Gas Chimneys Research Articles

Shallow gas accumulations and seepage in the sediments of the Northeast Persian Gulf

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

Deep‐sourced gas seepage and methane‐derived carbonates in the Northern Adriatic Sea

AbstractIn the Northern Adriatic Sea, the occurrence of gas seepage and of unique rock outcrops has been widely documented. The genesis of these deposits has recently been ascribed to gas venting, leading to their classification as methane‐derived carbonates. However, the origin of seeping gas was not clearly constrained. Geophysical data collected in 2009 reveal that the gas‐enriched fluid vents are deeply rooted. In fact, the entire Plio‐Quaternary succession is characterized by widespread seismic anomalies represented by wipe‐out zones, and interpreted as gas chimneys. They commonly root at the base of the Pliocene sequence but also within the Palaeogene succession, where they appear to be associated to deep‐seated faults. We suggest that there is a structural control on chimney distribution. Chimneys originate and terminate at different stratigraphic levels; commonly they reach the seafloor, where authigenic carbonate deposits form locally. Gas analyses of some gas bubble streams just above the rock outcrops reveal that gas is composed mainly of methane. Geochemical analyses performed at four selected outcrop sites show that these deposits formed as a consequence of active gas venting. In particular, geochemical analyses indicate carbonate precipitation from microbial oxidation of methane‐rich fluids, although a straightforward correlation with the source depth of gas feeding the authigenic carbonates cannot yet be clearly defined.

The evolution of methane vents that pierce the hydrate stability zone in the world's oceans

AbstractWe present a one‐dimensional model that couples the thermodynamics of hydrate solidification with multiphase flow to illuminate how gas vents pierce the hydrate stability zone in the world's oceans. During the propagation phase, a free‐gas/hydrate reaction front propagates toward the seafloor, elevating salinity and temperature to three‐phase (gas, liquid, and hydrate) equilibrium. After the reaction front breaches the seafloor, the temperature gradient in the gas chimney dissipates to background values, and salinity increases to maintain three‐phase equilibrium. Ultimately, a steady state is reached in which hydrate formation occurs just below the seabed at a rate necessary to replace salt loss. We show that at the Ursa vent in the Gulf of Mexico, the observed salinity and temperature gradients can be simulated as a steady state system with an upward flow of water equal to 9.5 mm yr−1 and a gas flux no less than 1.3 kg m−2 yr−1. Many of the world's gas vents may record this steady state behavior, which is characterized by elevated temperatures and high salinities near the seafloor.

Control of Quaternary sea‐level changes on gas seeps

AbstractGas seeping to the seafloor through structures such as pockmarks may contribute significantly to the enrichment of atmospheric greenhouse gases and global warming. Gas seeps in the Gulf of Lions, Western Mediterranean, are cyclical, and pockmark “life” is governed both by sediment accumulation on the continental margin and Quaternary climate changes. Three‐dimensional seismic data, correlated to multi‐proxy analysis of a deep borehole, have shown that these pockmarks are associated with oblique chimneys. The prograding chimney geometry demonstrates the syn‐sedimentary and long‐lasting functioning of the gas seeps. Gas chimneys have reworked chronologically constrained stratigraphic units and have functioned episodically, with maximum activity around sea level lowstands. Therefore, we argue that one of the main driving mechanisms responsible for their formation is the variation in hydrostatic pressure driven by relative sea level changes.

Q-compensated reverse-time migration

Reduced amplitude and distorted dispersion of seismic waves caused by attenuation, especially strong attenuation, always degrades the resolution of migrated images. To improve image resolution, we evaluated a methodology of compensating for attenuation ([Formula: see text]) effects in reverse-time migration ([Formula: see text]-RTM). The [Formula: see text]-RTM approach worked by mitigating the amplitude attenuation and phase dispersion effects in source and receiver wavefields. Source and receiver wavefields were extrapolated using a previously published time-domain viscoacoustic wave equation that offered separated amplitude attenuation and phase dispersion operators. In our [Formula: see text]-RTM implementation, therefore, attenuation- and dispersion-compensated operators were constructed by reversing the sign of attenuation operator and leaving the sign of dispersion operator unchanged, respectively. Further, we designed a low-pass filter for attenuation and dispersion operators to stabilize the compensating procedure. Finally, we tested the [Formula: see text]-RTM approach on a simple layer model and the more realistic BP gas chimney model. Numerical results demonstrated that the [Formula: see text]-RTM approach produced higher resolution images with improved amplitude and phase compared to the noncompensated RTM, particularly beneath high-attenuation zones.

Distribution and geological control of mud volcanoes and other fluid/free gas seepage features in the Mediterranean Sea and nearby Gulf of Cadiz

Existing knowledge on the distribution of mud volcanoes (MVs) and other significant fluid/free gas-venting features (mud cones, mud pies, mud-brine pools, mud carbonate cones, gas chimneys and, in some cases, pockmark fields) discovered on the seafloor of the Mediterranean Sea and in the nearby Gulf of Cadiz has been compiled using regional geophysical information (including multibeam coverage of most deepwater areas). The resulting dataset comprises both features proven from geological sampling, or in situ observations, and many previously unrecognized MVs inferred from geophysical evidence. The synthesis reveals that MVs clearly have non-random distributions that correspond to two main geodynamic settings: (1) the vast majority occur along the various tectono-sedimentary accretionary wedges of the Africa-Eurasia subduction zone, particularly in the central and eastern Mediterranean basins (external Calabrian Arc, Mediterranean Ridge, Florence Rise) but also along its westernmost boundary in the Gulf of Cadiz; (2) other MVs characterize thick depocentres along parts of the Mesozoic passive continental margins that border Africa from eastern Tunisia to the Levantine coasts, particularly off Egypt and, locally, within some areas of the western Mediterranean back-arc basins. Meaningfully accounting for MV distribution necessitates evidence of overpressured fluids and mud-rich layers. In addition, cross-correlations between MVs and other GIS-based data, such as maps of the Messinian evaporite basins and/or active (or recently active) tectonic trends, stress the importance of assessing geological control in terms of the presence, or not, of thick seals and potential conduits. It is contended that new MV discoveries may be expected in the study region, particularly along the southern Ionian Sea continental margins.

Submarine gas seepage in a mixed contractional and shear deformation regime: Cases from the Hikurangi oblique‐subduction margin

Gas seepage from marine sediments has implications for understanding feedbacks between the global carbon reservoir, seabed ecology, and climate change. Although the relationship between hydrates, gas chimneys, and seafloor seepage is well established, the nature of fluid sources and plumbing mechanisms controlling fluid escape into the hydrate zone and up to the seafloor remain one of the least understood components of fluid migration systems. In this study, we present the analysis of new three‐dimensional high‐resolution seismic data acquired to investigate fluid migration systems sustaining active seafloor seepage at Omakere Ridge, on the Hikurangi subduction margin, New Zealand. The analysis reveals at high resolution, complex overprinting fault structures (i.e., protothrusts, normal faults from flexural extension, and shallow (<1 km) arrays of oblique shear structures) implicated in fluid migration within the gas hydrate stability zone in an area of 2 × 7 km. In addition to fluid migration systems sustaining seafloor seepage on both sides of a central thrust fault, the data show seismic evidence for subseafloor gas‐rich fluid accumulation associated with proto‐thrusts and extensional faults. In these latter systems fluid pressure dissipation through time has been favored, hindering the development of gas chimneys. We discuss the elements of the distinct fluid migration systems and the influence that a complex partitioning of stress may have on the evolution of fluid flow systems in active subduction margins.

Estimating randomness using seismic disorder

The new seismic disorder attribute quantitatively describes the degree of randomness embedded in 3D poststack seismic data. We compute seismic disorder using a filter operation that removes simple structures including constant values, constant slopes, and steps in axial directions. We define the power of the filtered data as the seismic disorder attribute, which approximately represents data randomness. Seismic data irregularities are caused by a variety of reasons, including random reflection, diffraction, near-surface variations, and acquisition noise. Consequently, the spatial distribution of the seismic disorder attribute may help hydrocarbon exploration in several ways, including identifying geologic features such as fracture zones, gas chimneys, and terminated unconformities; indicating the signal-to-noise ratio to assess data quality; and providing a confidence index for reservoir simulation and engineering projects. We present three case studies and a comparison to other noise-estimation methods.

Formation of pockmarks and submarine canyons associated with dissociation of gas hydrates on the Joetsu Knoll, eastern margin of the Sea of Japan

This study, based on 3.5kHz SBP, 3D seismic data and long piston cores obtained during MD179 cruise, elucidated the timing and causes of pockmark and submarine canyon formation on the Joetsu Knoll in the eastern margin of the Sea of Japan. Gas hydrate mounds and pockmarks aligned parallel to the axis on the top of the Joetsu Knoll are associated with gas chimneys, pull-up structures, faults, and multiple bottom-simulating reflectors (BSRs), suggesting that thermogenic gas migrated upward through gas chimneys and faults from deep hydrocarbon sources and reservoirs. Seismic and core data suggest that submarine canyons on the western slope of the Joetsu Knoll were formed by turbidity currents generated by sand and mud ejection from pockmarks on the knoll. The pockmark and canyon formation probably commenced during the sea-level fall, lasting until transgression stages. Subsequently, hydropressure release during the sea level lowering might have instigated dissociation of the gas hydrate around the base of the gas hydrate, leading to generation and migration of large volumes of methane gas to the seafloor. Accumulation of hydrate caps below mounds eventually caused the collapse of the mounds and the formation of large depressions (pockmarks) along with ejection of sand and mud out of the pockmarks, thereby generating turbidity currents. Prolonged pockmark and submarine canyon activities might have persisted until the transgression stage because of time lags from gas hydrate dissociation around the base of the gas hydrate until upward migration to the seafloor. This study revealed the possibility that submarine canyons were formed by pockmark activities. If that process occurred, it would present important implications for reconstructing the long-term history of shallow gas hydrate activity based on submarine canyon development.

Submarine landslide identified in MD179 cores, off-Joetsu area, eastern margin of the Sea of Japan

Gas hydrate is exposed on the sea floor and is buried in shallow sediments in the off-Joetsu area at the eastern margin of the Sea of Japan. Sediment cores recovered from topographic highs of the Joetsu Knoll and Umitaka Spur show pockmarks and mounds formed by gas hydrate dissociation, but those from the Un-named ridge have no such topographic features. All topographic highs and pockmarks mainly comprise bioturbated layers interbedded with thinly laminated (TL) layers, which are common Sea of Japan sediments. Recovered sediments are, however, mostly disturbed by submarine landslides, showing tilted horizons, faults, slump folds, and breccia, except that from the Un-named ridge. The timing of events is well constrained by identification of the number of TL layers in some sediment cores. Landslides occurred both during the cold glacial period of the late MIS3 to the last glacial maximum (LGM) and during the warm interglacial period of the post-LGM. All were caused by the explosive rise of gas hydrate formed at very shallow depths of the sea bottom by the supply of gas from the depth of the gas hydrate stability zone through gas chimney passages developed under the pockmarks. Seismic activity demands consideration as a factor because the off-Joetsu area is tectonically active.

Distribution of subsurface fluid-flow systems in the SW Barents Sea

The SW Barents Sea is a large hydrocarbon-prone epi-continental Sea of the Norwegian Arctic region. A significant portion of the hydrocarbon gases generated in deep source rocks has leaked or migrated into the shallow subsurface and is now trapped in gas hydrate and shallow gas reservoirs. The evolution of sedimentary basins of this region has controlled the leakage of these fluids through marine sediments. Understanding the distribution of various fluid-flow systems may enhance our knowledge of the evolution of different basins in the SW Barents Sea and could help find potential targets for future hydrocarbon exploration. We analyze approximately 3000 2D multi-channel seismic profiles and data from 60 wells covering the entire SW Barents Sea, to identify and classify fluid-flow features, and study their relationship to tectonic elements and geological history. Gas chimneys are the most abundant feature among various other fluid-flow features such as fluid leakage along faults and fractures, seepage pipes, and high amplitude anomalies potentially indicating trapped fluids. Large fluid-flow features, covering areas as large as 600 km2, occur close to known hydrocarbon fields such as Snøhvit, Skrugard, and Havis. The fluid-flow features occur above major deep-seated faults in the area suggesting a close relation to it. The number of fluid-flow features in the western part of the study area is significantly higher than in the eastern part. The amount of net erosion in the study area shows no direct control over the distribution of fluid-flow features, suggesting that the faults and distribution of mature source rocks control the fluid flow. The strong correlation between the locations of fluid-flow features and structural elements indicates that extensional tectonics, uplift and glaciations could have played major roles in the timing and activity of the fluid leakage, although erosion might have had an added effect.

Focused fluid flow in the Baiyun Sag, northern South China Sea: implications for the source of gas in hydrate reservoirs

The origin and migration of natural gas and the accumulation of gas hydrates within the Pearl River Mouth Basin of the northern South China Sea are poorly understood. Based on high-resolution 2D/3D seismic data, three environments of focused fluid flow: gas chimneys, mud diapirs and active faults have been identified. Widespread gas chimneys that act as important conduits for fluid flow are located below bottom simulating reflections and above basal uplifts. The occurrence and evolution of gas chimneys can be divided into a violent eruptive stage and a quiet seepage stage. For most gas chimneys, the strong eruptions are deduced to have happened during the Dongsha Movement in the latest Miocene, which are observed below Pliocene strata and few active faults develop above the top of the Miocene. The formation pressures of the Baiyun Sag currently are considered to be normal, based on these terms: 1) Borehole pressure tests with pressure coefficients of 1.043–1.047; 2) The distribution of gas chimneys is limited to strata older than the Pliocene; 3) Disseminated methane hydrates, rather than fractured hydrates, are found in the hydrate samples; 4) The gas hydrate is mainly charged with biogenic gas rather than thermogenic gas based on the chemical tests from gas hydrates cores. However, periods of quiet focused fluid flow also enable the establishment of good conduits for the migration of abundant biogenic gas and lesser volumes of thermogenic gas. A geological model governing fluid flow has been proposed to interpret the release of overpressure, the migration of fluids and the formation of gas hydrates, in an integrated manner. This model suggests that gas chimneys positioned above basal uplifts were caused by the Dongsha Movement at about 5.5 Ma. Biogenic gas occupies the strata above the base of the middle Miocene and migrates slowly into the gas chimney columns. Some of the biogenic gas and small volumes of thermogenic gas eventually contribute to the formation of the gas hydrates.

Gas hydrate and free gas petroleum system in 3D seismic data, offshore Angola

Evidence of gas hydrate and free gas occurrences in a 3D seismic volume from the West-Central Coastal Province of the Congo Fan, offshore Angola, illustrates all the components of a complete petroleum system. Analysis and interpretation are based on the information in attributes calculated from three 3D time-migrated common-angle seismic volumes; the attributes include seismic amplitude, spectral components, dip magnitude, amplitude variation with angle, and instantaneous frequency. The source is organic-rich muds associated with late Cretaceous to early Tertiary channels, the migration paths are along growth faults, and the traps are partly defined by the gas hydrate stability zone (for the gas hydrate), partly by the bottom-simulating reflector (for the subhydrate free gas), and partly by faults (for both). The spatial distribution of free gas is further supported by the associated seismic bright spots, and also by the attenuation of high frequencies of P-waves that traverse the gas-saturated zone. Locally higher temperatures, associated with upward fluid circulation along fault zones, facilitate gas transmission through the gas hydrate and forms gas chimneys that extend to the sea floor.

Normal faults and gas migration in an active plate boundary, southern Taranaki Basin, offshore New Zealand

The function of normal faults in upsequence flow of gas has been examined using two-dimensional and three-dimensional seismic-reflection data from the southern Taranaki Basin, New Zealand. The spatial distributions of late-stage normal faults, gas chimneys, thickness of the Oligocene mudstone-rich seal (Otaraoa Formation), and modeled hydrocarbon expulsion volumes are compared. Gas chimneys are most common above Cretaceous source rocks modeled to have expelled hydrocarbons. Most (70%) of the observed gas chimneys follow, and/or are rooted in, late-stage normal faults. These faults are the primary seal bypass mechanism for hydrocarbons, where they displace the seal (or intersect faults that displace the seal) and the seal is thick (e.g., more than 340 m [1115 ft]). Active vertical gas flow through the seal commenced after the onset of faulting (3.6 Ma), whereas subseal lateral flow started significantly earlier at approximately 15 Ma and resulted in an early charge of structural highs. Gas flow up along faults in low-permeability mudstones (1 md) is channelized with steep chimneys commonly occurring close to fault tips and relay ramps. In these cases, gas flow may be focused by the presence of high densities of open fractures locally elevating upsequence bulk permeabilities to approximately 1 to 400 md.

Active gas migration systems offshore northern Israel, first evidence from seafloor and subsurface data

An active gas migration system across the eastern Mediterranean continental shelf, offshore northern Israel, is described for the first time. Seven vintages of industrial multi-channel seismic reflection and high-resolution single-channel data are integrated with data from a deep borehole, bathymetry and backscatter maps. Combined interpretation, aided by seismic attribute analysis, exhibits a variety of seismic gas signatures across the subsurface and seabed: acoustic blanking, bright spots, phase reversals, gas chimneys, reflector smearing, pockmarks and presumed gas release into the water column. Sub-seafloor gas stems from three main sources: (1) Mid-Pleistocene, (2) Last Glacial Maximum unconformities and (3) a series of incised channels in-between which are also related to glacial maxima. Close to the seafloor, gas accumulates across an area of 72km2 at depths ranging from 37 to 112m below sea-level, while ∼720 pockmarks pierce the seafloor mainly at the gas front periphery. Active bubble emission from one seep at the seafloor was recorded repeatedly over 3 years. We hypothesize that the well developed active gas system shown here for the first time is not unique to the continental shelf off northern Israel. Conversely, it represents the usually underestimated marine methane contribution from mid-latitude continental shelves into the marine environment and maybe into the atmosphere. This study hypothesizes that global warming and sea level rise induces a negative, restraining feedback for gas emission across mid-latitude continental margins and thus being inverse to high latitudes, where this feedback is most likely positive due to the stronger impact of bottom water warming.

Evolution of fluid expulsion and concentrated hydrate zones across the southern Hikurangi subduction margin, New Zealand: An analysis from depth migrated seismic data

Identification of methane sources controlling hydrate distribution and concentrations in continental margins remains a major challenge in gas hydrate research. Lack of deep fluid samples and high quality regional scale seismic reflection data may lead to underestimation of the significance of fluid escape from subducting and compacting sediments in the global inventory of methane reaching the hydrate zone, the water column and the atmosphere. The distribution of concentrated hydrate zones in relation to focused fluid flow across the southern Hikurangi subduction margin was investigated using high quality, long offset (10 km streamer), pre‐stack depth migrated multichannel seismic data. Analysis of low P wave velocity zones, bright‐reverse polarity reflections and dim‐amplitude anomalies reveals pathways for gas escape and zones of gas accumulation. The study shows the structural and stratigraphic settings of three main areas of concentrated hydrates: (1) the Opouawe Bank, dominated by focused periodic fluid input along thrust faults sustaining dynamic hydrate concentrations and gas chimneys development; (2) the frontal anticline, with a basal set of proto‐thrusts controlling permeability for fluids from deeply buried and subducted sediments sustaining hydrate concentrations at the crest of the anticline; and (3) the Hikurangi Channel, with buried sand dominated channels hosting significant amounts of gas beneath the base of the hydrate zone. In sand dominated channels gas injection into the hydrate zone favors highly concentrated hydrate accumulations. The evolution of fluid expulsion controlling hydrate formation offshore southern Hikurangi is described in stages during which different methane sources (in situ, buried and thermogenic) have been dominant.

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.

Seismic velocity and attenuation structures of the Queen Charlotte Basin from full-waveform tomography of seismic reflection data

We applied viscoacoustic waveform tomography to four seismic reflection lines from the central and northern part of the Queen Charlotte sedimentary basin and, using frequencies of 7–12 Hz, we estimated the compressional velocity and attenuation above a depth of approximately 1.2 km. We refined our previously published inversion strategy by alternating between phase-only and amplitude-plus-phase velocity inversion for the first two pairs of frequencies used, and added a second step, in which we inverted for attenuation from the lowest frequency using the final recovered velocity model and an initial homogeneous [Formula: see text]-model. Our recovered velocity and attenuation models demonstrated an overall good correlation with the available sonic and gamma-ray logs. Modeled seismic data matches the field data well and 1D velocity and attenuation profiles extracted at line intersections show a good correlation, thus demonstrating the robust nature of the results. Recovered velocities aid in interpreting shallow structures not readily identifiable on the conventional migration such as Quaternary strata and Pliocene faulting. Recovered attenuation values in the sedimentary rocks are generally consistent with saturated sandstones and consistent with the geology interpreted from well logs. Localized regions of elevated attenuation and associated low velocities correlate with siltstones and shales, the presence of hydrocarbons, or inferred increases in porosity due to fracturing. Seafloor pockmarks, where venting of gas occurs, are underlain by low velocities and an anomalous attenuation variation, and pipe-like gas chimneys are interpreted in two other areas of Hecate Strait. Igneous basement is associated with high velocity and high attenuation in its uppermost part, suggesting the presence of volcanic rocks, but the elevated attenuation may also be due to scattering and elastic mode conversions not included in the viscoacoustic inversion.

Gas hydrates associated with gas chimneys in fine-grained sediments of the northern South China Sea

Drilling for gas hydrates at the northern continental margin of the South China Sea (SCS) provides a unique insight into the formation of the gas hydrate system in fine-grained sediments, even though most high concentrations of hydrates are found in coarse sediments. Detailed studies of 3D seismic data from a deepwater basin (Baiyun Depression) of this region reveal the presence of overpressured fluid flow, mostly manifested in gas chimneys. Gas chimneys are commonly characterized by disrupted reflections (DR), dim amplitude anomalies (DA), and enhanced reflections (ER) on conventional seismic profiles. Gas chimneys are also characterized by chaotic, low-continuity and low-frequency on instantaneous frequency and instantaneous amplitude profiles. Gas hydrates and a prominent BSR were discovered in the region of gas chimney occurrence. Logging data and sample analyses from drill holes over the chimney structures indicate close relationships between active fluid flow and the formation and accumulation of gas hydrates. Our results document that gas chimneys consisting of a connected network of fractures provide a passageway along which fluids ascend beneath the gas hydrate stability zone (GHSZ) for hydrate formation in fine-grained sediments of the northern South China Sea. Fluids accumulate in uppermost part of the slope where they are trapped beneath the prograding submarine delta sequence, which is not permeable enough for fluids to migrate through. Gas-charged fluids may originate from deep-seated hydrocarbon reservoirs, which are indicated by the molecular and isotopic signatures of gases in gas hydrates occurrence zone. We suppose that the fractures extend into the GHSZ, creating a space for hydrate condensation in the fine-grained sediments of the slope. Gas chimneys may account for the overpressured fluid activity and, most likely, the gas hydrate enrichment in the northern SCS.

Heat flow derived from BSR and its implications for gas hydrate stability zone in Shenhu Area of northern South China Sea

Bottom simulating reflectors (BSRs), known as the base of gas hydrate stability zone, have been recognized and mapped using good quality three-dimensional (3D) pre-stack migration seismic data in Shenhu Area of northern South China Sea. Additionally, seismic attribute technique has been applied to better constrain on the distribution of gas hydrate. The results demonstrate that gas hydrate is characterized by “blank” zone (low amplitude) in instantaneous amplitude attribute. The thickness of gas hydrate stability zone inferred from BSR ranges from 125 to 355 m with an average of 240 m at sea water depth from 950 to 1,600 m in this new gas hydrate province. The volume of gas in-place bound in hydrate is estimated from 1.7 × 109 to 4.8 × 109 m3, with the most likely value of around 3.3 × 109 m3, using Monte Carlo simulation. Furthermore, geothermal gradient and heat flow are derived from the depths of BSRs using a conductive heat transfer model. The geothermal gradient varies from 35 to 95°C km−1 with an average of 54°C km−1. Corresponding heat flow values range from 43 to 105 mW m−2 with an average of 64 mW m−2. By comparison with geological characteristics, we suggest that the distribution of gas hydrate and heat flow are largely associated with gas chimneys and faults, which are extensively distributed in Shenhu Area, providing easy pathways for fluids migrating into the gas hydrate stability zone for the formation of gas hydrate. This study can place useful constraints for modeling gas hydrate stability zone from measured heat flow data and understanding the mechanism of gas hydrate formation in Shenhu Area.