Seepage Dynamics Research Articles

Experiment and Prediction of Enhanced Gas Storage Capacity in Depleted Gas Reservoirs for Clean Energy Applications

Utilizing depleted gas reservoirs for gas storage is the most efficient method. However, the impact of multi-cycle injection-production and water invasion on rock properties and gas-water seepage dynamics remains unclear. This study investigates such reservoirs' multi-cycle stress sensitivity and gas-water displacement behavior. Results show that multi-cycle injection-production and water invasion increase the stress sensitivity of the gas and damage gas storage space and seepage channels. This study also proposed a new unsteady gas-water relative permeability testing method and a prediction model based on in-situ X-ray CT scanning, improving measurement accuracy by 30%. The results found that the gas's relative permeability is directly proportional to the injection rate and inversely proportional to the initial gas saturation. A gas-water relative permeability model was established, demonstrating that gas-water interference intensifies as the number of cycles increases. In a gas-water transition zone in depleted gas reservoirs, gas relative permeability damage reaches 81%, reducing the movable pore space and impacting injection-production rates and capacity. This study offers valuable recommendations for optimizing production and dynamic prediction in depleted gas reservoirs, contributing to clean energy applications.

Advanced modeling of seepage dynamics and control strategies in thick coal seams under high-confined aquifer conditions: A case study

The hydraulic behavior of the connection between the floor failure area and the aquifer water-conductive zone is considered to be the root cause of mine water inrush disasters. Therefore, unraveling the floor failure mechanism is particularly important for safe coal mining above the high-confined aquifer. This paper estimates the depth of the baseplate failure to be 18.4–27.3 m by combining network parallel electrical methods with drilling visualization technology. The FLAC3D-based numerical model considering the strain hardening of caved rock was established with rigorous calibration and verification. The results showed that the depth of damage to the floor is 23.1 m, and the dominating floor failure mechanism is shear failure caused by the vertical stress exceeding the rock bearing capacity. Moreover, the stress recovery process of the baseplate does not alter the failure morphology of the baseplate. Based on the above research findings, the in-situ floor control technique of the working face No. 4305 is proposed and practiced in the field. Field measurements show that floor control performance is satisfactory with water inflow in the goaf being roughly stable at 50 m3/h. Our results can provide useful reference for safe mining above confined aquifer and prevention and mitigation of water-related hazards.

Indications of preferential groundwater seepage feeding northern peatland pools

Groundwater seepage from underlying permeable glacial sedimentary structures, such as eskers, has been hypothesized to directly feed pools in northern peat bogs. These hypotheses directly contradict classical peat bog models for ombrogenous systems, wherein meteoric water is the sole water input to these systems. Variations in the underlying mineral sediment in contact with the peat imply that unrecognized hydrogeologic connectivity may exist with pools in northern peat bogs, particularly where high permeability materials are in contact with the peat. Seepage dynamics originating from these structural variations were investigated using a suite of thermal and hydrogeophysical methods deployed around pools in a peat bog of northeastern Maine, USA. Thermal characterization methods mapped anomalies that were confirmed as matrix seepage or preferential flow pathways (PFPs). Geochemical methods were employed at identified thermal anomalies to confirm upwelling of solute-rich groundwater. Conduits around pools were associated with surficial terminations of suspected peat pipes, based on the inference of pathways extending down into the peat, that focus flow through PFPs in the peat matrix. Discharge also occurred through the peat matrix adjacent to suspected pipe structures and matrix seepage rates were quantified using analysis of diurnal temperature signals recorded at multiple depths. Seepage rates, with a maximum of nearly 0.4 m/d, were measured at localized points around pools. Periods of synchronized temperatures paired with highly muted diurnal temperature signals, recorded in diurnal temperature with depth data, were interpreted qualitatively as activation of strong upward discharge rates through suspected peat pipes. These time periods correlated strongly with local precipitation events around the peatland. Ground-penetrating radar surveys revealed discontinuities in the low permeability glacio-marine clay at the mineral sediment-peat interface, interpreted to be regional glacial esker deposits, which were located beneath and around pools. Heat tracing, specific conductance contrasts, seepage rates, and trace metal concentrations all imply groundwater seepage originating from underlying permeable glacial esker deposits and directly sourcing pools. Preferential groundwater inputs into northern peat bogs may play a key role in developing and maintaining pool systems, with enhanced solute transport impacting peatland ecology, water resources, and carbon cycling.

Contrasting Methane Seepage Dynamics in the Hola Trough Offshore Norway: Insights From Two Different Summers

AbstractThis study investigates the temporal variations in methane concentration and flare activity in the Hola trough (offshore Norway) during May 2018 and June 2022. Between these time periods, methane seep activity exhibits 3.5 times increase, as evidenced by hydroacoustic measurements. As the seep area in the Hola trough is constantly within the hydrate stability zone, the observed increase cannot be attributed to migration of its shallow boundary due to temperature increase. However, a combination of low tide conditions resulting in a lower sediment pore pressure and a bottom water temperature increase resulting in a lower methane solubility is likely to explain the increase in the number of seeps observed in June 2022. The hypothesis of tide influence is supported by data collected from a piezometer deployed and recovered during the cruise showing that the tidal effect was observed 3 m below the seafloor. Despite the numerous methane seeps detected, methane concentration and gas flow rates near the seafloor were low (<19 nM and <70 mL min−1, respectively) compared to other areas with methane seep activity. This is likely due to strong currents rapidly dispersing methane in the water column. Sub‐seafloor investigations identified pathways for gas migration in methane seep areas, influenced by topography. This study provides valuable insights into the temporal dynamics of methane concentrations, flare activity, and gas distribution in the Hola trough, contributing to our understanding of offshore methane dynamics in the region.

Coupled DSSAT and HYDRUS-1D Simulation of the Farmland–Crop Water Cycling Process in the Dengkouyangshui Irrigation District

(1) Background: Effective water management in agricultural systems poses a significant challenge, particularly in the Dengkouyangshui irrigation district. Inefficiencies and insufficient detail in water usage across crop growth stages have resulted in suboptimal water cycling. Recent infrastructure improvements and technological interventions necessitate a reevaluation of water usage, especially concerning changes in irrigation and seepage dynamics. (2) Methods: This study addresses these concerns by employing an integrated modeling approach that combines the DSSAT with the HYDRUS-1D soil hydrology model to simulate complex interactions among soil, crop growth, and irrigation practices within the district. Observational data were used to calibrate and validate the integrated model, including soil moisture, LAI, and crop yields from the 2022 and 2023 agricultural seasons. (3) Results: The simulation results strongly align with the empirical data, highlighting the ability of the model to capture the intricate dynamics of soil–water–atmosphere–plant interactions. (4) Conclusions: The soil’s retention and moisture-holding characteristics exhibited resilience during periods without water supplementation, with measurable declines in soil moisture at various depths, indicating the soil’s capacity to support crops in water-limited conditions. This study delineates water consumption by maize crops throughout their growth cycle, providing insights into evapotranspiration partitioning and quantifying seepage losses. An in-depth analysis of water balances at different growth stages informs irrigation strategies, suggesting optimal volumes to enhance efficiency during critical crop development phases. This integrative modeling approach is valuable for providing actionable data to optimize the water cycling process and improve agricultural sustainability in the Dengkouyangshui irrigation district.

Local Seismicity and Sediment Deformation in the West Svalbard Margin: Implications of Neotectonics for Seafloor Seepage

AbstractIn the Fram Strait, mid‐ocean ridge spreading is represented by the ultra‐slow system of the Molloy Ridge, the Molloy Transform Fault and the Knipovich Ridge. Sediments on oceanic and continental crust are gas charged and there are several locations with documented seafloor seepage. Sedimentary faulting shows recent stress release in the sub‐surface, but the drivers of stress change and its influence on fluid flow are not entirely understood. We present here the results of an 11‐month‐long ocean bottom seismometer survey conducted over the highly faulted sediment drift northwards from the Knipovich Ridge to monitor seismicity and infer the regional state of stress. We obtain a detailed earthquake catalog that improves the spatial resolution of mid‐ocean ridge seismicity compared with published data. Seismicity at the Molloy Transform Fault is occurring southwards from the bathymetric imprint of the fault, as supported by a seismic profile. Earthquakes in the northern termination of the Knipovich Ridge extend eastwards from the ridge valley, which together with syn‐rift faulting identified in seismic reflection data, suggests that a portion of the currently active spreading center is buried under sediments away from the bathymetric expression of the rift valley. This hints at the direct link between crustal rifting processes and faulting in shallow sediments. Two earthquakes occur close to the seepage system of the Vestnesa Ridge further north from the network. We suggest that deeper rift structures, reactivated by gravity and/or post‐glacial subsidence, may lead to accommodation of stress through shallow extensional faults, therefore impacting seepage dynamics.

Visualizing water seepage dynamics in grotto relics via atom-based representative model

Water seepage in grotto relics, i.e., Yungang Grottoes, Dazu Rock Carvings, is a key issue to accurately describe the deterioration and weathering process of grotto rock mass. Considering rainfall infiltration, Finite element simulation was performed for studying the water flow through macro-channel of fractured rock in the 4th cave of Yungang Grottoes, where a group of joints with directions of S62°E and N5°W are widely developed. A 3D atom-based representative model was derived from X-ray diffraction (XRD) patterns and the related semi-quantitative calculation of grotto rock powders, for visualizing the associated seepage characteristics through micro-channel by means of molecular dynamics simulation, for the first time. By analyzing various properties, ranging from the configuration and energetic behaviors to the dynamic characteristics, the calculated water flux and mass flow rate were equal to 270 ns−1 and 8.10 × 10–12 g s−1, respectively. A dynamic process of water transport from the entrance region to the exit region was examined and it is consistent with the relative concentration profiles at the corresponding stage. The tagged O atoms experienced a zigzag movement instead of linear motion as expected, roughly exhibited the same target direction. The seepage characteristics in grotto relics experienced a complex evolution process and three types can be summarized: water infiltrates through micro-channels with a low flow rate; it flows through fracture with a relatively high flow rate; it turned into a kind of analogous pipe flow in inter-connected fracture network, resulting in water seepage hazard. Current simulation studies provide helpful insights for understanding the water flow-infiltration behavior of fractured rock in grotto relics.

Non-Dorcian seepage equilibrium analysis of splitting grouting fluid in smooth single fracture

In this paper, the slurry seepage dynamics model is established, the critical conditions for instability of the seepage dynamics model are discussed, and the effects of power index, effective mobility and non-Darcian flow factors on the seepage velocity are analyzed. The results show that in the 2-D logarithmic parameter space, the boundary between the stability zone and the instability zone of seepage is a straight line, and the absolute value of the slope of the straight line decreases with the increase of the power index

Impact of methane seepage dynamics on the abundance of benthic foraminifera in gas hydrate bearing sediments: New insights from the South China Sea

The stable isotopic composition of benthic foraminifera tests is widely used to trace past methane release events. However, gaps in our knowledge on the ecology of benthic foraminifera at methane seeps impede the utility of test composition as proxy of methane emission histories and the influence of redox zonation on the local fauna. Here, we use sediment geochemistry and foraminiferal assemblages of a seep area of the South China Sea to reveal that seepage duration and intensity affect benthic foraminifera. Taxa tolerating eutrophic conditions like Uvigerina thrive at times of reduced seepage when aerobic methanotrophy is favored. In contrast, intense methane seepage leads to high sulfide levels, and a positioning of the sulfate-methane transition zone close to the seafloor. At these times, bottom waters may become oxygen depleted and benthic foraminifera will decline. This study provides (1) the means to understand how seepage dynamics control the distribution of benthic foraminifera and (2) new constraints on the use of foraminifera as paleoceanographic proxies, including possible mechanisms of mass mortality in the geological past.

Deciphering the geochemical link between seep carbonates and enclosed pyrite: A case study from the northern South China sea

Sulfate-driven anaerobic oxidation of methane (SD-AOM) is an important microbial process at methane seeps, which consumes most of the methane migrating from sediments and can lead to the formation of seep carbonate and pyrite. Geochemical proxies archived in seep carbonates have been long used to uncover the nature of seepage. Recently, the sulfur isotopic composition of enclosed pyrite shows ideal prospects for targeting the seepage dynamics, but it remains challenging due to a possible decoupled precipitation of carbonate and pyrite. In order to decipher the geochemical link between seep carbonates and enclosed pyrite, multiple geochemical proxies of both carbonate (major elements, rare earth elements, δ13C) and pyrite (contents and δ34Spy) were analyzed from two seep sites of the northern South China Sea. Multiple carbonate phases were identified via X-ray diffraction and transmission electron microscopy, including microcrystalline aragonite and botryoidal aragonite from Haima seeps, and micritic high-magnesium calcite (HMC), weakly ordered dolomite and botryoidal aragonite from site TVG11. The δ13C values for carbonates from both sites exhibit very light carbon isotopic signatures (from −59.2 to −35.8‰), confirming that the carbon source is mainly derived from the oxidation of biogenic methane. For pyrite enclosed in carbonates, their contents and sulfur isotopic compositions (δ34Spy) reveal generally positive correlations with Mg/Ca ratios, where HMC and dolomite show higher pyrite contents and δ34Spy values, but microcrystalline aragonite and botryoidal aragonite display lower pyrite contents and negative δ34Spy values. This is consistent with our knowledge that HMC and dolomite tend to precipitate at a deeper sulfate-methane transition zone (SMTZ) where the replenishment of seawater sulfate is limited, whereas microcrystalline aragonite and botryoidal aragonite tend to form at shallow SMTZ with enhanced seawater sulfate supplement. Thus, the δ34S values of pore water sulfate greatly affect the resulting sulfur isotopic composition of pyrite, i.e., the higher δ34Spy values in HMC and dolomite are attributed to the 34S enrichment in pore water sulfate at deeper SMTZ. This is supported by SIMS results that pyrite entrapped in botryoidal aragonite displays lower δ34SSIMS values than those in HMC. Our results suggest that geochemical proxies archived in both carbonate and pyrite point to the similar sedimentary environments, suggesting an intrinsic link between these two archives in our study sites.

Miocene Seep-Carbonates of the Northern Apennines (Emilia to Umbria, Italy): An Overview

The natural emission of methane-rich fluids from the seafloor, known as cold seepage, is a widespread process at modern continental margins. The studies on present-day cold seepages provide high-resolution datasets regarding the fluid plumbing system, biogeochemical processes in the sediment, seafloor seepage distribution and ecosystems. However, the long-term (hundreds of thousands to millions of years) evolution of cold seepage remains elusive. The identification and study of outcrop analogous now exposed on land represent a valuable method for better understanding the effects of geological processes and climate forcing on the development of cold seepage systems. Here, we provide an overview on Miocene seep-carbonate deposits of the northern Apennines (from Emilia to the Umbria-Marchean sector, Italy), based on decades of field research integrated with detailed sedimentological and geochemical investigations. We report a total of 13 seep-carbonate outcrops, which formed in three different structural settings of the paleo-accretionary wedge corresponding to wedge-top basins, outer slope and intrabasinal highs at the deformational front. We discuss the recurring lithostratigraphic occurrence of seep deposits and the main compositional features (carbonate facies, carbon and oxygen stable isotopes) in order to interpret the seepage dynamics, duration and infer the contribution of methane-rich fluids released by paleo-gas hydrates. The datasets presented in this study represent a valuable complete record of cold seepage spanning ~12 Myr, that can be used to better understand factors controlling the regional-scale spatial and temporal evolution of cold seepage systems at modern active continental margins.

Magnetic Mineral Diagenesis in a Newly Discovered Active Cold Seep Site in the Bay of Bengal

Diagenetically formed magnetic minerals at marine methane seep sites are potential archive of past fluid flow and could provide important constraints on the evolution of past methane seepage dynamics and gas hydrate formation over geologic time. In this study, we carried out integrated rock magnetic, and mineralogical analyses, supported by electron microscope observations, on a seep impacted sediment core to unravel the linkage between greigite magnetism, methane seepage dynamics, and evolution of shallow gas hydrate system in the K-G basin. Three sediment magnetic zones (MZ-1, MZ-2, and MZ-3) have been identified based on the down-core variations in rock magnetic properties. Two events of intense methane seepage are identified. Repeated occurences of authigenic carbonates throughout the core indicate the episodic intensification of anaerobic oxidation of methane (AOM) at the studied site. Marked depletion in magnetic susceptibility manifested by the presence of chemosynthetic shells (Calyptogena Sp.), methane-derived authigenic carbonates, and abundant pyrite grains provide evidences on intense methane seepage events at this site. Fracture-controlled fluid transport supported the formation of gas hydrates (distributed and massive) at this site. Three greigite bearing sediment intervals (G1, G2, G3) within the magnetically depleted zone (MZ-2) are probably the paleo-gas hydrate (distributed-type vein filling) intervals. A strong linkage among clay content, formation of veined hydrate deposits, precipitation of authigenic carbonates and greigite preservation is evident. Hydrate crystallizes within faults/fractures formed as the methane gas migrates through the gas hydrate stability zone (GHSZ). Formation of authigenic carbonate layers coupled with clay deposits restricted the upward migrating methane, which led to the formation of distributed-type vein filling hydrate deposits. A closed system created by veined hydrates trapped the sulfide and limited its availability thereby, causing arrestation of pyritization and favored the formation and preservation of greigite in G1, G2, G3.

Effect of displacement rates on fluid distributions and dynamics during water flooding in tight oil sandstone cores from nuclear magnetic resonance (NMR)

Water flooding is widely used as an economical and recyclable method to improve oil recovery in unconventional reservoirs. In this study, the effects of injection rates on oil distribution, the seepage dynamics and the contributions of increasing injection rates on enhancing oil recovery during water flooding are investigated. Typical tight oil sandstone cores with different permeabilities are obtained from Junggar Basin, Northwest China, for experimental studies that incorporate nuclear magnetic resonance analyses. Synthetic kerosene (viscosity = 11.2 mPa s) and heavy water (D2O) are used to simulate crude oil and formation water under formation conditions. The samples are first saturated with simulation water and thereafter displaced by synthetic oil until no water is produced to simulate the fully oil-saturated condition. To simulate the water flooding process, the samples are stably displaced by simulation water at injection rates increasing from 0.01 mL/min until no oil is produced. The T2 spectra, injection pressures, and produced oil at different injection rates and water injection volumes are measured simultaneously. The transverse relaxation time (T2) is converted for the corresponding pore-throat radius (r) by combining pore-throat size distributions (obtained from constant-rate mercury injection) and the fully oil saturated T2 spectra. It indicates that the amplitude of T2 spectra for macropore (>100 μm) and mesopore (10–100 μm) spaces decreases more rapidly than those for the micropore spaces (0–10 μm) as injection rate increases. The classical capillary number theory is introduced to represent the effect of injection rate and characterize the dynamics of seepage processes. Results show that the oil recovery factor can be considerably improved by increasing the injection rate in tight oil cores, and this significant improvement is primarily attributed to mesopores and micropores.

Iron speciation in sediment cores near the Jiulong Methane Reef and its implication

Variability in iron species in marine deposits are excellent geochemical archives for exploring variations in redox conditions during early diagenesis and seepage dynamics over time. Two sediment cores (DS07 and DS08) were collected in the Jiulong Methane Reef, a typical cold seep site that is located in the South China Sea (SCS), and analyzed for iron species using Mössbauer spectrometry and magnetic procedures. Excluding the surface layer (0-10 cm) in DS07, Fe2+/Fe3+ ratios indicated that core sediments were under anoxic conditions. The Fe2+/Fe3+ and frequency-dependent susceptibility (χfd) at this site were higher while the total organic matter content (TOC) and magnetic susceptibility (χ) were significantly lower at approximately 170 cm depth. Combined with chromium reducible sulfur (CRS), Fe/Al and sulfur to organic content (S/C) ratios suggested that there was a sulfate-methane transition zone (SMTZ) at approximately 170 cm. Moreover, thermomagnetic analyses indicated that magnetite was the dominant ferrimagnetic mineral at 21–23 cm, 165–185 cm, 213–233 cm and 285–295 cm depths, however, magnetite content was relatively lower between the 165 and 185 cm depth, which suggested that magnetite had been largely transformed to paramagnetic iron minerals during the anaerobic oxidation of methane (AOM) process. The DS08 sediment core was also mostly anoxic , however, no obvious methane seepage activity was observed because the maximum values of Fe2+/Fe3+, Fe/Al, CRS, S/C and the minimum values of TOC and χ did not simultaneously occur at the same depth. The study revealed that the speciation of Fe in sediment cores varied under different redox conditions which was of interest when utilizing iron speciation and magnetic minerals as indicators of historical environmental conditions and seepage activities.

Non-Darcian seepage stability analysis of non-Newtonian fluid

In this paper, the two-phase fluid composed of fine geologic particles and water is considered to be a non-Newtonian fluid, and the seepage dynamics model of the two-phase medium in the rock-soil structure is constructed. Based on the hypothesis, the boundary conditions and solving methods of the model are given and the critical conditions of the model instability are also discussed in detail. It is shown that the existence of the equilibrium state of the kinetic model is determined by the power exponent, the effective fluidity and the non-Darcian flow factor.

Authigenic carbonate formation revealed by lipid biomarker inventory at hydrocarbon seeps: A case study from the Okinawa Trough

Abstract Authigenic carbonates were recovered from the northern Okinawa Trough at 540–700 m water depth. Development of microbial communities and seepage dynamics driving the precipitation of authigenic carbonates remains poorly constrained, even though the source of methane-rich fluids, formation of Fe-rich carbonates, and the potential driving forces were previously reported. Here, petrologic observations, stable carbon and oxygen isotopic compositions, mineralogy, and lipid inventories of authigenic carbonates from the northern Okinawa Trough were analyzed. The carbonate minerals were comprised predominantly of aragonite, high-magnesium calcite, and siderite. The presence of molecular fossils diagnostic for anaerobic methane oxidizing archaea (ANME) and sulfate-reducing bacteria (SRB) with pronounced 13C depletions (as low as −134‰), together with highly negative δ13Ccarb values (as low as −55.8‰), suggest that the carbonates precipitated from anaerobic oxidation of methane (AOM) with microbial methane as the predominant carbon source. The difference between measured and calculated δ18Ocarb values (based on 0‰ of sea water δ18O vs V-SMOW) was higher than +3‰ in one of the carbonates (GGD16), reflecting an origin from gas hydrate dissociation. Most of the carbonates revealed δ18O offsets between measured and calculated values within +3‰, or a lower measured δ18O than calculated value, suggesting a mixture of methane-derived carbonates (low δ13C/high δ18O) and detrital origin carbonates (high δ13C/low δ18O). Biomarker patterns varied significantly among samples. A suite of 13C-depleted biomarkers indicated the predominance of an ANME-1 assemblage for all samples, suggesting carbonates formed at low to medium methane flux. The predominance of ANME-1, abundant detrital minerals, and allochthonous biomarkers, as well as low carbonate contents indicate that most samples formed at greater depth within the sediment column. Only one carbonate sample in this study is interpreted to have precipitated at relatively shallower depth, as indicated by the occurrence of crocetane, dominance of aragonite, and higher overall carbonate content.

Using chemical compositions of sediments to constrain methane seepage dynamics: A case study from Haima cold seeps of the South China Sea

Cold seeps frequently occur at the seafloor along continental margins. The dominant biogeochemical processes at cold seeps are the combined anaerobic oxidation of methane and sulfate reduction, which can significantly impact the global carbon and sulfur cycles. The circulation of methane-rich fluids at margins is highly variable in time and space, and assessing past seepage activity requires the use of specific geochemical markers. In this study, we report multiple sedimentary proxy records for three piston gravity cores (QDN-14A, QDN-14B, and QDN-31) from the Haima seep of the South China Sea (SCS). By combining total organic carbon (TOC), total inorganic carbon (TIC), total nitrogen (TN), total sulfur (TS), acid insoluble carbon and sulfur isotope (δ13Corganic carbon and δ34Sacid-insoluble), and δ34S values of chromium reducibility sulfur (δ34SCRS), as well as carbon isotopes of TIC (δ13CTIC) in sediments, our aim was to provide constraints on methane seepage dynamics in this area. We identified three sediment layers at about 260–300 cm, 380–420 cm and 480–520 cm sediment depth, characterized by particular anomalies of low δ13CTIC values and high TS content, high TS and CRS contents, and high δ34Sacid-insoluble and δ34SCRS values, respectively. On this basis, we propose that these sediment horizons correspond to distinct methane release events preserved in the sediment record. While the exact mechanisms accounting for the presence (or absence) of these particular geochemical signals in the sediment are not known, we propose that they correspond to variations in methane flux and their duration through time. Overall, our results suggest that sedimentary carbon and sulfur and their isotopes are useful tracers for better understanding of methane seepage dynamics over time.

Environmental controls on sulfur isotopic compositions of sulfide minerals in seep carbonates from the South China Sea

Authigenic carbonates and pyrite associated with sulfate-driven anaerobic oxidation of methane (AOM) at methane seeps provide archives to explore the biogeochemical processes involved and seepage dynamics over time. The wide range and extremely high δ34S values of pyrite (δ34Spy) have been used to trace the AOM-related processes. However, the detailed mechanism for this phenomenon is not well understood. We propose that the characteristics of δ34Spy were mainly controlled by the competition between sulfate reduction and sulfate supply, as well as the redox condition. To test this hypothesis, we investigated Sr/Ca and Mg/Ca ratios, trace element compositions, pyrite contents and sulfur isotopic compositions in seep carbonates from Site F and Haima in the northern South China Sea. Calcite and aragonite contents were distinguished through the Sr/Ca and Mg/Ca ratios. The data show that aragonites are always associated with relatively low δ34Spy values compared to calcites. The Mo contents show a good correlation with pyrite contents in calcites and aragonites, and the slope in aragonites is larger than that in calcites. This relationship indicates that the aragonite precipitated in a relatively open system with higher Mo availability. Thus, we conclude that sulfides with low δ34S values formed at high supply of sulfate under the relatively open system with respect to diffusive replenishment of sulfate, where the carbonate precipitation occurred close to the seafloor due to a strong methane flux. Under vigorous methane flux simultaneously, the high potential of less anoxic conditions, which could limit the additional pyrite accumulation and/or favor the microbial disproportionation, could also be the cause of the low δ34Spy, as supported by samples from the Haima sites. Evidence for this assumption is based on the occurrence of bivalve shells and less enrichment in As and Sb. Conversely, the positive δ34Spy values result from near to complete exhaustion of dissolve sulfate via AOM within a deeper sulfate-methane transition zone, where Mo is less available. The combination of a detailed elemental study of authigenic carbonates with sulfur isotopic composition of sulfide minerals in carbonates is a promising tool for reconstructing the dynamics of seep intensities at modern and, potentially, geological seep sites.

Authigenic carbonates from newly discovered active cold seeps on the northwestern slope of the South China Sea: Constraints on fluid sources, formation environments, and seepage dynamics

Authigenic carbonates recovered from two newly discovered active cold seeps on the northwestern slope of the South China Sea have been studied using petrography, mineralogy, stable carbon and oxygen isotopic, as well as trace element compositions, together with AMS 14C ages of shells of seep-dwelling bivalves to unravel fluid sources, formation conditions, and seepage dynamics. The two seeps (ROV1 and ROV2), referred to as ‘Haima seeps’ herein, are approximately 7 kilometers apart, and are typified by abundant carbonate rocks represented bycrusts and nodules. Aragonite and high-Mg calcite are the main carbonate minerals. Based on low δ13Ccarbonate values ranging from −43.0‰ to −27.5‰ (V-PDB) methane is apparently the predominant carbon source of seep carbonates. The corresponding δ18O values, varying from 2.5‰ to 5.8‰ (V-PDB), mostly are higher than calculated values representing precipitation in equilibrium with seawater (2.5‰ to 3.8‰), which probably reflects past destabilization of locally abundant gas hydrates. In addition, we found that carbonates with bivalve shells are generally aragonite-dominated, and bear no barium enrichment but uranium enrichments, reflecting shallow formation depths close to the seafloor. In contrast, carbonate crusts without bivalve shells and nodules contain more calcite, and are characterized by major molybdenum enrichment and different degrees of barium enrichment, agreeing with precipitation at greater depth under strictly anoxic conditions. AMS 14C ages suggest that a major episode of carbonate precipitation occurred between 6.1 ka and 5.1 ka BP at the Haima seeps, followed by a possibly subordinate episode from approximately 3.9 ka to 2.9 ka BP. The common occurrence of dead bivalves at both sites indicates that chemosynthesis-based communities flourished to a greater extent in the past, probably reflecting a decline of seepage activity in recent times. Overall, these results confirm that authigenic carbonates from gas hydrate-bearing areas can provide insight into long-term seepage dynamics and the genesis and fate of marine gas hydrate reservoirs.

Evidence of intense methane seepages from molybdenum enrichments in gas hydrate-bearing sediments of the northern South China Sea

The application of molybdenum (Mo) enrichment to sediments can potentially provide unique constraints on the methane seepage dynamics at continental margins. We report herein elemental and isotopic measurements for authigenic carbonates and sediment samples in drill cores (~95m long; Site GMGS2-08 at ~800m water depth) from gas hydrate-bearing sediments in the northern South China Sea. Five intervals that were impacted by methane seepages in the sediment profile were identified by the presence of strongly 13C-depleted carbonates (δ13C values as low as −56.8‰). The carbonates often cement chemosynthetic bivalve shell fragments and have anomalous δ18O positive values. These observations suggest that methane seepage was intense during those time intervals and that carbonate precipitation occurred in the uppermost sediments, probably due to gas hydrate dissociation. Interestingly, the five intervals were all characterized by strong Mo enrichments. We thus hypothesize that the presence of strong Mo enrichments is a good indicator of past episodes of methane seepage. Such an environment results in sulfidic conditions that occur in a narrow zone in proximity to seawater close to the seafloor, leading to resultant Mo enrichments in associated sediments. In contrast, environments with low methane flux would lead to the distribution of Mo throughout a wide sulfidic zone and, consequently, an absence of Mo enrichment in the sediments. The insights into the environment for Mo enrichments at seeps have significant implications for tracing past methane seepages, their intensities, and even possible occurrences of gas hydrate dissociation.