Seismic Chimneys Research Articles

Geophysical evidence and inferred triggering factors of submarine landslides on the western continental margin of the Ulleung Basin, East Sea

Submarine landslides form very complex depositional and erosional features on the seafloor, and their dynamics and triggering processes are yet to be understood completely. Numerous studies are being undertaken both because of the scientific significance but also for their potential harm to seafloor infrastructure and coastal areas. This study investigates the styles and causes of landsliding along the western margin of the Ulleung Basin in the East Sea, based on multiple sparker, subbottom profiler, multibeam echosounder and sediment core datasets collected in 2015. The bathymetric analyses indicate that the southern slope of the Ulleung Basin has experienced at least seven submarine failures. These failures left clear arcuate-shaped scarps that initiated at water depths of ~600 m. The observed headwall scarps have heights that exceed 60 m and appear to be the result of retrogressive-type failures. Seismic reflection data clearly image the basal sliding surface that is characterized by a prominent high-amplitude reflector. Chaotic-to-transparent seismic facies occur immediately downslope of the headwall scarps; these represent ~20 m thick landslide deposits. Gravity cores taken from areas adjacent to the scars suggest that these slides are older than ca. 97 ka. Interpretation of the present data shows that faults appear to cut recent sediments upslope of scarps, and that the slope may still be in an active phase of failure. Seismic data also image various overpressurized gases and/or gas fluids, as evidenced by the occurrence of pockmarks and seismic chimneys in upslope or adjacent areas of the scarps. Hence, earthquakes associated with tectonic activity and development of fluid overpressure may have acted as the main conditioning factor for destabilizing the slope sediments. Geotechnical stability analyses indicate that the sampled slope sediments are exceptionally stable under present-day conditions, even under seismic loading. This finding points to additional forces such as excess pore pressure caused by gas fluids at the times of slide emplacement.

Control of active faults and sea level changes on the distribution of shallow gas accumulations and gas-related seismic structures along the central branch of the North Anatolian Fault, southern Marmara shelf, Turkey

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

Simulating seismic chimney structures as potential vertical migration pathways for CO2 in the Snøhvit area, SW Barents Sea: model challenges and outcomes

Carbon capture and storage (CCS) activities at the Snohvit field, Barents Sea, will involve carrying out an analysis to determine which parameters affect the migration process of CO2 from the gas reservoir, to what degree they do so and how sensitive these parameters are to any changes. This analysis will aim to evaluate the effects of applying a broad but realistic range of reservoir, fault and gas chimney properties on potential CO2 leakage at various depths throughout the subsurface. Fluid flow might take place through parts of or the entire extent of the overburden. One of the aims of the analysis is assessing the potential of CO2 reaching the seabed. Using the Snohvit gas reservoir and overburden in the Barents Sea, a series of geological models were built using seismic and well-log data. We then performed numerical simulations of CO2 migration in focused fluid flow structures. Identification of potential migration pathways and their extent, such as gas chimneys and faults, and their incorporation into these models and simulations will provide a realistic insight into the migration potential of CO2. In the simulations the CO2 is injected over a 20 year period at a rate of 0.7 Mt/year and migration is allowed to take place over a 2000 year time frame for domains of approximately 21 km2 for the caprock fault models, 24 km2 for the realistic gas chimney models and 35 km2 for the generic gas chimney models, in a layered sedimentary succession. The total mass of CO2 injected in the reservoir during the 20-year injection period is 14 Mt. There is a strong interaction between the various parameters but the parameter that had the most influence on the CO2 migration process was probably the permeability of the reservoirs, especially the average permeability (k). Also, for the faulted caprock scenarios, it should be noted that at near surface depths the permeability of 765 mD is already adequate for a good CO2 flow. At the chimney top level (600 m) however, a further increase in permeability has an additional effect on improving CO2 flow. Overall, considering the slow upward migration velocity of the plume, this geological setup can be regarded as a suitable storage site.

Distribution and origin of seismic chimneys associated with gas hydrate using 2D multi-channel seismic reflection and well log data in the Ulleung Basin, East Sea

Analysis of multi-channel seismic reflection and well-log data from the Ulleung Basin, East Sea reveals that two distinct types of seismic chimneys have been identified in the basin, based on their geometries, seismic reflection patterns, well log response, and lithology. Type-I is a mound-like feature with a transparent or chaotic reflection pattern. Well log interpretation and lithology of this type indicate that Type-I consists of mobilized homogeneous mud including fracture-filling gas hydrate. This type is mainly located on the structural highs, and is mostly restricted to the Pliocene sedimentary succession. In contrast, Type-II is a pipe-like feature with vertically stacked distorted-reflectors. Based on well log data and lithology, Type-II filled with fracture-filling gas hydrate without breaking up primary sedimentary structure. This type appears along the deep-seated fault, and mainly developed within the Quaternary deposit. Our primary conclusion is that Type-I resulted from the subsurface sedimentary remobilization in response to regional uplift in the Pliocene whereas Type-II resulted from the focused vertical fluid flow through fractures triggered by reactivation of deep-seated fault during the Quaternary.

Gas migration pathways, controlling mechanisms and changes in sediment acoustic properties observed in a controlled sub-seabed CO2 release experiment

Carbon capture and storage (CCS) is a key technology to potentially mitigate global warming by reducing carbon dioxide (CO2) emissions from industrial facilities and power generation that escape into the atmosphere. To broaden the usage of geological storage as a viable climate mitigation option, it is vital to understand CO2 behaviour after its injection within a storage reservoir, including its potential migration through overlying sediments, as well as biogeochemical and ecological impacts in the event of leakage.The impacts of a CO2 release were investigated by a controlled release experiment that injected CO2 at a known flux into shallow, under-consolidated marine sediments for 37 days. Repeated high-resolution 2D seismic reflection surveying, both pre-release and syn-release, allows the detection of CO2-related anomalies, including: seismic chimneys; enhanced reflectors within the subsurface; and bubbles within the water column. In addition, reflection coefficient and seismic attenuation values calculated for each repeat survey, allow the impact of CO2 flux on sediment acoustic properties to be comparatively monitored throughout the gas release. CO2 migration is interpreted as being predominantly controlled by sediment stratigraphy in the early stages of the experiment. However, either the increasing flow rate, or the total injected volume become the dominant factors determining CO2 migration later in the experiment.

Seismic chimneys in the Southern Viking Graben – Implications for palaeo fluid migration and overpressure evolution

Detailed understanding of natural fluid migration systems is essential to minimize risks during hydrocarbon exploration and to evaluate the long-term efficiency of the subsurface storage of waste water and gas from hydrocarbon production as well as CO2. The Southern Viking Graben (SVG) hosts numerous focused fluid flow structures in the shallow (<1000 m) subsurface. The seismic expressions of vertical fluid conduits are variously known as seismic chimneys or pipes. Seismic pipes are known to form large clusters. Seismic chimneys have so far been described as solitary structures. Here, we show that the study area in the SVG hosts more than 46 large-scale vertical chimney structures, which can be divided in three categories implying different formation processes. Our analysis reveals that seal-weakening, formation-wide overpressure and the presence of free gas are required to initiate the formation of vertical fluid conduits in the SVG. The presence of numerous vertical fluid conduits implies inter-stratigraphic hydraulic connectivity, which significantly affects the migration of fluids in the subsurface. Chimney structures are important for understanding the transfer of pore pressure anomalies to the shallow parts of the basin.

Distribution and origin of natural gas leakage in the Colorado Basin, offshore Argentina Margin, South America: seismic interpretation and 3D basin modelling

The detailed analysis of a dense 2D seismic reflection dataset and data from 8 exploration wells, allowed us to identify, map out and characterize possible indications of past and present-day hydrocarbon leakage (i.e. gas chimneys, gas pockets, and seafloor mounds and pockmarks) on the continental shelf and slope of the Colorado Basin, offshore Argentina, where Permian, Jurassic and Early Cretaceous source rocks are potentially present and may be currently mature. Identified gas leakage features, developed both in the syn-rift and post-rift successions, were also analysed in relation to the structural the stratigraphic elements of the basin. A family of seabed pockmarks, located close to an array of submarine channels, was identified on the distal slope of the basin. These pockmarks are overlying a series of sub-vertical to vertical seismic chimneys in the subsurface. A calibrated basin-wide 3D petroleum system model comprising generation and migration of hydrocarbons was carried out and compared with the observations from the seismic analysis. Preliminary results from this model indicate that although synrift and early Cretaceous source rock (SR) intervals may be depleted in the central areas of the basin, an active kitchen from the Aptian SR may be present below the slope areas. Hydrocarbon migration pathways predicted by the 3D model (Hybrid method) coincide with the interpreted seismic chimneys underlying the observed seabed slope pockmarks. Hence, our results indicate that thermogenic gas may be currently generated in the distal slope of the basin from mature early post-rift source rocks within the Early Cretaceous (Aptian) sequences and migrates vertically, due to seal failure, through the stratigraphic column. This migrating thermogenic gas is feeding the seafloor pockmarks identified in the distal slope of the basin, although up-dip lateral migration along stratigraphic layers to the more proximal slope areas cannot be ruled out. The present work represents the first published study integrating detailed seismic analysis and 3D basin modelling linking observed gas-leakage indicators and associated seepage pathways, to their relative abundance, distribution and feeding systems offshore Argentina’s continental margin.

Seafloor mounds, craters and depressions linked to seismic chimneys breaching fossilized diagenetic bottom simulating reflectors in the central and southern Scotia Sea, Antarctica

Based on an extensive dataset including swath bathymetry, chirp sub-bottom profiler (TOPAS) and multichannel seismic reflection profiles obtained during four cruises in the Scotia Sea aboard the R/V Hespérides, we report a variety of seismic and morphological structures related to focused fluid flow in the Scan Basin (southern Scotia Sea) and the central Scotia Sea (Antarctica). We show that both positive-relief (mounds) and negative-relief (craters and elongated depressions) seafloor morphologies are associated with deep seismic chimneys that link the deep source zone to the subsurface structures through a network of fractures that progressively breach sub-horizontal bands of anomalously high-amplitude reflections. Based on the recognition that these bands of reflections generally mimic the seafloor topography and locally cross-cut the stratigraphic seismic reflections, we recognize three different bottom simulating reflectors (BSRs). According to the theoretical model for hydrate and silica diagenesis stability conditions in the central and southern Scotia Sea and the calculations of temperature and seismic polarity for the three BSRs, we infer that BSR-2 and BSR-3 are reflections caused by the transformation between Opal-A/Opal-CT and Opal-CT/Quartz, respectively. We thus postulate that the successive diagenetic fronts were caused by significantly high geothermal gradients during the early-middle Miocene. In contrast, the low temperatures calculated for the depth of the BSR-1 event rule out its diagenetic origin but delineate the base of the gas hydrate stability zone (GHSZ).An evolutionary model is proposed to explain the plumbing system and chimney structures that help the focused flow of gas-rich fluids to migrate into the subsurface. Firstly, the formation of silica transformation zones may have acted as reservoir traps during Neogene times. Secondly, the progressive decrease of heat flow during the late Pliocene and Quaternary favored the development of the networks of polygonal faults forming collapses and downward tapering chimneys. Finally, seafloor mounds are formed as a result of the continuous injection of gas-enriched fluids through these networks of fractures; they are transformed into gas hydrates above the present base of the GHSZ and move upwards by buoyancy drive as they lose density and increase their volume. We present these structures as type cases that might represent highly concentrated hydrates around local seafloor fluid venting structures. Furthermore, they may be one of the most important conduits into the ocean–atmosphere system for deep methane in the Antarctic seafloor. The breach of BSRs influenced by global warming may induce the catastrophic release of greenhouse gases to the ocean–atmosphere system and, in turn, impact on the Earth's evolution.

Seismic evidence of shallow gas from Lake Van, eastern Turkey

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

Occurrence and seismic characteristics of gas hydrate in the Ulleung Basin, East Sea

Multi-channel seismic reflection and well-log data from the Ulleung Basin, East Sea reveal several seismic signatures indicative of gas-hydrate occurrence in the Ulleung Basin that are associated with vertically and/or laterally stacked mass-transport complexes. The seismic indicators include (a) a bottom simulating reflector (BSR), (b) a seismic chimney, (c) high amplitude reflections within the gas hydrate stability zone (GHSZ), (d) acoustic blanking, (e) enhanced reflections below the BSR, and (f) seafloor gas-escape features. The BSR, associated with enhanced reflections below, is most commonly found over much of the basin indicating a physiochemical boundary of gas hydrates overlying free gas. Seismic chimneys are characterized by velocity pull-up and reduced reflectivity on the seismic sections, which appear to be caused by active migration of fluid gas vertically into the GHSZ. The logging data retrieved from the seismic chimneys showed elevated electrical resistivity (>80 Ohm-m) and P-wave velocity (>2000 m/s), indicating the presence of gas hydrate. Another seismic characteristic observed in gas hydrate bearing sediments is the strong amplitude reflections, defined by the relatively high reflectivity within the GHSZ. Acoustic blanking is likely to be the result of hydrate accumulation in the sediments causing a significant reduction of acoustic impedance contrast between sedimentary layers. Where the upward migrating gas seeps into the deep water column, seafloor pockmarks and mud mounds may be formed.Gas hydrate was recovered from the Ulleung Basin, East Sea in 2010 during the Second Ulleung Basin Gas Hydrate Drilling Expedition (UBGH2) under the Korean National Gas Hydrate Program. Based on the results, gas-fluids migrate into the GHSZ through two distinct pathways: (1) structural conduits which include fault and fracture systems associated with seismic chimneys and (2) stratigraphic conduits associated with inclined turbidite/hemipelagic layers. Two types of gas-hydrate occurrence were identified in the basin: (1) a stratally-bound type (pore filling) within turbidite sand layers and (2) a locally concentrated type (massive, nodule or fracture filling) within upward-growing chimneys associated with near vertical faults. Relatively high concentrations of gas hydrate, however, tend to occur in localized seismic chimneys, rather than in the strata-related features. The successful recovery indicates that the Ulleung Basin provides favorable conditions for gas-hydrate formation.

Fracture orientation and induced anisotropy of gas hydrate-bearing sediments in seismic chimney-like-structures of the Ulleung Basin, East Sea

During drilling in 2010 in the Ulleung Basin, East Sea, Logging-While-Drilling (LWD) was conducted for thirteen sites. LWD data shows various characteristics indicating presence of gas hydrate-bearing sediments. In particular, a series seismic chimney sites are characterized by anomalous log data (i.e. high resistivity and velocity values), compared to surrounding marine sediments. At chimney sites, the resistivity and velocity log values are over 200 Ω-m and 3000 m/s, respectively. Moreover, log values of low density (less than 1.1 g/cm3) indicating the presence of massive hydrates also correlate with intervals with the highest resistivity and velocity. Gas hydrates at the seismic inferred chimney sites occurred within inclined fractures in the mud dominated sediments. These gas hydrate-filled fractures were identified on LWD resistivity images and X-ray images of pressure cores. The gas hydrate-filled fracture intervals coincide with high measured resistivity intervals at three sites (Sites UBGH2-3, UBGH2-7, UBGH2-11). In most cases, high measured resistivity translated into high hydrate saturations via Archie's relationship; but the high saturations derived from the Archie's relationship appear to overestimate gas hydrate saturations when compared to pressure core samples and acoustic log measurements. Also, in intervals with inclined gas hydrate-filled fractures (e.g., UBGH2-3), there is notable separation between phase-shift and attenuation resistivity logs, with 2 MHz resistivity measurements being significantly higher than 400 kHz resistivity measurements. In this study, the analysis of fractures with dip angles greater than 30° on the resistivity log-images show dip angles between 43 and 63° in average for the three sites examined. The dip azimuth of the fractures at Sites UBGH2-3 and UBGH2-7 dominate westerly to southwestern direction, while the fractures at Site UBGH2-11 are characterized by no preferred orientation. This fracture pattern indicates that maximum horizontal stress direction at the time of fracture formation was not constant. In addition, the differential compaction of the sediments after deposited may be contributed to the development of fracture.

Linking Methane Seepage to Fluid Flow Mechanisms: Evidence from AVO Characteristics of Bottom Simulating Reflectors

The presence of gas hydrates over continental margins may be inferred by various seismic indicators, including the bottom simulating reflector (BSR). Recently, the occurrence of two BSRs have been reported from many regions of the world. In this study we estimate the uncertainty in amplitude versus offset (AVO) behaviour of the single BSR and double bottom simulating reflector (DBSR) observed over two geological provinces; the Kerala-Konkan Basin, offshore India and Green Canyon, offshore USA, and attempt to infer a mechanism for the observed anomalies from the AVO patterns. Anomalous behaviour of seismic velocities within the gas hydrate stability zone (GHSZ) associated with the occurrence of DBSRs, low amplitude seismic chimneys and bright spots, indicates increased hydrate concentration and fluid venting structures underneath the DBSR locations. Such structures, if extended upward into the regional GHSZ through discrete fracture networks, may act as a passage for methane escape into the ocean. Our analysis indicates that the variability in AVO signatures for gas hydrate saturated sediments is potentially linked to the discrete zones of steeply inclined fractures that are responsible for the migration of deep gas and its escape through the seabed. Key Words : Methane hydrate; Gas dynamics; Seismic reflections; Plumbing system; Amplitude versus offset (AVO); Bottom simulating reflector

Fluid flow pipes triggered by lateral pressure transfer in the deepwater western Niger Delta

Using three-dimensional (3D) seismic data, we establish a simple model for the development of vertical fluid flow pipes in the deepwater western Niger Delta. We analyse two examples of fluid flow pipes that form vertical seismic chimneys that are 400–600 m wide and ∼2000–2500 m in height, terminate at the current seabed and have bases located at the crest of rollover anticlines. In both cases we identify buried deepwater channels-complexes located below the pipes that formed prior to the growth of the rollover anticlines. The development of the anticlines caused tilting of these channel complexes and differential loading. We propose the channel complexes represent connected permeable reservoir intervals and that lateral pressure transfer caused the pore pressure at the crest of the structures to reach critical levels, leading to hydraulic fracturing of the overburden. Although hydrocarbons may migrate upwards through the chimney systems, they are not necessarily indicators that the channel complexes were gas or oil charged.

Identification and numerical modelling of hydrocarbon leakage in the Lower Congo Basin: Implications on the genesis of km-wide seafloor mounded structures

We present a combined approach of interpretation of 2D seismic-reflection data and numerical modelling of hydrocarbon generation and migration across the southern slope of the Lower Congo Basin, in order to investigate the factors controlling timing and distribution of hydrocarbon leakage in this area. We identified three main families of past and present-day leakage features: (1) Mid-Upper Miocene seismic chimneys concentrated basinwards and ending up on buried pockmarks, (2) Plio-Pleistocene chimneys, rather clustered to the east of the study area and ending up in seafloor pockmarks, and (3) fewer scattered chimneys identified within the Miocene sequences ending up in shallow enhanced reflectors (“Flat spots”). Stratigraphic and structural elements seem to control the distribution of these features. At least two major events of leakage occurred during the Middle–Late Miocene and intermittently during the Pliocene-Present. External factors as sediment supply are associated to the Miocene leakage event, whilst internal structural elements probably triggered the Pliocene to present-day leakage. A major seabed morphological feature, represented by a margin-paralleled belt of more than 1-km-wide mounds, was identified above growth faults to the east of the study area. Data-constrained 2D HC generation and migration modelling suggests a genetic link between these structures and vertical migration/leakage of thermogenic methane sourced from either currently mature Oligo-Miocene source rocks or secondary cracking and further expulsion from over-mature Upper-Cretaceous source rocks. Hence, the mounds are likely to represent a lineation of methane-derived carbonate build-ups. Despite the natural limitations of a 2D migration model, when combined and calibrated with observations from seismic data, it can be used as a valid tool to assess petroleum migration routes in sedimentary basins. To the best of our knowledge, this is the first integrated approach combining both seismic observations and numerical modelling carried out in the Angola basin.

Distribution of hydrocarbon leakage indicators in the Malvinas Basin, offshore Argentine continental margin

Abstract The Malvinas Basin is located in the southernmost Argentinian continental margin. Despite the lack of commercial hydrocarbon accumulation discoveries, the presence of thermogenic gas in gravity cores and seafloor oil slicks points to the existence of an active petroleum system in this basin. Based on the analysis of over 1000 2D industrial seismic-reflection profiles, covering the shelf and upper-slope of the Malvinas Basin offshore the southernmost Argentinian margin, we document the presence of buried and present-day features including subsurface seismic chimneys, seabed and buried pockmarks, and buried-mounded structures which are probably indicators of long-term leakage history of both liquid and gaseous hydrocarbons since the Eocene to the Present. Based on their distribution and likely controlling factors, these leakage features were classified into four areas of leakage: area I to area IV. Area I is located in the centre of the basin and contains seismic anomalies as pipes originating above or in a polygonal faulted Pliocene–Miocene interval accompanied by bright spots and seabed pockmarks. Area IIa/b is located in the south of the basin and contains pipes and buried pockmarks located close to the southern transpressional deformation front. Area III is located to the east of the basin and consists of pipes hosted in a Mid-Cretaceous deltaic-fan. Area IV, located in the western part of the basin, consists of buried Eocene mounded structures located near the Rio Chico High and above basement highs and faults. They are interpreted as authigenic carbonate mounds possibly derived from oxidation of thermogenic methane that leaked upwards along basement-rooted faults. A reversed-polarity seismic reflection showing a lineation of bright spots has been identified at an average depth of 170 m below seafloor in water depths of about 500 m. We interpret this reflection as a bottom simulating reflector (BSR), which enables us to estimate a geothermal gradient of 23.9 ± 2.0 °C/km for the area. Near and above the thrust faults of a transpressional deformation front, the vertical pipes in area II cross-cut possible hydrate deposits, suggesting that there is a current breaching of these deposits due to tectonically-driven upward focused fluid flow and heat transport. The gas source for the features observed in areas I, IIa/b and IV is most likely leakage from the uppermost Jurassic–Barremian reservoir Springhill Fm., although a biogenic gas source for leakage indicators in area I cannot be ruled out. The leakage indicators in area III are possibly sourced from the Mid-Cretaceous sediments of the Middle Inoceramus Fm.

Constraints on seismic reflections and mode conversions at bottom simulating reflectors associated with gas hydrates

Bottom simulating reflectors (BSRs) are the primary seismic signatures of subsurface gas hydrate accumulations. Inference about the presence of gas hydrates only from P wave data hampers realist assertion and the quantification of resource estimates. Owing to uncertainty about conversion of seismic waves at the BSR interface and its identification of shear wave signature in most of the conventional seismic surveys studies of BSRs as being wave conversion boundaries are relatively sparse. In view of arriving at synthetic response and validate the assumptions that BSR works as mode conversion boundary we for the first time generate seismic reflections data and identify different conversion modes in relation gas hydrate exploration. The modeling results suggest that the hydrate and free gas interface acts as principal boundary for mode conversion from compressional (P) waves into converted shear waves (PS). It also appears that wave propagation effects in the overburden significantly influences the P-wave amplitude of BSR. To validate the modeling results we analyze seismic data from the Green Canyon, offshore USA and the Kerala-Konkan (KK) basin, India. The presence of BSRs, enhanced seismic reflection below BSR, seismic chimneys and hydrate mounds are found as the key seismic indicators for the occurrence of gas hydrates. The occurrence of double bottom simulating reflector (DBSR) over Green Canyon, Gulf of Mexico, Offshore, USA is originated with the migration of free gas through discrete layers of fractured sediments.

Three‐dimensional seismic analysis of the morphology and spatial distribution of chimneys beneath the Nyegga pockmark field, offshore mid‐Norway

ABSTRACTThree‐dimensional (3D) seismic data analysis has identified 415 seismic chimneys in the Nyegga area, on the mid‐Norwegian continental margin. The majority of the seismic chimneys connect with pockmarks at the modern seafloor, suggesting that the seismic chimneys represent the acoustic imprint of highly focused, cross‐stratal fluid flow. A semi‐quantitative spatial analysis of the seismic chimney complex, measuring parameters describing the dimension, geometry and root zone, provides constraints on the geological controls on chimney formation. Most of the seismic chimneys in the Nyegga region descriptively fall into the categories of ‘blowout chimneys’, because they diagnostically link to topography‐controlled leakage locations within strata showing acoustic indications of free gas. We find that approximately 60% of the fluid‐escape chimneys emanate from free gas layers below the gas hydrate stability zone. The timing of highly focused fluid expulsion through chimneys and the formation of the Nyegga pockmark field postdate the last glacial maximum at 25 000 years before present.

Linking a linear pockmark train with a buried Palaeozoic structure: a case study from the St. Lawrence Estuary

The 15-km-long Matane pockmark train belongs to a series of NNE-striking alignments of pockmarks mapped on the seafloor of the St. Lawrence Estuary. It includes 109 pockmarks that show a complete transition from well-defined, relatively deep (up to 8.6 m), crater-like depressions to subtle, partly buried morphological features, suggesting that pockmarks have formed at different periods along the whole alignment and that the location of fluid release has changed through time. On seismic profiles, pockmarks are characterized by vertical seismic chimneys that root in the (fractured?) hinge zone of an open anticline within the autochthonous Palaeozoic rocks of the St. Lawrence Platform. In absence of a geochemical characterization of expelled gas, the relationship between the Matane pockmark train and the anticline in a domain characterized by mature source rocks is the strongest evidence for the genetic link between pockmarks and the release of gas from an active hydrocarbon system or a reservoir located in the Palaeozoic succession.

The structures beneath submarine methane seeps: Seismic evidence from Opouawe Bank, Hikurangi Margin, New Zealand

The role of methane in the global bio-geo-system is one of the most important issues of present-day research. Cold seeps, where methane leaves the seafloor and enters the water column, provide valuable evidence of subsurface methane paths. Within the New Vents project we investigate cold seeps and seep structures at the Hikurangi Margin, east of New Zealand. In the area of Opouawe Bank, offshore the southern tip of the North Island, numerous extremely active seeps have been discovered. High-resolution seismic sections show a variety of seep structures. We see seismic chimneys either characterised by high-amplitude reflections or by acoustic turbidity and faults presumably acting as fluid pathways. The bathymetric expression of the seeps also varies: There are seeps exhibiting a flat seafloor as well as a seep located in a depression and small mounds. The images of the 3.5 kHz Parasound system reveal the near-surface structure of the vent sites. While high-amplitude spots within the uppermost 50 m below the seafloor (bsf) are observed at the majority of the seep structures, indicating gas hydrate and/or authigenic carbonate formations with an accumulation of free gas underneath, a few seep structures are characterised by the complete absence of reflections, indicating a high gas content without the formation of a gas trap by hydrates or carbonates. The factors controlling seep formation have been analysed with respect to seep location, seep structure, water depth, seafloor morphology, faults and gas hydrate distribution. The results indicate that the prevailing structural control for seep formation at Opouawe Bank is the presence of numerous minor faults piercing the base of the gas hydrate stability zone.

Seismic indicators of gas hydrate and associated gas in the Ulleung Basin, East Sea (Japan Sea) and implications of heat flows derived from depths of the bottom-simulating reflector

Analysis of multi-channel seismic reflection data from the Ulleung Basin, East Sea (Japan Sea), reveals various seismic indicators of gas hydrate and associated gas, including the bottom-simulating reflector (BSR), enhanced reflections below the BSR, and seismic chimneys. The recent recovery of massive gas hydrate by drilling, together with these seismic indicators, strongly suggests favorable conditions for the formation of gas hydrate in the area. The BSR is most common and of a wide range of amplitude and continuity. Seismic chimneys, characterized by columnar zones of amplitude reduction, terminate mostly within the gas hydrate stability zone (GHSZ), but a few extend to pockmarks or mounds on the seafloor. Seismic chimneys with seafloor expressions probably represent vertical vents for gas originating from below the GHSZ. The increase in the degree of amplitude reduction in seismic chimneys with increasing seismic frequency may be due to seismic attenuation by gas bubbles trapped in hydrate-filled/coated fractures in the GHSZ. BSR-derived heat flows range from about 65 to over 115 mW/m 2, comparable to those from direct measurements. Heat flows in the northern part of the study area are very high for the age of the basin, suggesting hotter than normal mantle temperature. The area of high (> 105 mW/m 2) heat flows also coincides largely with the distribution of the presumed incipient oceanic crust, formed during the earliest stage of seafloor spreading.