2021 eruption of La Palma, the most active island in the Canary archipelago, to study its effects on the structure in a post-eruptive stage. To date, most of the studies, whether in volcanic environments or other geological systems, focus on determining seismic velocity changes that can be associated with precursory signals. In our study, we are interested in localizing the medium changes that permit constraining the depth of the most affected structure and suggesting possible mechanisms capable of inducing such alterations. The auto- and cross-correlation functions were computed using the phase cross-correlation strategy. The correlations were linearly stacked using a 3-day sliding window. The combination of these two approaches proved to render the best results. The analysis of 3 years of data resulted in the detection of occasional decorrelation before the eruption, followed by a well-defined decoherence period after the eruption. In addition, the relationship between the waveform correlation and lag time, using autocorrelations from before and after the eruption, permits identifying phase shifts and waveform distortion, which are sensitive to different parameters and, thus, have great importance in inferring the possible mechanism. Phase shifts occur when there is only velocity change without changing the structure, whereas waveform distortion is caused by a structural (geological) change. We also inferred the depths at which the most significant medium alterations occur. We observed that the decorrelation occurs at lag times corresponding to changes localized in depth. The surface structure appears to not have undergone significant medium changes for depths until approximately 8 km, either before or after the eruption. [ABSTRACT FROM AUTHOR]

The internal fault architecture is crucial in assessing the significance of faults in fluid migration. The development of overlapping zones between segments and subsidiary structures is characteristic of a strike–slip faults. However, their internal architectures and roles in fluid migration are still poorly understood. The Tarim Basin's recently identified strike–slip faults imply that the petroleum resource is hosted in caves that were formed by subsequent dissolution after the formation of the fault zones in carbonate rocks, indicating that the internal fault architecture may be closely linked to the accumulation of petroleum. We investigated the architecture of the strike–slip fault zone using field, geochemical, seismic and well‐logging data. The results revealed that the strike–slip faults contain flower‐like structures in their vertical profiles and an en échelon and 'X' conjugate pattern in their horizontal slices. The fault core may become more complex because of the flower structure as fault breccia, slip surfaces, hydrothermal veins, dissolved pores and caves develop, and the damage zone contains multiple stages of fractures with high dip angles. Compared with 'X' pattern conjugate faults, NE‐trending strike–slip faults have a more developed and connected fault zone. The fault core acts as a fast conduit for fluid transport and experiences significant elemental losses, and the elemental variations in the damage zone may relate in long‐term and relatively lower‐level fluid–rock interactions. Three fault zone architecture models were created, namely, a releasing bend, a restraining bend and a single segment, and their controlling impacts on fluid migration were addressed accordingly. Our findings imply that fluid migration and accumulation are more favourable at the releasing bend than at the restraining bend and single segment. [ABSTRACT FROM AUTHOR]

Pulsing emanations of native hydrogen (H 2) have been observed at the surface of emitting structures, specifically "Fairy circles" in Minas Gerais State, Brazil. However, the underlying cause of these H 2 pulses remains unclear. Possible controlling factors include deep migration processes, interactions with the atmosphere and near-surface conditions. In this study, we examine the mechanisms that may trigger pulsating fluid migration and the resulting periodicity, with particular attention to the influence of deep geological processes. We employed a numerical model to simulate the migration of a constant deep fluid flow. To ensure the accuracy of the model in solving complex fluid flows, we conducted initial simulations to compare the morphology and amplitude of 2D thermal anomalies induced by buoyancy-driven water flow within a fault zone with another numerical model. Following the validation of our model in capturing complex fluid flows, we proceeded to model the H 2 gas flow along a 1-km draining fault, intersected by a low permeable rock layer acting as a "pressure relief valve" system. Our objective is to investigate the conditions that give rise to a pulsing regime. Our results indicate that sustained surface bursts in the model only occur if: (I) a permeability with an effective-stress dependency is considered, (II) a significant contrast in permeability exists between different zones, (III) an adequately high initial effective stress state is present at the base of the low-permeability layer, and (IV) the incoming and continuous fluid flow of H 2 at depth is sufficiently low to prevent instant "opening" of the low-permeability layer due to overpressure. The predicted periodicity for surface H 2 pulses regulated by the fault-valve mechanisms is expected to range from 100 to 300 days, representing a considerably longer duration compared to the measurements observed up to now in Brazilian Fairy circles. • Examined natural H 2 pulsing flow along a fault via numerical simulation. • H 2 pulse generation linked to stress-dependent deep permeability zones. • H 2 pulse intervals span months, longer than prior São Francisco basin data. [ABSTRACT FROM AUTHOR]

Natural gas hydrates are a clean and efficient new energy source with the potential to replace conventional energy, holding significant production value. Studying hydrate accumulation systems is fundamental for hydrate resource evaluation and target reservoir selection. The Shenhu area of the South China Sea has abundant hydrate resources, and drilling data show that the hydrate distribution in this area exhibits noticeable heterogeneity. Aiming at this phenomenon, we used a novel thermo-flow/chemical coupled simulator to simulate the hydrate accumulation system based on the actual geological strata and provide a detailed depiction of the evolution of components in the process. The results indicate that favorable migration conditions can accelerate hydrate formation. However, to achieve the rapid formation of thick, high-saturation hydrate layers, the fluid migration conditions must be complemented by effective fluid aggregation conditions. Furthermore, a sensitivity analysis of the fault morphology was conducted, revealing that larger-scale faults are more conducive to rapid hydrate accumulation. In summary, this study provides a quantitative analysis of the hydrate accumulation process and its key influence factors using a novel simulator, offering theoretical support for resource evaluation and an exploration of hydrate distribution.

Structures that facilitate fluid migration are common in sedimentary basins. We document several possible hydrothermal and/or volcanic vents located above a >157 km2, late Cretaceous volcanic field in the Great South Basin, offshore New Zealand. Three of the four vents are vertically stacked, suggesting episodic re‐use of the same fluid pathway between ca. 75 and 56 Ma. A palaeo‐pockmark dated to ca. 49 Ma and free gas occurring within strata ca. 21 Myr old are located directly above these stacked vents. The spatial association of the vents, pockmark and free gas further suggests re‐use of the fluid migration pathway(s) extended for over 54 Myr. Our results imply that reutilization of fluid flow pathways can affect the distribution of fluids within basins over prolonged periods, potentially impacting hydrocarbon/geothermal exploration and geohazard assessment. [ABSTRACT FROM AUTHOR]

Ambient noise correlation analyses are largely used to monitor temporal medium changes generally associated with stress field variations and/or fluid movement. Here, we analyze the 2021 eruption of La Palma, the most active island in the Canary archipelago, to study its effects on the structure in a post-eruptive stage. To date, most of the studies, whether in volcanic environments or other geological systems, focus on determining seismic velocity changes that can be associated with precursory signals. In our study, we are interested in localizing the medium changes that permit constraining the depth of the most affected structure and suggesting possible mechanisms capable of inducing such alterations. The auto- and cross-correlation functions were computed using the phase cross-correlation strategy. The correlations were linearly stacked using a 3-day sliding window. The combination of these two approaches proved to render the best results. The analysis of 3 years of data resulted in the detection of occasional decorrelation before the eruption, followed by a well-defined decoherence period after the eruption. In addition, the relationship between the waveform correlation and lag time, using autocorrelations from before and after the eruption, permits identifying phase shifts and waveform distortion, which are sensitive to different parameters and, thus, have great importance in inferring the possible mechanism. Phase shifts occur when there is only velocity change without changing the structure, whereas waveform distortion is caused by a structural (geological) change. We also inferred the depths at which the most significant medium alterations occur. We observed that the decorrelation occurs at lag times corresponding to changes localized in depth. The surface structure appears to not have undergone significant medium changes for depths until approximately 8 km, either before or after the eruption.

Abstract Mt. Ontake is an active volcano in central Japan where phreatic eruption activity is prominent. A shallow subsurface structure in the summit region of this volcano has been scarcely studied despite its importance. Our study examines the structure from three seismic analyses: a typical P-wave velocity from the semblance of the vertical propagation of intermediate-depth earthquakes (IDEs), a layered velocity model from P- and S-wave arrival times of shallow volcanic earthquakes, and a pseudoreflection profile from the autocorrelation functions of the IDEs. Our results consistently indicate the presence of three layers, which are interpreted as younger (> 0.1 Ma) and older (0.39–0.78 Ma) eruptive deposits and the basement. A comparison of the structure with hypocentres and deformation sources suggests fluid migration controlled by the structure. Graphical Abstract

Carbonate rocks exhibit significant heterogeneity as both a source rock and reservoir. Stylolite formation plays a crucial role in the enrichment of organic matter and the migration of geofluids within carbonate rocks. In order to study the enrichment mechanism of organic matter and the geofluid migration mode in the stylolites developed in carbonate rocks, stylolite-bearing core samples from the Dachigan structural belt in the eastern Sichuan Basin were collected. The stylolites and matrix were subjected to the total organic carbon (TOC) test and Rock-Eval pyrolysis, thin-section observation under fluorescent light, whole-rock X-ray diffraction, carbon and oxygen isotope analysis, and scanning electron microscopy. The organic matter occurring in the stylolites is mainly in the form of three types: soluble organic matter, pyrobitumen, and bitumen. This suggests that the organic matter within the stylolites mainly consists of secondary migrated organic matter. The stylolites also exhibit well-developed secondary dolomite and pyrite resulting from late-diagenetic recrystallization. These minerals contribute to the preservation of intercrystalline pore spaces and fractures, providing favorable conditions for oil and gas accumulation and migration. The strong cementation observed at the contact between the stylolites and matrix, along with the presence of secondary minerals nearby, may be attributed to the fractionation of light and heavy oil components during the migration of hydrocarbon fluids from the matrix to the stylolites. The thicknesses of the stylolites vary within the bulk, indicating severe diagenesis in thinner areas. Consequently, this leads to significant fractionation effects. The fractionation of crude oil components by stylolites poses challenges for the study of definitive oil–source correlations. To overcome these challenges, future research could investigate biomarker compounds to attempt oil–source correlations. Additionally, future efforts should take into consideration the spatial variation in the crude oil properties. Understanding the role of stylolites in organic matter enrichment and geofluid migration is crucial for optimizing exploration strategies in the Sichuan Basin, a region of growing importance in the energy industry. Moreover, our findings shed light on the complex interactions within stylolite-bearing rocks, which are not limited to this specific basin. These insights offer valuable contributions to the broader field of geology and reservoir characterization, enhancing our ability to predict and interpret similar geological formations globally. [ABSTRACT FROM AUTHOR]

The Mugagangri Group (MG), located at the southern margin of the Qiangtang terrane in Tibet, is a crucial research target for understanding the subduction and accretion history of the Meso-Tethys Ocean. Extensional crack-seal veins restricted within sandstone blocks from the broken formation in the MG (Gaize) formed synchronously in the mélange formation. The primary inclusions trapped in the veins recorded multiple pieces of information during the formation of the accretionary wedge. To precisely constrain the MG subduction–accretion processes, we investigated the trapping temperature, salinity, density, and composition of the fluid inclusions within the crack-seal veins derived from the broken formation in the MG (Gaize). The primary inclusions indicate that the crack was sealed at ~151–178 °C. The salinity of the primary inclusions exhibited a well-defined average of 3.3 ± 0.7 wt% NaCl equivalent, slightly lower than the average of seawater (3.5 wt%). There were no nonpolar gases, and only H2O (low salinity) was detectable in the primary inclusions. These characteristics suggest that the syn-mélange fluids were a type of pore fluid in the shallow subduction zone, with the principal source being pore water from sediments overlying the oceanic crust. Because of mineral dehydration and compaction, the pore fluids became more diluted with H2O and fluid overpressure owing to a pore fluid pressure that was greater than the hydrostatic pressure. Subsequently, the creation of cracks through hydraulic fracturing provided a novel pathway for the flow of fluids which, in turn, contributed to the décollement step-down and underthrusting processes. These fractures acted as conduits for fluid movement and played a crucial role in facilitating these peculiar occurrences of quartz veins. The depth (~5 km) and temperature estimates of the fluid expulsion align with the conditions of the décollement step-down, thereby leading to the trapping of fluids within the sandstone blocks and their subsequent underplating to the accretionary complex. In our preferred model, such syn-mélange fluids have the potential to provide valuable constraints on the subduction–accretion processes occurring in other accretionary complexes. [ABSTRACT FROM AUTHOR]

The present study emphasizes the seismic characteristics of earthquake swarm activity of the Palghar region that occurred in the hard strata of Deccan traps of the Indian peninsula. This activity is well monitored by National Centre for Seismology (NCS) from December 2018 to October 2020. The related data inferred that initially before the swarm activation there was a barricade to stop the entry of fluids into the permeable zone. Later the fluids are injected into the fault zone at a feasible place may be near the surface at the time of swarm initiation. Further the associated fluids percolated through several channels in many directions forming a multi-dimensional system of cracks and fissures at the level of swarm progression through two distinctive clusters during the time of present study along normal fault orientations. During this process, the heterogeneous material within the subsurface layers might have experienced a decrease in the effective stress due to the increase in the pore pressure which is the resultant of the percolation of fluids within the permeable zone. The outcome of the results shows a comparable strategic increase in the seismicity after the soil saturation during and after the rainfall in the affected zone. Also, the study of the seismicity accompanying fluid injections indicates that the fluid triggering seismicity is demonstrated by the increased slope of the magnitude frequency distribution in the form of spatial b-value which has been found reliable for a catalogue for Palghar swarm activity. The coulomb stress change is also examined to find out the consistency of stress patterns with the earthquake occurrences in the Palghar region which gives the characteristics of swarm sequence. The study emphasized the fundamental and geo-mechanical behavior of fluid migration through multi-directional cracks and fissures as the cause for the evolution and further growth of the Palghar swarm activity.

It is difficult to exploit low-permeability reservoirs, and CO2 flooding is an effective method to improve oil recovery from low permeability reservoirs. However, in the process of CO2 flooding, acidic fluids dissolved in formation water will react with rock to cause dissolution and precipitation, resulting in pores and precipitates, changing the evolution law of seepage channels, destroying formation integrity, and affecting the effect of CO2 oil displacement. The change in rock's physical properties and the mass transfer law between CO2-water-rock are unclear. This paper considers the coupling effects of seepage, mechanics, and chemistry when CO2 is injected into the formation. The mass transfer model of CO2-water-rock in the geochemical reaction process is established on this basis. The physical properties of the reservoir after CO2 injection are quantitatively studied based on the microscopic mechanism of chemical reaction, and the migration law of solute in the reservoir rock during CO2 flooding under the coupling effects of multiple fields is clarified. The experimental results show that with the increase in reaction time, the initial dissolution reaction of formation rocks will be transformed into a precipitation reaction of calcite, magnesite, and clay minerals. The porosity and permeability of the rocks near the well first increase and then decrease. The far well end is still dominated by dissolution reactions, and the average values of formation porosity and permeability show an upward trend. Although the dissolution reaction of CO2-water-rock can improve the physical properties of reservoir rocks to a certain extent, the mutual transformation of the dissolution reaction and precipitation reaction further exacerbates the heterogeneity of formation pore structure, leading to the instability of CO2 migration, uneven displacement, and destruction of formation stability. The research results of this paper solve the problem of quantitative calculation of physical parameters under the coupling effect of multiple fields after CO2 injection into reservoirs and can predict the changes in formation physical properties, which can provide a certain theoretical basis for evaluating formation integrity and adjusting CO2 injection under the condition of CO2 flooding. [ABSTRACT FROM AUTHOR]

For underground mining, efficient groundwater management is one of the critical mining economics components. The region of interest, known as Tharisa Mine, is situated on the western limb of the Bushveld Igneous Complex, which is home to South Africa's premier platinum-group metal resources. This work aimed to provide the findings from the investigation and imaging of the near-subsurface hydrogeological architecture in a shallow profile using stable isotopes of water (18O and 2H) and radioactive water isotopes (3H). Regarding isotope data, 18O varied from −3.5 to 1.5‰; 2H from −24 to 4.7‰; and 3H from 2.0 to 3.4 T.U. Utilizing combined geophysical techniques, the results were verified. Additionally, the geophysical methods, including seismic refraction tomography, multichannel analysis of surface waves, electrical resistivity tomography, and magnetics, helped identify the fluid's pathways and lineaments during migration to verify the isotope results. The groundwater inflow volumes into the open pit were initially determined by integrating the following findings: the delineation of fracture systems/zones and fluid migration pathways; mining activities enhance the storage and transmission ability of the aquifer; and the main sources of water in the mine include mixing of surface and deep water sources, recycling of water possibly via lineaments, and tailings dam seepages. [ABSTRACT FROM AUTHOR]

Mt. Ontake is an active volcano in central Japan where phreatic eruption activity is prominent. A shallow subsurface structure in the summit region of this volcano has been scarcely studied despite its importance. Our study examines the structure from three seismic analyses: a typical P-wave velocity from the semblance of the vertical propagation of intermediate-depth earthquakes (IDEs), a layered velocity model from P- and S-wave arrival times of shallow volcanic earthquakes, and a pseudoreflection profile from the autocorrelation functions of the IDEs. Our results consistently indicate the presence of three layers, which are interpreted as younger (> 0.1 Ma) and older (0.39–0.78 Ma) eruptive deposits and the basement. A comparison of the structure with hypocentres and deformation sources suggests fluid migration controlled by the structure. [ABSTRACT FROM AUTHOR]

There are abundant bottom water reservoirs in China. Unlike conventional oil reservoirs, bottom water reservoirs have various problems, such as early water breakthrough, short water-free oil recovery period, and rapid water cut increase. For example, during water flooding, the injected water easily breaks into the bottom water and does not effectively displace the upper crude oil. The recovery rate is generally low. Based on this phenomenon, an experimental study of nitrogen foam flooding in bottom water reservoirs is conducted in this paper. The seepage characteristics of nitrogen foam in oil and water layers are studied through one-dimensional core tube experiments. Through two-dimensional plate oil displacement experiments, we have revealed the fluid migration and distribution characteristics in the plane and vertical directions during nitrogen foam flooding in bottom water reservoirs; additionally, we have summarized the mechanisms of nitrogen foam in bottom water reservoirs involved in improving oil recovery characteristics. The research results show that the seepage resistance of foam in the water layer is much greater than that in the oil layer, effectively increasing the displacement strength of the oil layer. During the development stage of bottom water flooding in bottom water reservoirs, the water cut increases rapidly, the bottom water coning is obvious, and the residual oil is mainly distributed between the oil wells and the upper part of the oil layer near the wellbore. During nitrogen foam flooding, the foam enters the water layer to form an effective plug so that the subsequent foam is diverted into the oil layer; additionally, the oil is displaced laterally to the production well for production. When the foam enters the oil layer, it defoams and floats to form a secondary gas cap; this effect causes displacement of the residual oil at the top and effectively improves the displacement efficiency by weeping volume of the injected fluid”

Mofettes are gas emission sites where high concentrations of CO2 ascend through conduits from as deep as the mantle to the Earth's surface and as such provide direct windows to processes at depth. The Hartoušov mofette, located at the western margin of the Eger Graben, is a key site to study interactions between fluids and swarm earthquakes. The mofette field (10 mofettes within an area of 100 m × 500 m and three wells of 28, 108, and 239 m depth) is characterized by high CO2 emission rates (up to 100 t/d) and helium signatures with (3He/4He)c up to 5.8 Ra, indicating mantle origin. We compiled geological, geophysical, geochemical, and isotopic data to describe the mofette system. Fluids in the Cheb basin are mixtures between shallow groundwater and brine (>40 g/L at a depth of 235 m) located at the deepest parts of the basin fillings. Overpressured CO2-rich mineral waters are trapped below the mudstones and clays of the sealing Cypris formation. Drilling through this sealing layer led to blow-outs in different compartments of the basin. Pressure transients were observed related to natural disturbances as well as human activities. External (rain) and internal (earthquakes) events can cause pressure transients in the fluid system within hours or several days, lasting from days to years and leading to changes in gas flux rates. The 2014 earthquake swarm triggered an estimated excess release of 175,000 tons of CO2 during the following four years. Pressure oscillations were observed at a wellhead lasting 24 h with increasing amplitudes (from 10 to 40 kPa) and increasing frequencies reaching five cycles per hour. These oscillations are described for the first time as a potential natural analog to a two-phase pipe–relief valve system known from industrial applications. [ABSTRACT FROM AUTHOR]

The unprecedented and long-lasting seismicity of Triacastela (over 25 years) attracted the interest of the research community on this stable continental region (SCR), particularly after the anomalously high 5.1 and 4.9 Mw earthquakes compared to the regional standards. The high rainfall and tide rates of this region compared to the rest of the Iberian Peninsula, in addition to the recognized existence of thermal springs and crustal fluids, motivated us to test these sources of hydroseismicity as a potential trigger mechanism for the observed seismicity in Triacastela. Based on network upgrades, we have gathered the seismic catalog in two periods for analysis (before and after year 2002). Before 2002, neither a diffusion-type earthquake migration nor any significant statistical correlation between the seismicity rates and rainfall or tides is found. After 2002, some clusters migrate to the south, suggesting the presence of fluid migration during earthquake swarms, but no diffusion-type migration is observed on longer time scales. Furthermore, we find correlation coefficients close to zero, indicating that rainfall and tides can be excluded as driving mechanisms. However, the seismic upward migration, a high b-value (1.2), and a low aftershock-productivity parameter (α = 0.9) observed in this period support the hypothesis of upward fluid migration through fracture zones. The presence of Mantellic helium-3 along the seismogenic faults and the increase of geochemical precursors in the groundwater previously to 1995 and 1997 mainshocks further support deep fluids as a source for the observed induced fluid migration seismicity in Triacastela. [ABSTRACT FROM AUTHOR]

On March 10, 2011, an Ms. 5.8 earthquake struck Yingjiang City, western Yunnan, China, causing destructive damage. Due to the very sparse distribution of seismic stations on the southwestern border of China, its seismogenic structure and mechanism remain controversial. In this study, with the aid of machine-learning-based detection and location workflow and template matching technique, we detect 10,356 events ranging from December 1, 2010, to April 30, 2011. The high-precision earthquake catalog shows that the foreshocks initiated in the extensional stepover connecting the northeast and middle segments of the Dayingjiang fault and then bilaterally extended northeast and southwest, with migration fronts that can be simulated by fluid diffusion model with diffusivities of 0.8 m2/s and 0.19 m2/s, respectively. The mainshock occurred at the southwest end of the foreshock sequence and then probably activated the northwest-trending blind fault. In addition, we determine the full moment tensor solutions for the mainshock, six large foreshocks, and one aftershock, with magnitudes ranging from 3.03 to 5.80, in which the mainshock was characterized by an obvious negative isotropic (ISO) component. The static Coulomb failure stress change caused by five Mw ≥ 4.0 foreshocks on the mainshock fault plane is ∼24 kPa, reaching the typical static triggering threshold. Therefore, we suggest that both the fluid diffusion and stress perturbation contribute to triggering the mainshock. This study advances our understanding of the spatiotemporal evolution, seismogenic mechanism, and hazard implication for the Yingjiang Ms. 5.8 earthquake and provides additional evidence of natural fluid-triggered seismicity in western Yunnan. • We detect 10,356 events for the 2011 Yingjiang Ms. 5.8 earthquake sequence. • Our result reveals a foreshock sequence that could be related to fluid diffusion. • Both the fluid diffusion and stress perturbation contribute to triggering the mainshock. [ABSTRACT FROM AUTHOR]

Hydrate-based CO 2 sequestration (HBCS) under seafloor with huge storage capacity and long-term mechanical stability is attractive for large-scale carbon reduction. However, as representative geochemical constituents of marine sediments, swelling clay minerals still play ambiguous roles in hydrate phase transition and sedimentary structure stability. In this study, pore-scale behaviors of hydrate deposition and reservoir structure evolution in the presence of montmorillonite (MMT), a representative swelling clay, was investigated using low-field nuclear magnetic resonance (LNMR) technique. Total inter-particle interaction energy of 195.4 K b T ensured the dispersion of clay in 0.5 wt% MMT system, which facilitated homogeneous hydrate formation by providing nucleation sites and surface electric fields, improving hydrate conversion from 78.2 % to 89.6 %. Weak water activity caused by strong water absorption of swelling clay resulted in a 14.4 % reduction in CO 2 storage capacity of high-concentration (10.0 wt%) MMT reservoir. Hydrate phase transition accompanied by the migration of MMT particles and liquid water synergistically contributed to sedimentary structure evolution. Migration of MMT-rich fluid with high viscosity could cause irreversible geologic damages by exacerbating sedimentary skeleton deformation. This work reveals the interaction between swelling clay and CO 2 hydrate at microscopic scale, providing new perspectives for effective implementation of CO 2 geologic sequestration in marine sediments. [Display omitted] • Revealing the interaction of swelling clay with CO 2 hydrate from the pore scale. • The presence of montmorillonite improves the homogeneity of hydrate distribution. • Concentrated clay fluids provide mass transfer resistance for hydrate formation. • Clay agglomeration and migration can cause irreversible damage to the reservoir. • A small amount of clay allows for homogeneous and dense hydrate-based CO 2 storage. [ABSTRACT FROM AUTHOR]

Basin and petroleum systems modelling of the Wessex Basin, UK has been conducted to identify the maturation and migration events that charged the principal Bridport Sands and Sherwood Sandstone reservoirs at the Wytch Farm oil field. Modelling results have been compared with recent observations of magnetic enhancements at oil-water contacts (OWCs) and possibly paleocontacts (PCs) in Wytch Farm reservoirs, to assess the use of magnetic OWCs to help calibrate petroleum systems models and provide insights into the migration history of the Wessex Basin. The model predicts the Blue Lias source rock only reached maturity to the south of the Purbeck Fault, with hydrocarbon generation initiating in the Late Jurassic and peaking in the Late Cretaceous, requiring lateral migration to Wytch Farm using the Bridport Sands as the main carrier bed. Cross-fault and northward migration occurred through conduits at Creech, Bushey Farm and in offshore areas, which charged the principal structures at Wytch Farm. A ∼20 km wide Late Cretaceous juxtaposition between the Bridport Sands and Sherwood Sandstone in the hangingwall and footwall of the Purbeck Fault, respectively, led to the charging of the Sherwood Sandstone reservoir. A basin-wide Cenozoic easterly tilt of ≤1⁰ caused a westerly hydrocarbon remigration, has shifted the Bridport Sands and Sherwood Sandstone Wytch Farm structures to the west, and drastically reduced the size of Bridport Sands accumulations. There is a strong correlation between the predicted depths of Late Cretaceous and present-day OWCs with magnetic enhancements in drill cores. Multiple magnetic enhancements above the OWC at the Wareham oil field indicate the Cenozoic tilting event was periodic, forming multiple stable OWCs, with migration modelling suggesting a spill from Wytch Farm. • The Blue Lias source rock matured during the Early to Late Cretaceous in the hangingwall of the Purbeck Fault. • Wytch Farm was charged from the south by lateral migration routes through Creech, Bushey Farm and offshore conduits. • There is a strong correlation between the depths of magnetic enhancements in drill cores and OWCs identified in the model. • Modelling and magnetic analyses suggest a periodic basin-wide easterly Cenozoic tilt produced pulses of westerly remigration. [ABSTRACT FROM AUTHOR]

Bottom simulating reflections (BSRs) interpreted in seismic data are one of the most used indicators for the presence of gas hydrates. In numerous hydrate provinces, including the Hikurangi margin, east of New Zealand, distinct and anomalous gaps in reflectivity punctuate otherwise continuous BSRs. We undertake a seismic stratigraphic and structural interpretation of a dense grid of two‐dimensional reflection seismic data to investigate possible causes of widespread BSR gaps that occur near the hinge area of synclines. We explain these BSR gaps with a tectono‐sedimentary model where sedimentation into accretionary wedge synclines leads to an upward migration of the base of gas hydrate stability with respect to stratigraphy, causing dissociation of gas hydrates to water and free gas. A trough‐shaped syncline's radially dipping beds promotes along‐strata, upward migration of gas away from the hinge area, incrementally depleting synclinal hinges of gas and resulting in a BSR gap or muted BSR. This model may be applicable to observed gaps in BSRs in tectonically active slope basins worldwide. [ABSTRACT FROM AUTHOR]

It is difficult to exploit low-permeability reservoirs, and CO2 flooding is an effective method to improve oil recovery from low permeability reservoirs. However, in the process of CO2 flooding, acidic fluids dissolved in formation water will react with rock to cause dissolution and precipitation, resulting in pores and precipitates, changing the evolution law of seepage channels, destroying formation integrity, and affecting the effect of CO2 oil displacement. The change in rock’s physical properties and the mass transfer law between CO2-water-rock are unclear. This paper considers the coupling effects of seepage, mechanics, and chemistry when CO2 is injected into the formation. The mass transfer model of CO2-water-rock in the geochemical reaction process is established on this basis. The physical properties of the reservoir after CO2 injection are quantitatively studied based on the microscopic mechanism of chemical reaction, and the migration law of solute in the reservoir rock during CO2 flooding under the coupling effects of multiple fields is clarified. The experimental results show that with the increase in reaction time, the initial dissolution reaction of formation rocks will be transformed into a precipitation reaction of calcite, magnesite, and clay minerals. The porosity and permeability of the rocks near the well first increase and then decrease. The far well end is still dominated by dissolution reactions, and the average values of formation porosity and permeability show an upward trend. Although the dissolution reaction of CO2-water-rock can improve the physical properties of reservoir rocks to a certain extent, the mutual transformation of the dissolution reaction and precipitation reaction further exacerbates the heterogeneity of formation pore structure, leading to the instability of CO2 migration, uneven displacement, and destruction of formation stability. The research results of this paper solve the problem of quantitative calculation of physical parameters under the coupling effect of multiple fields after CO2 injection into reservoirs and can predict the changes in formation physical properties, which can provide a certain theoretical basis for evaluating formation integrity and adjusting CO2 injection under the condition of CO2 flooding.

For underground mining, efficient groundwater management is one of the critical mining economics components. The region of interest, known as Tharisa Mine, is situated on the western limb of the Bushveld Igneous Complex, which is home to South Africa’s premier platinum-group metal resources. This work aimed to provide the findings from the investigation and imaging of the near-subsurface hydrogeological architecture in a shallow profile using stable isotopes of water (18O and 2H) and radioactive water isotopes (3H). Regarding isotope data, 18O varied from −3.5 to 1.5‰; 2H from −24 to 4.7‰; and 3H from 2.0 to 3.4 T.U. Utilizing combined geophysical techniques, the results were verified. Additionally, the geophysical methods, including seismic refraction tomography, multichannel analysis of surface waves, electrical resistivity tomography, and magnetics, helped identify the fluid’s pathways and lineaments during migration to verify the isotope results. The groundwater inflow volumes into the open pit were initially determined by integrating the following findings: the delineation of fracture systems/zones and fluid migration pathways; mining activities enhance the storage and transmission ability of the aquifer; and the main sources of water in the mine include mixing of surface and deep water sources, recycling of water possibly via lineaments, and tailings dam seepages.

In a present context of sustainable energy and hazard mitigation, understanding fluid migration in sedimentary basins – large subsea provinces of fine saturated sands and clays – is a crucial challenge. Such migration leads to gas or liquid expulsion at the seafloor, which may be the signature of deep hydrocarbon reservoirs, or precursors to violent subsea fluid releases. If the former may orient future exploitation, the latter represent strong hazards for anthropic activities such as offshore production, CO$_2$ storage, transoceanic telecom fibers or deep-sea mining. However, at present, the dynamics of fluid migration in sedimentary layers, in particular the upper 500 m, still remains unknown in spite of its strong influence on fluid distribution at the seafloor. Understanding the mechanisms controlling fluid migration and release requires the combination of accurate field data, laboratory experiments and numerical simulations. Each technique shall lead to the understanding of the fluid structures, the mechanisms at stake, and deep insights into fundamental processes ranging from the grain scale to the kilometers-long natural pipes in the sedimentary layers. Here we review the present available techniques, advances and challenges still open for the geosciences, physics, and computer science communities.

• We built a high-precision earthquake catalog for the Weiyuan shale gas revealing detailed seismicity. • Earthquake swarms in this field indicate a complex fault system. • The 2019 Ms 5.2 earthquake may be triggered by to combination effects of fluid pressure and stress perturbation. The Weiyuan-Rongxian shale gas field in the Sichuan basin, China, has experienced numerous earthquakes in recent years, causing significant casualties and property loss. To investigate the spatial–temporal evolution of seismicity and seismic fault structures, in this study, we first utilize a deep-learning-based picking algorithm –(PhaseNet) to identify seismic phases from raw seismic data between March 2019 and October 2020, and then employ a Rapid Earthquake Association and Location technique (REAL) to associate these seismic picks. Finally, the VELEST and GrowClust methods are performed to build a high-precision earthquake catalog containing 18,912 earthquakes. Our results reveal that the spatial–temporal distribution of earthquake swarms indicates a complex fault system. In the northern part of Weidong block (R1), the earthquakes illuminate a shovel-shaped structure that the dip angle of the fault plane gradually decreases with depth. In the south of Weidong block (R2), the natural fractures with dip angles of about 80° control the distribution pattern of earthquakes. In the Weixi block (R3), the Molin fault and basement fault form a typical structural wedge that bounds a triangular or wedge-shaped fault block. The majority of earthquakes located below the buried depth of the shale gas and spatio-temporal correlations between seismicity and industry operations both suggest that they are probably induced by hydraulic fracturing. The migration front of the earthquakes can be modeled by fluid diffusion with the diffusivity D between 0.2 m2/s and 3.5 m2/s. The Ms 5.2 earthquake occurring on 18 December 2019 is located in a positive ∼10 kpa Coulomb failure stress area generated by the 8 September 2019, Ms 5.4 earthquake, reaching the typical static triggering threshold. Therefore, in addition to fluid diffusion, the stress perturbation caused by previous seismic ruptures may be considered as another factor of earthquakes occurring in the Weiyuan-Rongxian. Our study provides evidences for better understanding the seismicity characteristics, seismogenic mechanism, and fault structure in Weiyuan-Rongxian shale gas field. [ABSTRACT FROM AUTHOR]

Calcite veins hosted by extension fractures and faults are common in the outcrop belt of the Austin Chalk – Eagle Ford – Buda stratigraphic section across south-central and west Texas, and contain aqueous and liquid hydrocarbon fluid inclusions. The hydrocarbon fluid inclusions in these thermally immature rocks indicate long-distance migration of oils from mature source rock down-dip and possibly from deeper units along faults. The widespread spatial distribution and extensive variety of oil types indicated by the fluid inclusion fluorescence suggest a protracted series of migration events – possibly from multiple source areas – rather than a single event. Oil inclusions in fault and fracture hosted calcite veins provide evidence of both strata-bound and cross-stratal oil migration along fault and fracture networks that are pervasive in the Austin Chalk, Eagle Ford, and Buda, as well as stratigraphically deeper units, in the subsurface and outcrop belt. Aqueous fluid inclusion microthermometry of the calcite veins provides evidence for significant exhumation of rocks in the Austin Chalk – Eagle Ford – Buda outcrop belt. Approximately 1.25–1.79 km of overburden was removed along the Balcones fault zone outcrop belt of south-central Texas, while in outcrop belt of the Devils River Uplift as much as 2.44–3.50 km of overburden was removed. In the productive region of the deeper basin down-dip from the outcrop belt, fluid inclusions in a deep core indicate that fluids were trapped during burial, but do not record maximum burial conditions. • Faults and fractures in immature Cretaceous strata of Texas host calcite veins. • Oil inclusion fluorescence in calcite veins records variety of trapped oils. • Protracted migration from distant and/or multiple source areas is indicated. • Migration from deep basin and/or deeper units along fault and fracture networks. • Exhumation across outcrop belt ranges from 1.25 to 3.50 km. [ABSTRACT FROM AUTHOR]

This document is a corrigendum for an article titled "Insights into fluid migration during the 2021 La Palma eruption using seismic noise interferometry." The corrigendum addresses errors in the funding statement of the published article and provides the correct funding information. The authors apologize for the errors and state that it does not affect the scientific conclusions of the article. The corrigendum also includes information about the authors and their affiliations. [Extracted from the article]

The Dayingezhuang orogenic gold deposit, located in the northwestern Jiaodong Peninsula, is hosted by the Zhaoping detachment fault, but the paleostress regime during the mineralization period remains poorly understood. In this study, a series of numerical modeling experiments with variable stress conditions were carried out using FLAC3D software to determine the orientation of paleostress and the fluid migration processes during the ore-forming period. The results show that the simple compression or tension stress model led to fluid downward or upward flow along the fault, respectively, accompanying the expansion deformation near the hanging wall or footwall of the Zhaoping fault, which is inconsistent with the known NE oblique mineralization distribution at Dayingezhuang. The reverse and strike-slip model shows that the shear stress was distributed in the gentle dip sites of the fault, and the expansion space occurred in the geometric depression sites of the fault, which is also inconsistent with the known mineralization distribution. The normal and strike-slip model shows that shear stress was distributed in the sites where the fault geometry transforms from steep to gentle. In addition, the expansion deformation zones appeared at sites with dip angles of 35~60° in the footwall and extended along with the NE-trending distribution from shallow to deep levels. The numerical results are quite consistent with the known mineralization, suggesting that the fault movement during the mineralization stage is a combination of the local strike-slip and the NW–SE extension in the Dayingezhuang deposit. Under this stress regime (σ1 NE–SW, σ2 vertical, σ3 NW–SE), the NE dilation zones associated with fault deformation served as channels for the ore-forming fluid migration. Based on the numerical modeling results, the deeper NE levels of the No. 2 orebody in the Dayingezhuang deposit have good prospecting potential. Thus, our study not only highlights that gold mineralization at Dayingezhuang is essentially controlled by the detachment fault geometry associated with certain stress directions but also demonstrates that numerical modeling is a robust tool for identifying potential mineralization. [ABSTRACT FROM AUTHOR]

Most of the developing oil and gas fields in Russia are located in Arctic regions and constructed on permafrost, where recent environmental changes cause multiple hazards for their infrastructure. The blowing-up of pingos, resulting in the formation of gas emission craters, is one of the disastrous processes associated both with these external changes and, likely, with deep sources of hydrocarbons. We traced the channels of fluid migration which link a gas features reservoirs with periglacial phenomena associated with such craters with the set of geophysical methods, including common depth point and shallow transient electromagnetic methods, on an area of a prospected gas field. We found correlated vertical anomalies of acoustic coherence and electrical resistivity associated with gas chimneys in the upper 500–600 m of the section. The thickness of the ice-bonded permafrost acting as a seal for fluids decreased in the chimney zone, forming 25–50 m deep pockets in the permafrost base. Three pingos out of six were located above chimneys in the study area of 200 km2. Two lakes with parapets typical for craters were found. We conclude that the combination of applied methods is efficacious in terms of identifying this type of hazard and locating potentially hazardous objects in the given territory. [ABSTRACT FROM AUTHOR]

The success of recently developed geological and geophysical methods and technologies for monitoring of mud volcanoes in the Kerch-Taman region is eventually based on the fundamental scientific results accumulated at the turn of the century under the overall leadership of the Academician E.F. Shnyukov. The results of geological and geophysical studies of the Mount Karabetov mud volcano (Taman Peninsula) featuring the passive seismoacoustic sounding method are presented. New data on the spatial configuration of subvertical fluid-saturated structures associated with the volcano's feeding system were obtained. Subsequent common interpretation of the geophysical data and the results of the structural-geomorphologic observations allowed one to formulate a consistent hypothesis on the possible deep mechanism of mud volcanic activity responsible for the mostly paroxysmal nature of eruptions of the Mount Karabetov.

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Polygonal faults (PFs) have been widely found in over 100 sedimentary basins worldwide, mainly in marine setting on continental margins and intracratonic sedimentary basins. PFs are characterized by layer-bound minor normal faults arranged in polygonal patterns and multi-direction strikes, most of which were developed in fine-grained sediments rich in claystone. The genesis mechanisms of PFs have been recognized as non-tectonic, mainly including shear failure, syneresis, density inversion, low coefficients of friction and gravity sliding theory. Previous studies often regarded PFs as conduits playing a role in favouring fluid migration, which were also regarded as fluid source expelled during PF generation. However, some researchers proposed that the PF-bearing layer is generally impermeable and may act as a seal. To better understand the role of PFs in fluid migration and their contrition to the shallower hydrocarbon accumulation, a geophysical review of global PFs case studies is conducted in this paper. A comprehensive analysis is carried out based on abundant case studies, with key parameters analyzed including the PFs characteristics (e.g., buried depth, lithology, size, etc.), formation mechanisms of PFs, tectonic-sedimentary setting, and the spatial relationship of PFs, nearby major fluid migration pathways and high amplitude anomalies. The compilation of global PF case studies of shows that hydrocarbon fluids from the deep is more likely to migrate upward through normal big faults, gas chimneys or unconformities surface around PFs. In most cases, PFs play limited roles in fluid migration, such as in South China Sea where hydrocarbon accumulations have been observed under the PFs and the PF-bearing layer may act as a seal trapping the free gas underneath. Generally, the role of PFs in fluid migration should be determined through a comprehensive analysis of the characteristics of PFs, surrounding amplitude anomalies, hydrocarbon distribution, and even the sealing ability of PFs which needs further study. It is more likely that the PFs play a limited role in vertical fluid migration and related shallower hydrocarbon accumulation and distribution.

Mofettes are gas emission sites where high concentrations of CO2 ascend through conduits from as deep as the mantle to the Earth’s surface and as such provide direct windows to processes at depth. The Hartoušov mofette, located at the western margin of the Eger Graben, is a key site to study interactions between fluids and swarm earthquakes. The mofette field (10 mofettes within an area of 100 m × 500 m and three wells of 28, 108, and 239 m depth) is characterized by high CO2 emission rates (up to 100 t/d) and helium signatures with (3He/4He)c up to 5.8 Ra, indicating mantle origin. We compiled geological, geophysical, geochemical, and isotopic data to describe the mofette system. Fluids in the Cheb basin are mixtures between shallow groundwater and brine (>40 g/L at a depth of 235 m) located at the deepest parts of the basin fillings. Overpressured CO2-rich mineral waters are trapped below the mudstones and clays of the sealing Cypris formation. Drilling through this sealing layer led to blow-outs in different compartments of the basin. Pressure transients were observed related to natural disturbances as well as human activities. External (rain) and internal (earthquakes) events can cause pressure transients in the fluid system within hours or several days, lasting from days to years and leading to changes in gas flux rates. The 2014 earthquake swarm triggered an estimated excess release of 175,000 tons of CO2 during the following four years. Pressure oscillations were observed at a wellhead lasting 24 h with increasing amplitudes (from 10 to 40 kPa) and increasing frequencies reaching five cycles per hour. These oscillations are described for the first time as a potential natural analog to a two-phase pipe–relief valve system known from industrial applications.

The shallowest sediment of three mud volcanoes (MVs) located in the middle slope of the Gulf of Cádiz, Anastasya, Pipoca and St. Petersburg, have been seasonally studied during June and December 2016. These structures are locally important contributors of many biogeochemical active substances to the water column, with a special attention in the emission of methane (CH4). Along this study, the role of organic matter diagenesis and its contribution to the diffusive fluxes estimated in the sediment–water interface from the three MVs have been investigated mainly. For this, the first combined analyses of sediment properties (granulometry, porosity, density, organic carbon) and pore water chemistry (major elements, dissolved inorganic carbon (DIC), nutrients, CH4 and nitrous oxide) have been carried out. Anastasya and St. Petersburg MVs presented similar behaviours, with a slight concentration variation for the chemical variables in the top layers of the sedimentary record followed by a more intense vertical variation in deeper layers. Anastasya MV showed some typical processes of these structures, such as the clay dehydration, the dissolution of halite and the precipitation of authigenic carbonates, which was also observed in St. Petersburg MV. Organic matter diagenesis clearly altered the biogeochemical profiles, except for Pipoca MV. Moreover, mud breccia with a mousse-like texture has been identified in the deepest levels of Anastasya and St. Petersburg MVs. Here, anaerobic oxidation of methane is linked to the decrease of SO 4 2 - and the increases of CH4 and DIC with the core depth. Changes across the sediment–water interface presented low diffusive fluxes in the three MVs due to the presence of other processes as the irrigation by benthic macrofauna, the action of bottom currents and/or the upward fluid migration. From the obtained results, we can conclude that Anastasya MV presents a certain venting activity at present. [ABSTRACT FROM AUTHOR]

Multi-channel seismic reflection and sub-bottom profiling data reveal landward-dipping normal faults as potential conduits for mantle-derived fluids in the coseismic slip area of the 2011 Tohoku earthquake (Mw9.0). Normal faults below the helium isotope anomaly sites appear to develop through the forearc crust (i.e., the seafloor sedimentary section and Cretaceous basement) and to evolve to lower dip angles as extension progresses deeper, potentially extending down to the mantle wedge, despite their intermittently continuous reflections. The faults are characterized by high-amplitude, reverse-polarity reflections within the Cretaceous basement. Moreover, deep extension of the faults connecting to a low-velocity region spreading from the Cretaceous basement into the mantle wedge across the forearc Moho suggests that the faults are overpressured by local filling with mantle-derived fluids. The locations of the normal faults are roughly consistent with aftershocks of the 2011 Tohoku earthquake, which show normal-faulting focal mechanisms. The 2011 Tohoku mainshock and subsequent aftershocks can lead the pre-existing normal faults to be reactive and more permeable so that locally trapped mantle fluids can migrate up to the seafloor through fault fracture zones. The reactivated normal faults may be an indicator of shallow coseismic slip to the trench. Locally elevated fluid pressures can decrease the effective normal stress for the fault plane, facilitating easier slip along the fault and local tsunami. The landward-dipping normal faults developing from the seafloor down into the Cretaceous basement are predominant in the middle slope region of the forearc. A possible shear zone with high-amplitude, reverse-polarity reflections above the plate interface, which is almost localized to the middle slope region, suggests more intense basal erosion of the overlying plate in that region.

Aftershock cascades play an important role in forecasting seismicity in natural and human‐made situations. While their behavior including the spatial aftershock zone has been the focus of many studies in tectonic settings, this is not the case when fluid flows are involved. Using high‐quality seismic catalogs, we probe aftershocks dynamics in five settings influenced by fluids: (a) induced seismicity in Oklahoma and Kansas, (b) natural swarms in California and Nevada, and (c) suspected swarms in the Yuha Desert (California). All settings exhibit significant aftershock behavior highlighting the importance of event‐event triggering processes. The spatial aftershock zones scale with mainshock magnitude as expected based on the rupture length. While (a) and (b) show a rapid decay beyond their rupture length, (c) exhibits long‐range behavior suggesting that fluid migration might not be the dominant mechanism. We also find that the scaling of aftershock productivity with mainshock magnitude together with the Gutenberg‐Richter b‐value might allow to distinguish between natural swarms and induced seismicity. Plain Language Summary: While it is known that fluid injection operations can induce seismic activity, it has remained unclear how this activity compares to their natural counterpart, seismic swarms driven by natural fluid flows. The latter are typically characterized by the absence of a dominant event within the seismic sequence, while exhibiting other characteristics consistent with tectonic sequences including aftershock triggering. Our analysis of high‐quality seismic catalogs for both types of fluid‐driven seismicity shows that both exhibit a significant amount of aftershocks arising from "secondary" processes (i.e. stress‐based event‐event triggering as an indirect consequence of fluid injections) leading to spatially localized aftershocks zones. Yet, the trade‐off between the seismic productivity relation, which refers to the average increase in the number of aftershocks with the magnitude of their trigger, and the distribution of earthquake magnitudes controls the relative role of small compared to large triggers and we find that aftershock triggering is much more dominated by smaller events in the induced setting. Both findings are of direct importance for earthquake forecasting and seismic hazard assessment. Key Points: Significant event‐event triggering is present in both natural swarms and induced seismicityBoth fluid‐driven settings are characterized by narrow aftershock zonesAftershock triggering is dominated by smaller triggers in induced seismicity but much less so for natural swarms [ABSTRACT FROM AUTHOR]

Since the discovery of >570 methane flares on the northern U.S. Atlantic margin between Cape Hatteras and Georges Bank in the last decade, the acquisition of thousands of kilometers of additional water column imaging data has provided greater coverage at water depths between the outer continental shelf and the lower continental slope. The additional high-resolution data reveal >1400 gas flares, but the removal of probable duplicates from the combined database of new flares and those recognized in 2014 yields ∼1139 unique sites. Most of these sites occur in clusters of 5 or more seeps, leaving about 275 unique locations (including 47 clusters) for seepage along the margin. As a function of depth, seep distribution is heavily skewed toward the upper continental slope at water depths shallower than 400 m on the southern New England margin and ∼ 550 m in the Mid-Atlantic Bight, with additional seeps clustered at ∼1100 m and just deeper than ∼1400 m in both sectors. Despite little ongoing tectonic deformation or active faulting on this passive margin, a variety of processes driven from below the seafloor (e.g., migration of fluids along faults or through permeable strata, seepage above diapirs or other pre-existing structures) and from above (e.g., erosion, sapping, unroofing) contribute to the development of seeps in different settings along the margin. In addition, the prevalence of seeps on promontories overlooking shelf-breaking canyons may be directly related to the three-dimensional nature of the hydrate stability zone in these locations. As a function of depth, the parts of the slope at the contemporary landward limit of gas hydrate stability are devoid of seeps, and the upper slope zones with the most concentrated seepage were not within the gas hydrate stability zone even during the Last Glacial Maximum. Thus, if the large number of upper slope seeps is at least partially sourced in gas hydrate degradation, the gas emitted at these seeps must have migrated there from greater depths on the continental slope. • The number of known cold seep sites on the U.S. Atlantic margin has now doubled. • Seeps and clusters are found at ∼275 locations, mostly north of Cape Hatteras. • Most seeps are shallower than the landward limit of gas hydrate stability. • Faulting, erosion, unroofing, and hydrologic sapping dominate seep evolution. • Complex hydrate distribution leads to promontory seeps next to shelf-break canyons. [ABSTRACT FROM AUTHOR]

Abstract Fluid release from subducting oceanic lithosphere is a key process for subduction zone geodynamics, from controlling arc volcanism to seismicity and tectonic exhumation. However, many fundamental details of fluid composition, flow pathways, and reactivity with slab‐forming rocks remain to be thoroughly understood. In this study we investigate a multi‐kilometer‐long, high‐pressure metasomatic system preserved in the lawsonite‐eclogite metamorphic unit of Alpine Corsica, France. The fluid‐mediated process was localized along a major intra‐slab interface, which is the contact between basement and cover unit. Two distinct metasomatic stages are identified and discussed. We show that these two stages resulted from the infiltration of deep fluids that were derived from the same source and had the same slab‐parallel, updip flow direction. By mass balance analysis, we quantify metasomatic mass changes along this fluid pathway and the time‐integrated fluid fluxes responsible for them. In addition, we also assess carbon fluxes associated with these metasomatic events. The magnitude of the estimated fluid fluxes (104–105) indicates that major intra‐slab interfaces such as lithological boundaries acted as fluid channels facilitating episodic pulses of fluid flow. We also show that when fluids are channelized, high time‐integrated fluid fluxes lead to carbon fluxes several orders of magnitude higher than carbon fluxes generated by local dehydration reactions. Given the size and geologic features of the investigated metasomatic system, we propose that it represents the first reported natural analogue of the so‐called high permeability channels predicted by numerical simulations.

Long‐term slow slip events (L‐SSEs) often excite short‐term slow slips events (S‐SSEs) and tectonic tremor in the zone of episodic tremor and slip (ETS). However, the factors controlling the occurrence of primary versus excited tremor events remain unclear. To elucidate these factors, we analyzed tectonic tremor events in and around the Bungo Channel (Nankai subduction zone), where L‐SSEs are known to excite tremor and S‐SSEs in the ETS zone. We focused on the spatial distribution of the characteristic amplitude (CA) of tremor, determined from the duration‐amplitude distributions of tremor events, as an indicator of the properties of the tremor source. CAs are large in L‐SSE slip areas and small in adjacent areas. The difference between CA values during tremor‐excitation periods (L‐SSEs) and the intervening periods (ΔCA) is positive in the slip area, negative in adjacent areas, and tends toward zero in the far field. We suggest that the heterogeneous distributions of CA and ΔCA reflect the heterogeneous effective strengths of tremor patches, which might be related to petrological properties, and stress and pore‐fluid pressure variations induced by L‐SSEs, respectively. The upward migration of fluid from the ETS zone along the plate interface might modulate the effective stress and strength states of tremor patches during L‐SSEs. Plain Language Summary: In subduction zones, slow earthquakes have inspired great interest in the connection between slow and megathrust earthquakes. Long‐term slow slip events (L‐SSEs) are known to excite short‐term slow slip events and tectonic tremor. To understand the factors controlling the occurrence of primary and excited tremors, we investigated the characteristic amplitude (CA) of tremor events (representing the properties of a tremor source) during tremor‐excitation periods and the intervening periods. CAs are larger in L‐SSE slip areas than in adjacent areas during both periods. This contrast may result from differences in pore‐fluid pressure arising from differences in petrological properties between the two areas. Moreover, relative to CAs during the intervening periods, CAs during tremor‐excitation periods are larger in the slip area, smaller in adjacent areas, and almost the same far from the slip area. These variations in CA are the combined effects of (1) stress changes accompanying L‐SSEs and (2) upward fluid migration along the plate interface from the tremor source area during L‐SSEs, because fluid migration reduces the pore‐fluid pressure and increases the strength of tremor source areas. Our findings emphasize that CA can be a useful tool for monitoring fluid migration in the source areas of tremor events. Key Points: Long‐term slow slip events change characteristic amplitudes of tectonic tremor in the episodic tremor and slip zoneVariations in characteristic amplitude reflect the heterogeneous effective strengths of tremor patchesFluid migration during long‐term slow slip events might control the stress state and strength of tremor patches [ABSTRACT FROM AUTHOR]