Upward Fluid Migration Research Articles

An intraslab seismic sequence activated by the 2011 Tohoku‐oki earthquake: Evidence for fluid‐related embrittlement

AbstractUsing quantitative analysis of waveforms, we investigate a seismic cluster that occurred at a depth of 67 km in the Philippine Sea slab 8 months after the 11 March 2011 megathrust Tohoku‐oki earthquake (Mw 9.0). The sequence started with an M 4.1 normal‐fault event on 14 November in the deepest part of the cluster, and subsequent earthquakes migrated upward by 6 km along a narrow conduit‐like zone. The earthquakes have stress drops of 0.5–40 MPa, and groups of earthquakes with coherent waveforms are observed. We explain these observations in terms of fluid‐related embrittlement and the migration of overpressured fluids. The low‐permeability plate interface of the Pacific plate may have been broken by the coseismic or postseismic slips of the Tohoku‐oki earthquake, with the fluids subsequently being liberated from the underlying crust of the Pacific slab and migrating into the Philippine Sea slab due to the pore pressure gradient. The tensional stresses generated by the coseismic slip promoted the efficient upward migration of fluids and also acted to enhance the deviatoric stress around the source area. The heightened pore pressures and the resulting reduced effective normal stress lowered the strength of the faults sufficiently to bring the system into the brittle regime under the enhanced deviatoric stress. The lag of 8 months may represent the time needed for the unsealing of the plate interface, the upward migration of fluids, and the increase in pore pressure to become sufficient to overcome the lithostatic pressure.

Mass-transport deposits and fluid venting in a transform margin setting, the eastern Demerara Plateau (French Guiana)

The eastern Demerara Plateau offshore French Guiana was surveyed in 2003 during the GUYAPLAC cruise (multibeam bathymetry and acoustic imagery, 6-channel seismic reflection and 3.5 kHz echo-sounding). The data show the “post-transform” Cenozoic that the series located on the outer part of the plateau (below c. 2000 m) contain at least twelve stacked mass transport deposits (MTDs) that have recorded a history of large-scale slope failure, as well as two main normal fault sets that provide possible pathways for upward fluid migration through the series, reaching at high as the uppermost MTDs. Seabed data show that the area above the failures is characterized by circular-to-elongate (slope-parallel) depressions interpreted as fluid seeps (pockmarks), some of them have been modified by along slope currents. We suggest that the development of the MTDs to results from the combinaiton of the presence of fluid overpressure at depth the geometry of the margin's deep structure, in particular the existence of a 'free borderlateral border' on the outermost plateau. Our results also emphasise the role of stratigraphic décollements within the Cenozoic series.

Groundwater helium: An indicator of active tectonic regions along Narmada River, central India

Results of a survey of dissolved helium, fluoride and electrical conductivity in groundwater from across the main stem of the Narmada River, between Bharuch in the west and Amarkantak in the east, are reported. This survey was undertaken to identify active tectonic regions, based on locations of high helium concentrations, interpreted as indicative of upward migration of deep fluids. Existence of deep fluids in this region has been hypothesised earlier based on various geophysical studies in the Narmada Rift Basin — a major tectonic feature in central India. Samples with high helium concentration are clustered in two broad regions with known intersecting faults, indicating the possibility of plumes with high helium concentration being injected into shallower groundwater in these regions, facilitated by these faults and fractures. Locations of groundwater samples having excess fluoride remarkably correspond to these two clusters of excess helium. This suggests a possible commonality between the causal factor of excess helium and higher fluoride in groundwater, which needs to be further investigated.

Fluid evolution and authigenic mineral paragenesis related to salt diapirism – The Mercator mud volcano in the Gulf of Cadiz

The formation of mud volcanoes in the Gulf of Cadiz is closely linked to diapirism in the deep subsurface. The Mercator mud volcano (MMV) is a rare example where diapiric emplacement, in addition to being key for upward fluid migration, is also an important zone for fluid and mineral diagenesis. The most intriguing findings in the near-surface muds of the MMV are extremely high salinities of up to 5.2M of NaCl from diapiric and evaporitic halite dissolution and the occurrence of authigenic gypsum and anhydrite crystals, both of which have not been observed to date in the Gulf of Cadiz. Employing a thermodynamic model we elucidate how the interplay of temperature pulses, strong salinity gradients, and fluid flow dynamically drive mineral dissolution and re-formation. The strong increase in salinity in the pore fluids has important implications for thermodynamic equilibria by significantly lowering the activity of water, thereby raising the gypsum–anhydrite transition zone from >1km to about 400m sediment depth at the MMV. This transition is further shifted to immediately below the seafloor during intervals of active mud and fluid expulsion when the MV surface temperature is heated up to at least 30°C. As a consequence, precipitation of authigenic gypsum near the sediment surface (1–2 mbsf) has been linked to the dissolution of evaporites below the MMV. More precisely, the mechanisms generating supersaturation in the ascending gypsum-saturated MMV fluids are (1) the slow and constant cooling of these fluids along the geothermal gradient during their ascent leading to formation of ubiquitous micro-crystals and (2) the more rapid cooling after a heat pulse or transport from greater and warmer depth during an active mud volcano phase leading to the precipitation of cm-scale gypsum crystals or even fist-size concretions. The MMV fluids approaching the salt diapir from farther below have experienced a genesis similar to those of other mud volcanoes in the Gulf of Cadiz located above deep-rooted faults. These processes include clay mineral dewatering, thermogenic degradation of organic matter and deep high-temperature leaching of terrigenous sediments or continental crust.

Geochemical and geological constraints on the composition of marine sediment pore fluid: Possible link to gas hydrate deposits

Pore water sulfate consumption in marine sediments is controlled by microbially driven sulfate reduction via organo-clastic and methane oxidation processes. In this work, we present sediment pore fluid compositions of 10 long sediment cores and high resolution seismic data from the Krishna–Godavari (K–G) basin, Bay of Bengal. Our results show occurrence of transient (S and kink types) and steady state (quasi-linear) sulfate concentration profiles which are attributed partly to the anaerobic oxidation of methane (δ13CCH4: −84.8 to −100.1‰ VPDB) and organo-clastic sulfate reduction. Influence of AOM on alkalinity is evident from the presence of authigenic carbonate layers with highly depleted carbon isotope ratios in core MD161-8. The authigenic carbonates represent the paleo-SMTZs and suggest marked fluctuation in vertical methane flux. Our geophysical data show the acoustic signatures of upward fluid migration from shallow sub-surface, whereas, coring during NGHP expedition-01 confirms the presence of sub-surface gas hydrate deposits in K–G basin which can be linked to deep methane sources. The geochemical analysis suggests that shallow methane source can be attributed to high burial flux of labile organic matter due to high sedimentation rate. Sampling sites with high methane flux from the shallow gas source are characterized by quasi-linear sulfate concentration profile and a shallow sulfate methane transition zone (SMTZ) and may not be necessarily linked to deeper gas hydrate deposits. In contrast, the deep methane source results in a transient kink type sulfate profile and a deeper SMTZ. We have observed a close link between the occurrence of gas hydrate and the S/kink type sulfate profile. We interpret the short lived ‘kink’ in the sulfate profiles as a result of recent enhancement in vertical methane flux possibly driven by reactivation of fault-fractures systems which provide the conduits for fluid flow.

Shallow gas and focused fluid flow systems in the Pearl River Mouth Basin, northern South China Sea

Analysis of three-dimensional seismic data and multibeam data from the Pearl River Mouth Basin in the northern South China Sea, reveals numerous focused fluid flow features and associated widespread shallow gas from 500m to 2000m water depth. Shallow gas is usually indicated by acoustic anomalies, such as enhanced reflections, acoustic turbidity, and acoustic blanking. Two types of tectonically and stratigraphically controlled fluid flow related systems are identified. The first type can be observed in the deep strata, including gas chimneys, mud diapirs, mud volcanoes, pipes, and large normal faults, which are usually located over basement highs and are related to overpressure mainly caused by gas generation. The focused fluid flow structures serve as pathways for upward migration of thermogenic fluids from source rocks or gas reservoirs. The second type is a shallow fluid flow system consisting of minor normal faults, migrating canyons, mass transport deposits, contourite, and pockmarks. The minor faults and migrating canyons provide pathways for fluids of thermogenic origin transported by the deep fluid flow system and of biogenic origin generated in the shallow succession. The mass transport deposits and contourite mainly serve as caprock. Small-scale fluid seepage is observed at the modern seabed expressed as pockmarks located above shallow gas accumulations. The distribution of shallow gas is controlled by the combination of these two systems. The focused fluid flow features and shallow gas are poorly studied in the study area and we propose a 3D model of focused fluid flow and shallow gas distribution that can be used more widely in other passive and active continental margins. Our results are important to the understanding of resources (hydrocarbon and gas hydrate) exploration in such a petroliferous basin and they must be taken into account when assessing seabed stability.

Intraoceanic subduction of “heterogeneous” oceanic lithosphere in narrow basins: 2D numerical modeling

We provide 2D thermomechanical numerical models to constrain the subduction process of narrow (~ 600km wide) oceanic basins resulting in formation and exhumation of serpentinite-bearing high-pressure (HP) and ultrahigh-pressure (UHP) complexes presently exposed as dismembered massifs in orogenic belts. We simulated subduction of “heterogeneous” (i.e. non layered) oceanic lithosphere testing different serpentinite rheologies and varying other major parameters (initial slab dip, convergence rate, age and structure of oceanic lithosphere, distance subduction zone/continental margin, weak zone geometry). We show that serpentinite rheology is the most important parameter affecting both the geometry and behavior of serpentinite channels that form in the mantle-wedge as the result of upward migration of slab fluids. A strong serpentinite power-law rheology is responsible for the continuous accretion and underthrusting of slab slices at the base of mantle-wedge to form planar- or wedge-shape serpentinitic channels; the great part of sediments is buried only during continental crust subduction but is never exhumed; moreover slab slices do not mix with mantle wedge rocks. Exhumation of mafic/ultramafic rocks occurs from pressure not exceeding 15kbar. On the other hand, a weak Newtonian behavior of serpentine best describes subduction and exhumation dynamics. This rheology produces channels that usually widen to depth; slab and sediment slices are easily incorporated in mantle wedge serpentinite during ongoing convergence; in this case, slab and mantle-wedge serpentinite are mixed together and can be closely associated in the channel. The models running a Newtonian rheology highlight that slices of the oceanic overriding plate can be eroded and dragged in the channel till great depth. A weak power-law serpentinite rheology produces a transitional behavior between that of strong power-law and Newtonian rheology. The serpentinitic mélange, produced using a Newtonian rheology, is finally exhumed from pressure >20kbar during the final stages of continental crust subduction and collision. The rise to shallow depth of the HP mélange is driven by concurrent serpentinite buoyancy, slab roll-back and asthenospheric mantle upwelling. Moreover, we propose that the subduction/exhumation dynamics observed in the model acted during closure of the narrow Alpine Tethys ocean; in particular, we suggest that some slices presently outcropping in HP ophiolitic massifs of the Western Alps (Voltri and Monviso massifs) were tectonically coupled at slab/mantle interface and exhumed during collision in a soft, low-viscosity serpentinite channel.We finally investigated intensity and dynamics of magmatic processes during subduction. Our numerical experiments suggest that even subduction of narrow oceanic basins may indeed allow the development of a magmatic arc. In such relatively small short-living arcs, steeper, faster and younger slabs produce earlier onset of the volcanism after the beginning of subduction.

Dynamics of fault-fluid-hydrate system around a shale-cored anticline in deepwater Nigeria

[1] Gas hydrates were recovered by coring at the eastern border of a shale-cored anticline in the eastern Niger Delta. To characterize the link between faults and fluid release and to identify the role of fluid flow in the gas hydrate dynamics, three piezometers were deployed for periods ranging from 387 to 435 days. Two of them were deployed along a major fault linked to a shallow hydrocarbon reservoir while the third monitored the fluid pressure in a pockmark aligned above the same major fault. In addition, 10 ocean-bottom seismometers (OBS) were deployed for around 60 days. The piezometers simultaneously registered a prolonged fluid flow event lasting 90 days. During this time, OBS measurements record several episodic fluid release events. By combining and analyzing existing and newly acquired data, we show that the fluid-fault system operates according to the following three stages: (1) upward pore fluid migration through existing conduits and free gas circulation within several shallow sandy layers intersecting the major fault, (2) gas accumulation and pore pressure increases within sandy-silty layers, and (3) hydrofracturing and fluid pressure dissipation through sporadic degassing events, causing pore fluid circulation through shallow sandy layers and drawing overlying seawater into the sediment. This paper clearly demonstrates how an integrated approach based on seafloor observations, in situ measurements, and monitoring is essential for understanding fault-fluid-hydrate systems.

Grain-size distribution in the mantle wedge of subduction zones

[1] Mineral grain size plays an important role in controlling many processes in the mantle wedge of subduction zones, including mantle flow and fluid migration. To investigate the grain-size distribution in the mantle wedge, we coupled a two-dimensional (2-D) steady state finite element thermal and mantle-flow model with a laboratory-derived grain-size evolution model. In our coupled model, the mantle wedge has a composite olivine rheology that incorporates grain-size-dependent diffusion creep and grain-size-independent dislocation creep. Our results show that all subduction settings lead to a characteristic grain-size distribution, in which grain size increases from 10 to 100 μm at the most trenchward part of the creeping region to a few centimeters in the subarc mantle. Despite the large variation in grain size, its effect on the mantle rheology and flow is very small, as >90% of the deformation in the flowing part of the creeping region is accommodated by grain-size-independent dislocation creep. The predicted grain-size distribution leads to a downdip increase in permeability by ∼5 orders of magnitude. This increase is likely to promote greater upward migration of aqueous fluids and melts where the slab reaches ∼100 km depth compared with shallower depths, potentially providing an explanation for the relatively uniform subarc slab depth. Seismic attenuation derived from the predicted grain-size distribution and thermal field is consistent with the observed seismic structure in the mantle wedge at many subduction zones, without requiring a significant contribution by the presence of melt.

Geophysical signatures associated with fluid flow and gas hydrate occurrence in a tectonically quiescent sequence, Qiongdongnan Basin, South China Sea

AbstractInterpretation of high‐resolution two‐dimensional (2D) and three‐dimensional (3D) seismic data collected in the Qiongdongnan Basin, South China Sea reveals the presence of polygonal faults, pockmarks, gas chimneys and slope failure in strata of Pliocene and younger age. The gas chimneys are characterized by low‐amplitude reflections, acoustic turbidity and low P‐wave velocity indicating fluid expulsion pathways. Coherence time slices show that the polygonal faults are restricted to sediments with moderate‐amplitude, continuous reflections. Gas hydrates are identified in seismic data by the presence of bottom simulating reflectors (BSRs), which have high amplitude, reverse polarity and are subparallel to seafloor. Mud diapirism and mounded structures have variable geometry and a great diversity regarding the origin of the fluid and the parent beds. The gas chimneys, mud diapirism, polygonal faults and a seismic facies‐change facilitate the upward migration of thermogenic fluids from underlying sediments. Fluids can be temporarily trapped below the gas hydrate stability zone, but fluid advection may cause gas hydrate dissociation and affect the thickness of gas hydrate zone. The fluid accumulation leads to the generation of excess pore fluids that release along faults, forming pockmarks and mud volcanoes on the seafloor. These features are indicators of fluid flow in a tectonically‐quiescent sequence, Qiongdongnan Basin. Geofluids (2010) 10, 351–368

Acoustic detection of seabed hydrocarbon seepage in the north depression of South Yellow Sea Basin

Abstract The full-covered Side-Scan Sonar and high-resolution seismic survey method is initially used to research on the seabed hydrocarbon seepage in the survey area, for the purpose of the following: clarifying the shallow acoustic features related to seabed hydrocarbon seepage in the north depression area of South Yellow Sea Basin, analyzing the relations between the seepage and the structure, and providing evidence for selection of geochemistry sampling station and petroleum exploration. On the mosaic image of Side-Scan Sonar, the strong reflecting spots in strip-like distribution present the physiognomy of pockmark and dome. On the seismic profile, we can see the characteristics, such as enhanced amplitude, columnar disturbance, gas chimney, and blank reflection, which result from gas accumulation. The analysis results indicate that the widely developed faults that approach or connect to seabed provide the channel and source of hydrocarbon seepage. The accumulation and seepage of subsea shallow-layer gas results from the upward migration of fluids from deep-layer pores, and the sub-sea survey area is still at tiny seepage stage.

Simplified Model for CO2 Leakage and its Attenuation due to Geological Structures

For risk associated with storage of CO2 under the Earth's surface, the impervious cap is one of the most significant factors. Geological structures such as faults can provide conduits for CO2 to escape through the cap. When conductive faults intersect the storage formation and overlying permeable layers, CO2 leaks may exhibit three distinct behaviors: initial upward migration of fluids through the fault, lateral fluid movement through permeable layers, and continued movement of CO2 along the fault above the leakage pathways. To quantify this behavior, especially the attenuation, we develop a quasi-1D model for migration of buoyant fluid from a reservoir along a conductive fault. The fault can intersect multiple shallower formations. The model accounts for flow from the fault into a permeable formation using the concept of leakoff. The leakoff coefficient depends on the geometry and on the petrophysical properties of the formation. We use a commercial simulator (GEM from CMG) to run 2D verification studies of the 1D model. We present a series of examples that illustrate the controlling mechanisms for leakage rate from the reservoir and its attenuation by flux into shallower layers. Leakage flux and its attenuation vary nonlinearly with the permeability of the fault and the permeability of the shallower layers intersected by the fault. Permeable layers near the CO2 storage reservoir exhibit the greatest attenuation. While there is a nonlinear relationship between flux through the fault and the size of the leakoff coefficient, there is a linear relationship between the percentage which leaks off into neighboring formations and the ratio of fault permeability to leakoff coefficient. The rate of attenuation of CO2 leakage depends on geometric and/or petrophysical properties of highly permeable structures as well as reservoir properties. An analytical derivation of the leak-off coefficient based on Darcy's equation compares favorably with 2D simulations. On the other hand, in the case of tilted permeable layers, calculation of the leakoff coefficient is less straightforward due to preferential flow within the layer (lighter CO2 rising above heavier brine).

Morphogenesis of the SW Balearic continental slope and adjacent abyssal plain, Western Mediterranean Sea

We present the seafloor morphology and shallow seismic structure of the continental slope south-east of the Balearic promontory and of the adjacent Algero-Balearic abyssal plain from multibeam and chirp sonar data. The main purpose of this research was to identify the sediment pathways from the Balearic promontory to the Algero-Balearic deep basin from the Early Pliocene to the Present. The morphology of the southern Balearic margin is controlled by a SW–NE structural trend, whose main expressions are the Emile Baudot Escarpment transform fault, and a newly discovered WSW–ENE trend that affects the SW end of the escarpment and the abyssal plain. We relate the two structural trends to right-lateral simple shear as a consequence of the Miocene westward migration of the Gibraltar Arc. Newly discovered steep and narrow volcanic ridges were probably enabled to grow by local transtension along the transform margin. Abyssal plain knolls and seahills relate to the subsurface deformation of early stage halokinetic structures such as salt rollers, salt anticlines, and salt pillows. The limited thickness of the overburden and the limited amount of deformation in the deep basin prevent the formation of more mature halokinetic structures such as diapirs, salt walls, bulbs, and salt extrusions. The uppermost sediment cover is affected by a dense pattern of sub-vertical small throw normal faults resulting from extensional stress induced in the overburden by subsurface salt deformation structures. Shallow gas seismic character and the possible presence of an active polygonal fault system suggest upward fluid migration and fluid and sediment expulsion at the seafloor through a probable mud volcano and other piercement structures. One large debris flow deposit, named Formentera Debris Flow, has been identified on the lower slope and rise of the south Formentera margin. Based on current observations, we hypothesize that the landslide originating the Formentera Debris Flow occurred in the Holocene, perhaps in historical times.

Crustal structure of the ultra-slow spreading Knipovich Ridge, North Atlantic, along a presumed amagmatic portion of oceanic crustal formation

The ultra-slow, asymmetrically-spreading Knipovich Ridge is the northernmost part of the Mid Atlantic ridge system. In the autumn of 2002 a combined ocean-bottom seismometer multichannel seismic (OBS/MCS) and gravity survey along the spreading direction of the Knipovich Ridge was carried out. The main objective of the study was to gain an insight into the crustal structure and composition of what is assumed to be an amagmatic segment of oceanic crust. P-wave velocity and Vp/Vs models were built and complemented by a gravity model. The 190 km long transect reveals a much more complex crustal structure than anticipated. The magmatic crust is thinner than the global average of 7.1 ± 1.0 km. The young fractured portion of Oceanic Layer 2 has low seismic velocities while the older part has normal seismic velocities and is broken into several rotated fault blocks seen as thickness variations of Layer 2. The youngest part of Oceanic Layer 3 is also dominated by low velocities, indicative of fracturing, seawater circulation and thermal expansion. The remaining portion of Layer 3 exhibits inverse variations in thickness and seismic velocity. This is explained by a sequence of periods of faster spreading (estimated to be up to 8 mm/year from interpretation of magnetic anomalies) when more normal gabbroic crust was being generated and periods of slower spreading (5.5 mm/year) when amagmatic stretching and serpentinization of the upper mantle occurred, and crust composed of mixed gabbro and serpentinized mantle was generated. The volumetric changes and upward fluid migration, associated with the process of serpentinization in this part of the crust, caused disruption to the overlying sedimentary layers.

Scientific results from Gulf of Mexico Gas Hydrates Joint Industry Project Leg 1 drilling: Introduction and overview

The Gulf of Mexico Gas Hydrates Joint Industry Project (JIP) is a consortium of production and service companies and some government agencies formed to address the challenges that gas hydrates pose for deepwater exploration and production. In partnership with the U.S. Department of Energy and with scientific assistance from the U.S. Geological Survey and academic partners, the JIP has focused on studies to assess hazards associated with drilling the fine-grained, hydrate-bearing sediments that dominate much of the shallow subseafloor in the deepwater (>500 m) Gulf of Mexico. In preparation for an initial drilling, logging, and coring program, the JIP sponsored a multi-year research effort that included: (a) the development of borehole stability models for hydrate-bearing sediments; (b) exhaustive laboratory measurements of the physical properties of hydrate-bearing sediments; (c) refinement of new techniques for processing industry-standard 3-D seismic data to constrain gas hydrate saturations; and (d) construction of instrumentation to measure the physical properties of sediment cores that had never been removed from in situ hydrostatic pressure conditions. Following review of potential drilling sites, the JIP launched a 35-day expedition in Spring 2005 to acquire well logs and sediment cores at sites in Atwater Valley lease blocks 13/14 and Keathley Canyon lease block 151 in the northern Gulf of Mexico minibasin province. The Keathley Canyon site has a bottom simulating reflection at ∼392 m below the seafloor, while the Atwater Valley location is characterized by seafloor mounds with an underlying upwarped seismic reflection consistent with upward fluid migration and possible shoaling of the base of the gas hydrate stability (BGHS). No gas hydrate was recovered at the drill sites, but logging data, and to some extent cores, suggest the occurrence of gas hydrate in inferred coarser-grained beds and fractures, particularly between 220 and 330 m below the seafloor at the Keathley Canyon site. This paper provides an overview of the results of the initial phases of the JIP work and introduces the 15 papers that make up this special volume on the scientific results related to the 2005 logging and drilling expedition.

Insights on the March 1998 eruption at Piton de la Fournaise volcano (La Réunion) from microgravity monitoring

We investigate the temporal gravity changes associated with one of the major recent eruptions at Piton de la Fournaise volcano (March 1998) that occurred after an unusual five‐year quiet period and initiated a new eruptive cycle. Repeated microgravity surveys allowed us to measure residual gravity changes up to 100 μGal within the Enclos Fouqué caldera for four months before the start of the eruption. We first analyzed the temporal gravity changes and the height changes, also measured at the gravity benchmarks, on the basis of an intrusive dyke model previously proposed for this eruption from InSAR and tilt data. This analysis reveals that such simple model (finite rectangular shaped tensile dislocation) cannot fit both gravity and geodetic data and leads us to propose a dual source gravity model for this eruption. The gravity data inversion using a genetic algorithm search method allows us to quantify the mass change produced by the intrusive dyke (3.9 to 8.7 × 109kg) and to suggest an additional mass increase at sea level depth (4.6 to 7.2 × 1010kg). We interpret the latter as the effect of magma ascent into a reservoir that occurred within the four months before the eruption. We suggest that high concentrations of gas content in the magma reservoir account for a higher compressibility of the intruded magma and produce significant mass change with moderate surface deformation. The 1996 seismic crisis, interpreted as an upward migration of magmatic fluids into a reservoir located at sea level, might support such a hypothesis.

Origin and accumulation of carbon dioxide in the Huanghua depression, Bohai Bay Basin, China

The CO2 content in natural gas in the Huanghua depression, Bohai Bay Basin, China, is highly variable, ranging from 0.003 to 99.6%. Understanding the origin and distribution of the CO2 is important to assess risk prior to drilling. This study uses gas geochemistry to identify the origins of CO2 in the sedimentary basin and places these findings within a geologic context. Chemical compositions, , 3He/4He, and 40Ar/36Ar were measured for 50 gas samples collected from gas- and oil-producing wells located in different tectonic regions in the depression. From these analyses, we determined that the CO2 in the Huanghua depression originated from three sources: thermal decomposition of organic matter, thermal decomposition of carbonate minerals, and mantle degassing. Gases with low amounts (3%) of CO2 tend to be organogenic. This organogenic CO2 occurs in hydrocarbon accumulations and is characterized by values ranging from 20 to 10 and low 3He/4He (R/Ra 1, herein R and Ra represent the 3He/4He ratio of sample and air, respectively). Carbon dioxide originating from thermal carbonate decomposition occurs as a minor component (10%) in hydrocarbon gas accumulations and is characterized by a narrow range of (2 to +2) and R/Ra 1. Huanghua depression natural gases with CO2 content in excess of 15% resulted from mantle degassing and mainly occur at the intersection of faults. These gases have 3He/4He ratios in excess of atmospheric value (R/Ra 1) and ranging from 5 to 3. Volatiles from mantle degassing during the postmagmatic stage are the most likely major source for CO2 in these high-CO2-content reservoirs. Basement faults likely provide pathways for the upward migration of CO2-rich mantle fluids. Consequently, CO2-rich gas pools are locally concentrated in the Gangxi and Dazhongwang fault zones within the depression.

4D seismic time-lapse monitoring of an active cold vent, northern Cascadia margin

Two single-channel seismic (SCS) data sets collected in 2000 and 2005 were used for a four-dimensional (4D) time-lapse analysis of an active cold vent (Bullseye Vent). The data set acquired in 2000 serves as a reference in the applied processing sequence. The 4D processing sequence utilizes time- and phase-matching, gain adjustments and shaping filters to transform the 2005 data set so that it is most comparable to the conditions under which the 2000 data were acquired. The cold vent is characterized by seismic blanking, which is a result of the presence of gas hydrate in the subsurface either within coarser-grained turbidite sands or in fractures, as well as free gas trapped in these fracture systems. The area of blanking was defined using the seismic attributes instantaneous amplitude and similarity. Several areas were identified where blanking was reduced in 2005 relative to 2000. But most of the centre of Bullseye Vent and the area around it were seen to be characterized by intensified blanking in 2005. Tracing these areas of intensified blanking through the three-dimensional (3D) seismic volume defined several apparent new flow pathways that were not seen in the 2000 data, which are interpreted as newly generated fractures/faults for upward fluid migration. Intensified blanking is interpreted as a result of new formation of gas hydrate in the subsurface along new fracture pathways. Areas with reduced blanking may be zones where formerly plugged fractures that had trapped some free gas may have been opened and free gas was liberated.

Geochemical studies of pore fluid in surface sediment on the Daini Atsumi Knoll

We collected sediment samples and pore water samples from the surface sediment on the Daini Atsumi Knoll, and analyzed the sediments for CH 4, C 2H 6, and δ 13C CH4, and the pore fluids for CH 4, C 2H 6, δ 13C CH4, Cl −, SO 4 2−, δ 18O H2O, and δD H2O, respectively. A comparison of the measured concentration and isotopic composition of methane in pore water samples with those in sediment samples revealed that methane was present in the sediment samples at a higher concentration and was isotopically heavier than those in the pore water samples. It suggests that the effect of the release of a sorbed gas bound to organic particles when heated prior to analysis of hydrocarbons was larger than that of the degassing process. A large amount of a sorbed gas would be a significant source of natural gas. Two striking features are the chemical and isotopic composition of the pore water samples taken from the different sites around the Daini Atsumi Knoll. In the KL09, KL10, and KP07 samples, Cl − concentrations in the pore water samples showed depletion to a minimum of 460 mmol/kg, correspond to ∼ 17% dilution of seawater, however the latter was not enriched in CH 4. The isotopic compositions of pore water samples suggested the low-Cl − fluids in the pore water were not derived from dissociation of methane hydrate, but were derived from input of meteoric water. In contrast, in the KP05 samples from the north flank of the Daini Atsumi Knoll, pore water were characterized by CH 4 enrichment more than 370 μmol/kg, but not depleted in Cl − concentrations. The observed methane concentration in the KP05 samples is not sufficient for methane hydrate to form in situ, indicating that the existence of methane hydrate in the surface sediment is negligible, as supported by Cl − concentration. Based on the stable carbon isotope ratio of methane in the pore fluid from the KP05 site ( δ 13C CH4 < − 50‰PDB), methane is thought to be of microbial origin. The pore waters in the surface sediments in the north flank of the Daini Atsumi Knoll were not directly influenced by upward fluid bearing methane of thermogenic origin from a deeper part of the sedimentary layer. However, extremely high methane concentration in the north flank site as compared with the concentration of pore water taken from the normal seafloor suggests that the north flank site is not the normal seafloor. We hypothesize that upward migration of chemically-reduced fluids from a deeper zone of the sedimentary layer reduces chemically-oxidized solutes in the surface sediment. As a consequence methane production replaced sulfate reduction as the microbial metabolism in the reduced environment of the surface sediment.

Sediment injection in the pit of the Urania Anoxic brine lake (Eastern Mediterranean)

The Urania brine-filled, anoxic basin shows physical and geological characteristics which set it aside from the previously studied anoxic basins of the Eastern Mediterranean. The detailed study of a core raised from the western horn of the horseshoe-shaped basin provides evidence for the upward injection of old sediments into more recent, surficial anoxic mud. The oldest intruded sediments identified are Zanclean in age, and pelagic in facies (MPl3 and MPl4a biozones). Debris-flow or flow-in mechanisms are discussed, but discarded. The sediment injection agrees with the very high bottom water temperatures measured in the pit, the very high heat flow and a concave downwards pore water dissolved chlorosity gradient, all indicative of upward fluid migration. Mud expulsion features were recorded close to the location of the core. Movement along a fault plane could be responsible for the upward injection of old sediments and for their expulsion on the sea-floor.