Gas Chimneys Research Articles

Methane hydrate recycling offshore of Mauritania probably after the last glacial maximum

To what extent methane liberated from marine hydrate will enter the ocean during a warmer world is unknown. Although methane release due to hydrate dissociation has been modelled, it is unclear whether or not methane will reach the seafloor during a warmer world and therefore contribute to oceanic and atmospheric budgets. Here we show, using a new three-dimensional (3-D) seismic dataset, that some hydrate deposits surround the gas chimneys passing through the HSZ. Bottom water warming since the last glacial maximum (LGM) is interpreted to cause hydrate dissociation but critically some of the released methane was not vented to the ocean. The released gas caused seal failure and free gas entered the hydrate stability zone (HSZ) through vertical gas chimneys to where new hydrate accumulations formed. This process is a new evidence for methane recycling and could account in part for the lack of methane in ice core records that cover warming events during the late Quaternary. This research provides new insight into how methane could be recycled rather than vented during a warmer world.

Role of Faults in Hydrocarbon Leakage in the Hammerfest Basin, SW Barents Sea: Insights from Seismic Data and Numerical Modelling

Hydrocarbon prospectivity in the Greater Barents Sea remains enigmatic as gas discoveries have dominated over oil in the past three decades. Numerous hydrocarbon-related fluid flow anomalies in the area indicate leakage and redistribution of petroleum in the subsurface. Many questions remain unanswered regarding the geological driving factors for leakage from the reservoirs and the response of deep petroleum reservoirs to the Cenozoic exhumation and the Pliocene-Pleistocene glaciations. Based on 2D and 3D seismic data interpretation, we constructed a basin-scale regional 3D petroleum systems model for the Hammerfest Basin (1 km × 1 km grid spacing). A higher resolution model (200 m × 200 m grid spacing) for the Snøhvit and Albatross fields was then nested in the regional model to further our understanding of the subsurface development over geological time. We tested the sensitivity of the modeled petroleum leakage by including and varying fault properties as a function of burial and erosion, namely fault capillary entry pressures and permeability during glacial cycles. In this study, we find that the greatest mass lost from the Jurassic reservoirs occurs during ice unloading, which accounts for a 60%–80% reduction of initial accumulated mass in the reservoirs. Subsequent leakage events show a stepwise decrease of 7%–25% of the remaining mass from the reservoirs. The latest episode of hydrocarbon leakage occurred following the Last Glacial Maximum (LGM) when differential loading of Quaternary strata resulted in reservoir tilt and spill. The first modeled hydrocarbon leakage event coincides with a major fluid venting episode at the time of a major Upper Regional angular Unconformity (URU, ~0.8 Ma), evidenced by an abundance of pockmarks at this stratigraphic interval. Our modelling results show that leakage along the faults bounding the reservoir is the dominant mechanism for hydrocarbon leakage and is in agreement with observed shallow gas leakage indicators of gas chimneys, pockmarks and fluid escape pipes. We propose a conceptual model where leaked thermogenic gases from the reservoir were also locked in gas hydrate deposits beneath the base of the glacier during glaciations of the Hammerfest Basin and decomposed rapidly during subsequent deglaciation, forming pockmarks and fluid escape pipes. This is the first study to our knowledge to integrate petroleum systems modelling with seismic mapping of hydrocarbon leakage indicators for a holistic numerical model of the subsurface geology, thus closing the gap between the seismic mapping of fluid flow events and the geological history of the area.

Anomalous depressions in the northern Yellow Sea Basin: Evidences for their evolution processes

Depressions of both known and unknown origin are observed in continental margin basins and ocean floors worldwide. A field of anomalous depressions with dome structures within the centers are documented for the first time using sub-bottom seismic profiles, multibeam bathymetry data, and well data in the northern Yellow Sea Basin. They have complex seabed morphologies (circular, elliptical, polygonal, linear and crescent shapes) with kilometer-scale diameters and shallow depths (10 s decimeters to meters). Focused fluid flow pathways, such as gas chimneys, faults and pipes, and free gas (enhanced amplitude anomalies) are observed within the shallow strata. The depressions are interpreted as 'immature pockmarks', possibly caused by insufficient fluid supply compared to fully developed 'normal pockmarks'. They are suspected to still be in the middle stage of pockmark evolution. Their formation processes are suspected to be controlled by sub-seabed plumbing/fluid flux and bottom currents. This study is important for geologic hazard assessments in the study area and it also contributes to the full understanding of pockmark evolution.

Detecting gas leakage using high-frequency signals generated by air-gun arrays

Recent field experiments have demonstrated that marine air-gun arrays create acoustic energy greater than 1 kHz. We have suggested to use the high-frequency signal as a source to look for gas leakage at, for instance, a producing hydrocarbon field, or a [Formula: see text] storage site in which the field is covered by permanent acoustic sensors at the seabed, often referred to as a permanent reservoir monitoring field. The only needed modification is that the temporal sampling interval for the receivers is decreased to 0.1 ms (in contrast to the normal sampling interval of 1 or 2 ms), to ensure that the system is capable of recording signals up to 5 kHz. We suggest using numerous fixed receivers at the seabed to detect a gas chimney by simple high-pass filtering and subsequent transmission type analysis of the recorded signals. We think this method might serve as an elegant, precise, and very cost-effective way to detect gas leakage into the water layer.

Types, Characteristics and Significances of Migrating Pathways of Gas-bearing Fluids in the Shenhu Area, Northern Continental Slope of the South China Sea

The first marine gas hydrate expedition in China has been conducted by Guangzhou Marine Geological Survey in the Shenhu Area, northern continental slope of the South China Sea. Previous study has analyzed the P-T conditions, geophysical anomalies and saturation calculations of these gas hydrates, but has not documented in detail the migration of gas-bearing fluids in the study area. Based on the interpretations of 2D/3D seismic data, this work identified two types of migration pathways for gas-bearing fluids in the Shenhu area, i.e., vertical and lateral pathways. The vertical pathways (large-scale faults, gas chimneys and mud diapirs) presented as steep seismic reflection anomalies, which could be traced downward to the Eocene source rocks and may penetrate into the Late Miocene strata. The deeper gases/fluids might be allowed migrating into the shallower strata through these vertical conduits. However, the distributions showed distinct differences between these pathways. Large-scale faults developed only in the north and northeast of the Shenhu area, while in the drilling area gas chimneys were the sole vertical migration pathways. Since the Pliocene, normal faults, detachment faults and favorable sediments have constituted the lateral pathways in the Shenhu gas hydrate drilling area. Although these lateral pathways were connected with gas chimneys, they exerted different effects on hydrate formation and accumulation. Gas-bearing fluids migrated upward along gas chimneys might further migrate laterally because of the normal faults, thereby enlarging the range of the chimneys. Linking gas chimneys with the seafloor, the detachment faults might act as conduits for escaping gases/fluids. Re-deposited sediments developed at the early stage of the Quaternary were located within the gas hydrate stability zone, so hydrates would be enriched in these favorable sediments. Compared with the migration pathways (large-scale faults and mud diapirs) in the LW3-1 deep-sea oil/gas field, the migration efficiency of the vertical pathways (composed of gas chimneys) in the gas hydrate drilling area might be relatively low. Description and qualitative discrimination of migration pathways in the Shenhu gas hydrate drilling area are helpful to further understand the relationship between good-quality deep source rocks and shallow, mainly biogenically-produced, hydrates. As the main source rocks of the Baiyun sag, lacustrine mudstones in the Wenchang and Enping Formations may provide thermogenic methane. Gas chimneys with relatively low migration efficiency created the vertical pathways. Caused by the Dongsha tectonic movement, the release of overpressured fluids might reduce the vertical migration rates of the thermogenic methane. The thick bathyal/abyssal fine-grained sediments since the Late Miocene provided migration media with low permeability. These preconditions may cause carbon isotopic fractionation of thermogenic methane during long-distance vertical migrations. Therefore, although geochemical analyses indicate that the methane forming gas hydrate in the Shenhu area was mainly produced biogenically, or was mixed methane primarily of microbial origin, thermogenic methane still contribute significantly.

Seepage system of oil-gas and its exploration in Yinggehai Basin located at northwest of South China Sea

Seepage systems of oil-gas in Yinggehai Basin are divided into two types, namely: “micro-seepage”, which is presented by gas chimneys and pockmarks; and “macro-seepage”, which is also called oil-gas outflow; and, in addition, the combination of the two basic types. Among the oil seepage systems, the combined seepage system at Yingdong Slope of Yinggehai Basin is the most eye-catching, and gas chimneys and pockmarks micro-leakage systems in mud diapir zones in the central part of the basin are very common. Both the indications of large-scale oil-gas outflow at Yingdong Slope, which have been booming for a hundred years; and the occurrence of pockmarks at the central mud diapir belt, along with the chaotic seismic reflection of widely-distributed shallow gas chimneys—have shown that hydrocarbon in this area is sufficient and oil-gas is now in dynamic equilibrium of the processes of accumulation, migration, gathering and dispersing. It builds up good conditions for the accumulation, migration, gathering and reserving of oil and gas. However, it must be noted that the results of oil-gas exploration at Yingdong Slope didn't turn out to be satisfactory, despite the presence of oil-gas outflow and gas chimney combined seepage systems. So, strengthen synthesized analysis and study on oil-gas seepage systems and on the conditions for accumulation, migration, gathering and dispersing; the forecasting and evaluation to the advantageous conditions for enriched oil and gas zones; and trap preservation in accordance with the dynamic balance theories; are of significant importance for purposes of exploration.

The history and future trends of ocean warming‐induced gas hydrate dissociation in the SW Barents Sea

AbstractThe Barents Sea is a major part of the Arctic where the Gulf Stream mixes with the cold Arctic waters. Late Cenozoic uplift and glacial erosion have resulted in hydrocarbon leakage from reservoirs, evolution of fluid flow systems, shallow gas accumulations, and hydrate formation throughout the Barents Sea. Here we integrate seismic data observations of gas hydrate accumulations along with gas hydrate stability modeling to analyze the impact of warming ocean waters in the recent past and future (1960–2060). Seismic observations of bottom‐simulating reflectors (BSRs) indicate significant thermogenic gas input into the hydrate stability zone throughout the SW Barents Sea. The distribution of BSR is controlled primarily by fluid flow focusing features, such as gas chimneys and faults. Warming ocean bottom temperatures over the recent past and in future (1960–2060) can result in hydrate dissociation over an area covering 0.03–38% of the SW Barents Sea.

EVIDENCE OF COLD SEEPING IN PLIO-PLEISTOCENE SEDIMENTS OF SE PELOPONESE: THE FOSSIL CARBONATE CHIMNEYS OF NEAPOLIS REGION

Unique, upright standing, chimney-like structures outcropping in (Plio-)Pleistocene sediments of Neapolis region, SE Peloponese are studied thoroughly. The up to 5m high and 3m wide structures, previously interpreted either as “rhizoliths” or hypercalcified giant sponges or fossilized palm trees, display a wide range of morphological types and are composed of hard, several cm thick, carbonate tube. Their orifice is filled up with muddy, sandy or fine conglomerate material, which reflects the sedimentary nature of the surrounding formation, and is penetrated by numerous whitish, mm-thin, carbonate veins. Their formation and spatial occurrence is genetically associated with two intersecting systems of diaclases/fractures, also sealed with carbonate material. We interpret these structures as non-volcanic, fossil, gas chimneys, related to cold seeping (biogenic methane? seeping). The intersecting diaclases have been used as conduits for the upward fluid migration, thus, the gas seeping process must have taken place after the diagenesis of the host sedimentary formations. The impressive outcrops of carbonate gas chimneys in the Neapolis region is the first evident of cold seeping, eventually related with mud volcanism, in the Plio-Pleistocene sedimentary basins of the Hellenic Forearc.

Two-Dimensional Seismic Interpretation and Petroleum System Modelling in the Hammerfest Basin, SW Barents Sea

The Hammerfest Basin has undergone several structural and tectonic evolutions which affected its petroleum systems. Two-dimensional seismic interpretation and petroleum system modelling of the Hammerfest Basin was done in order to understand the petroleum system and effects of uplift and subsidence on hydrocarbon distribution, accumulations, maturity, migration and leakage. The central part of the Hammerfest Basin has been affected by Jurassic syn-rift faults forming a large graben structure in response to subsidence along the folded Asterias Fault Complex and Troms-Finnmark Fault Complex. The graben structure is characterized by gas chimney, bright spots and pockmarks. The modelling result shows that the Jurassic Hekkingen Formation is less mature (oil window) and that the Snadd and Kobbe Formations are mature to highly mature (gas window), respectively. The Sto and Tubaen reservoirs are enriched with oil and gas which migrated from the Snadd and Kobbe source rocks. The hydrocarbon accumulations show that the Sto Formation is the main reservoir in the Hammerfest Basin. Rapid burial during Permian and late Cretaceous was followed by uplift events. Three phases of erosion: the first two phases of erosion (75 to 60 Ma and 30 to 15 Ma) affected and eroded 200 m of Kveite Formation and 300 m of the Torsk Formation, respectively and the third phase of erosion (2.5 to 0.01 Ma) has eroded 100 m of Torsk Formation. The oil migration follows the deep seated faults along the Asterias Fault Complex, Troms-Finnmark Fault Complex and along the bounding faults of the graben structure at the center of the basin which leads to leakage. The migration of gas on the other hand, is vertically upward throughout the basin leaking out of the seal. The gas migration has been enhanced by the uplift and subsidence events which have been the main controlling factors of the hydrocarbon maturation and distribution within the Hammerfest Basin.

A PRELIMINARY STUDY OF SUBMARINE COLD SEEPS BY SEISMIC OCEANOGRAPHY TECHNIQUES

AbstractSubmarine cold seeps are widely distributed in the continental margin seas around the world. In this study, we apply a conventional multichannel seismic reflection (seismic oceanography) method to image the water column near the seafloor in order to detect cold seeps. In addition to analyzing the fluid escape structures, we also describe and discuss the development positions, seismic reflection characteristics as well as features of the cold seeps. The seismic reflection from the water column is very weak; therefore, the seismic sections above and below the seafloor are processed in two different sequences as follows: (1) geometry definition, direct wave attenuation and amplitude recovery, high‐pass filter, common midpoint sorting, constant velocity (seawater sound velocity) stack, and post‐stack FK filter in some sections; and (2) data quality control, amplitude recovery, 6∼100 Hz bandpass filter, multiple attenuation, deconvolution, velocity analysis, normal move‐out correction, common midpoint stack, post‐stack noise attenuation, 4∼70 Hz bandpass filter, and FX migration. The processed sections are then assembled together along the seafloor after carefully adjusting the color scale. The analysis shows that active cold seeps primarily present plume, broom, and/or irregular shapes that have weak and chaotic seismic reflections in the water column. The seismic reflection amplitude is enhanced at times; this could be attributed to the suspension of mud or particles in the water column. Cold seepage activities are typically associated with fluid escape structures, including mud diapirs, pipes, faults, fractures, gas chimneys, seabed pockmarks, and mud volcanoes. This indicates fluid migration from deep to shallow strata, fluid seepage or escape at the seafloor, as well as the formation of widespread cold seep activities. All the results herein are derived from a comprehensive interpretation of the seismic sections of the water columns and strata; however, further field studies, theoretical simulations, and experiments are required to confirm these conclusions.

Mechanisms initiating fluid migration at Snøhvit and Albatross fields, Barents Sea

The southwestern Barents Sea, part of the epicontinental Barents Sea located between Norway and the Svalbard archipelago, hosts several mature petroleum systems. Large gas chimneys, leaking faults and seabed pockmarks are proof that hydrocarbon leakage has taken place previously in this part of the Barents Sea. Glacial erosion, being one of the major episodes of erosion and uplift, has probably caused fluid leakage out of proven hydrocarbon reservoirs and fluid migration along permeable carrier beds. We use 3D seismic data from the Snohvit gas field to focus on the nature of the fluid transport systems and the origins and potential mechanisms for how fluids migrated from deep-seated reservoirs. Mapping of the trap of the reservoirs at Snohvit has allowed us to determine potential spill point locations, coinciding with major gas chimneys and several faults that extend into shallow strata. We explain how uplift and erosion caused gas expansion in the reservoirs allowing the gas to reach below the spill point located at the gas–water contact level at the bottom of the closure. Fluids, such as hydrocarbons, may have escaped through these spills points, leading to the emplacement and development of the aforementioned gas chimneys. Similarly, migration of fluids may have taken place along faults, reactivated by glaciotectonic processes and connected with spill points at reservoir depth. Both gas chimneys and faults have allowed gas to migrate into and be trapped in shallow gas accumulations within the Torsk Formation and below the upper regional unconformity.

Interpretation of gas chimney from seismic data using artificial neural network: A study from Maari 3D prospect in the Taranaki basin, New Zealand

The seismic interpretation in Maari 3D prospect of the Taranaki basin in New Zealand based on artificial neural network has brought out gas migration pathways from the source rock through the faulted reservoirs to the seabed. The findings correlate reasonably with the results of Moki-1 well drilled within the study region. The gas chimneys are analyzed using modern 3D visualization tools by displaying the chimney probability cube over different vertical seismic sections, horizon slices and time slices respectively. The training of multi-seismic attributes resulted into 0.4–0.6 normalized RMS error giving rise to 5.08–10.26% misclassification during the training and testing phases. Several fault intersection zones (weak zones) within the reservoirs exhibit high probability of gas chimneys. This study acts as an add-on-tool for understanding the petroleum system and provides preventive clues for mitigating hazards in future exploitation program.

Defining the 3D geometry of thin shale units in the Sleipner reservoir using seismic attributes

Defining the 3D geometry and internal architecture of reservoirs is important for prediction of hydrocarbon volumes, petroleum production and storage potential. Many reservoirs contain thin shale layers that are below seismic resolution, which act as impermeable and semi-permeable layers within a reservoir. Predicting the storage volume of a reservoir with thin shale layers from conventional seismic data is an issue due to limited seismic resolution. Further, gas chimneys indicative of gas migration pathways through thin shale layers, are not easily defined by conventional seismic data. Additional information, such as borehole data, can be used to aid mapping of shale layers, but making lateral predictions from 1D borehole data has high uncertainty. This paper presents an integrated workflow for quantitative seismic interpretation of thin shale layers and gas chimneys in the Utsira Formation of the Sleipner reservoir. The workflow combines the use of attribute and spectral analysis to add resolution to conventional seismic amplitude data. Detailed interpretation of these analyses reveals the reservoirs internal thin shale architecture, and the presence of gas chimneys. The comprehensive interpretation of the reservoirs internal structure is used to calculate a new reservoir storage volume. This is done based on the distribution of sand and interpreted shale layers within the study area, for this active CO2 storage site.

Interpretation of a gas chimney in the Polish Carpathian Foredeep based on integrated seismic and geochemical data

AbstractGas chimneys are common in offshore petroliferous basins, but little known on land where seismic columnar anomalies are often attributed as poor data quality or processing artefacts. This study utilizes high‐quality 3D seismic data to document a seismic columnar anomaly penetrating through the Miocene heterolithic submarine fan‐deltaic infill of the Carpathian Foredeep. The interpreted gas chimney exhibits vertically clustered velocity push‐down features throughout the attenuated amplitude column accompanied by gas shows in well tests, has its root in gas‐bearing Palaeozoic interval and culminates in an anomalous geochemical gas record at soil level. The chimney system, ca 2 km in height and 500‐m wide, begins above the flank of a rotational bedrock fault‐block and extends vertically along a fault‐controlled conduit. At shallower levels, it passes upwards into amplitude wipeout zones that spread laterally around and partly across thin, gas‐charged reservoirs showing bright spots associated with an AVO response. At shallow levels, gas pathways through muddy slope and deltaic clinoforms are not imaged in low‐fold regions of the seismic volume. The surface geochemical anomalies, in contrast to the microbial methane signature of the Miocene succession, show significant enrichment in higher alkanes and alkenes with C2H6/C3H8 ratios indicative of a deep‐sourced, thermogenic gas or gas condensate. These anomalies form a semi‐enclosed halo around the chimney. Despite the juxtaposition of biogenic and thermogenic methane, the chimney structure imaged on seismic data supports a causal link of gases derived from Palaeozoic source rocks ascending to the surface.

Three dimensional seismic studies of deep-water hazard-related features on the northern slope of South China Sea

Mass transport deposits and geological features related to fluid flow such as gas chimneys, mud diapirs and volcanos, pockmarks and gas hydrates are pervasive on the canyon dominated northern slope of the Pearl River Mouth basin of the South China Sea. These deposits and structures are linked to serious geohazards and are considered risk factors for seabed installations. Based on high resolution three dimensional seismic surveys, seismic characteristics, distributions and origins of these features are analyzed. A distribution map is presented and geometrical parameters and spatial distribution patterns are summarized. Results show that various groups of the mapped features are closely tied to local or regional tectonism and sedimentary processes. Mass transport complexes are classified as slides near the shelf break, initially deformed slumps on the flanks of canyons and highly deformed slumps on the lower slope downslope of the mouth of canyons. We propose them to be preconditioned by pore pressure changes related to sea level fluctuations, steep topography, and fluid and fault activities. Gas chimneys are mainly located in the vicinity of gas reservoirs, while bottom-simulating reflectors are observed within the gas chimney regions, suggesting gas chimneys serve as conduits for thermogenic gas. Mud diapirs/volcanos and pockmarks are observed in small numbers and the formation of pockmarks is related to underlying gas chimneys and faults. This study aims at reducing risks for deep-water engineering on the northern slope of South China Sea.

Understanding the seismic disorder attribute and its applications

While reflections associated with conformal sedimentary layers are usually coherent and continuous, other reflections, such as mass-transport complexes, karst collapse, and salt, may appear to be quite chaotic, without any specific orientation. We also may see chaotic events that have little to do with the target geology but rather are artifacts due to variations in the overburden and surface or budget limitations resulting in a suboptimum acquisition program. While some of these artifact issues can be handled at the time of processing, a certain level of randomness remains in most seismic data volumes. Geologic features of interpretational interest, such as fault damage zones, unconformities, and gas chimneys, often have randomness associated with them. The published work on randomness describes techniques such as cross correlation, which measures randomness associated with noise, the chaos attribute based on eigen-analysis of gradient covariance, and the seismic disorder attribute based on application of second derivative operators in the three axial directions in a 3D seismic volume. We apply the seismic disorder attribute to two different data sets and find that it is a useful attribute for assessing the signal-to-noise ratio and data quality, in addition to helping delineate damage zones associated with large faults and the interior of salt dome structures.

Gas hydrates distribution in the Shenhu Area, northern South China Sea : comparisons between the eight drilling sites with gas-hydrate petroleum system

The results of the first marine gas hydrate drilling expedition of Guangzhou Marine Geological Survey (GMGS-1) in northern continental slope of the South China Sea revealed a variable distribution of gas hydrates in the Shenhu area. In this study, comparisons between the eight sites with gas-hydrate petroleum system were used to analyze and re-examine hydrate potential. In the Shenhu gas hydrate drilling area, all the sites were located in a suitable low-temperature, high-pressure environment. Biogenic and thermogenic gases contributed to the formation of hydrates. Gas chimneys and some small-scale faults (or micro-scale fractures) compose the migration pathways for gas-bearing fluids. Between these sites, there are three key differences: the seafloor temperatures and pressures; geothermal gradient and sedimentary conditions. Variations of seafloor temperatures and pressures related to water depths and geothermal gradient would lead to changes in the thickness of gas hydrate stability zones. Although the lithology and grain size of the sediments were similar, two distinct sedimentary units were identified for the first time through seismic interpretation, analysis of deep-water sedimentary processes, and the Cm pattern (plotted one-percentile and median values from grain-size analyses), implying the heterogeneous sedimentary conditions above Bottom Simulating Reflectors (BSRs). Based on the analyses of forming mechanisms and sedimentary processes, these two fine-grained sedimentary units have different physical properties. Fine-grained turbidites (Unit I) with thin-bedded chaotic reflectors at the bottom acted as the host rocks for hydrates; whereas, finegrained sediments related to soft-sediment deformation (Unit II) characterized by thick continuous reflectors at the top would serve as regional homogeneous caprocks. Low-flux methane that migrated upwards along chimneys could be enriched preferentially in fine-grained turbidites, resulting in the formation of hydrates within Unit I.However, overlying fine-grained sediments related to soft-sediment deformation would hinder the further migration of gases/fluids, causing the extremely low methane concentration in Unit I. Three of the eight sites with hydrates from recovered core samples were located within sedimentary Unit I, and the other five sites were not. Because, the most significant difference between the eight sites is the nature and type of sedimentary deposits above the BSRs, it is suggested therefore that sedimentary conditions are the crucial factor controlling the formation and occurrence of gas hydrates in the Shenhu gas hydrate drilling area, northern South China Sea.

Gas hydrate accumulations related to focused fluid flow in the Pegasus Basin, southern Hikurangi Margin, New Zealand

The Hikurangi Margin, east of the North Island of New Zealand, is known to contain significant deposits of gas hydrates. This has been demonstrated by several multidisciplinary studies in the area since 2005. These studies indicate that hydrates in the region are primarily located beneath thrust ridges that enable focused fluid flow, and that the hydrates are associated with free gas. In 2009–2010, a seismic dataset consisting of 2766 km of 2D seismic data was collected in the undrilled Pegasus Basin, which has been accumulating sediments since the early Cretaceous. Bottom-simulating reflections (BSRs) are abundant in the data, and they are accompanied by other features that indicate the presence of free gas and concentrated accumulations of gas hydrate. We present results from a detailed qualitative analysis of the data that has made use of automated high-density velocity analysis to highlight features related to the hydrate system in the Pegasus Basin. Two scenarios are presented that constitute contrasting mechanisms for gas-charged fluids to breach the base of the gas hydrate stability zone. The first mechanism is the vertical migration of fluids across layers, where flow pathways do not appear to be influenced by stratigraphic layers or geological structures. The second mechanism is non-vertical fluid migration that follows specific strata that crosscut the BSR. One of the most intriguing features observed is a presumed gas chimney within the regional gas hydrate stability zone that is surrounded by a triangular (in 2D) region of low reflectivity, approximately 8 km wide, interpreted to be the result of acoustic blanking. This chimney structure is cored by a ∼200-m-wide low-velocity zone (interpreted to contain free gas) flanked by high-velocity bands that are 200–400 m wide (interpreted to contain concentrated hydrate deposits).

Deep-seated faults and hydrocarbon leakage in the Snøhvit Gas Field, Hammerfest Basin, Southwestern Barents Sea

High-quality 3D seismic data are used to analyze the history of fault growth and hydrocarbon leakage in the Snøhvit Field, Southwestern Barents Sea. The aim of this work is to evaluate tectonic fracturing as a mechanism driving hydrocarbon leakage in the study area. An integrated approach was used which include seismic interpretation, fault modeling, displacement analysis and multiple seismic attribute analysis.The six major faults in the study area are dip-slip normal faults which are characterized by complex lateral and vertical segmentation. These faults are affected by three main episodes of fault reactivation in the Late Jurassic, Early Cretaceous and Paleocene. Fault reactivation in the study area was mainly through dip-linkage. The throw-distance plots of these representative faults also revealed along-strike linkage and multi-skewed C-type profiles. The faults evolved through polycyclic activity involving both blind propagation and syn-sedimentary activity with their maximum displacements recorded at the reservoir zone. The expansion and growth indices provided evidence for the interaction of the faults with sedimentation throughout their growth history.Soft reflections or hydrocarbon-related high-amplitude anomalies in the study area have negative amplitude, reverse polarity and are generally unconformable with structural reflectors. The interpreted fluid accumulations are spatially located at the upper tips of the major faults and gas chimneys. Four episodes of fluid migration are inferred and are linked to the three phases of fault reactivation and Neogene glaciations. Hydrocarbon leakage in the Snøhvit Gas Field is driven by tectonic fracturing, uplift, and erosion. The interpreted deep-seated faults are the main conduits for shallow hydrocarbon accumulations observed on seismic profiles.

Coincidence or not? Interconnected gas/fluid migration and ocean–atmosphere oscillations in the Levant Basin

A growing number of studies on shallow marine gas/fluid systems from across the globe indicate their abundance throughout geological epochs. However, these episodic events have not been fully integrated into the fundamental concepts of continental margin development, which are thought to be dictated by three elements: tectonics, sedimentation and eustasy. The current study focuses on the passive sector of the Levant Basin on the eastern Mediterranean continental margin where these elements are well constrained, in order to isolate the contribution of gas/fluid systems. Single-channel, multichannel and 3D seismic reflection data are interpreted in terms of variance, chaos, envelope and sweetness attributes. Correlation with the Romi-1 borehole and sequence boundaries constrains interpretation of seismic stratigraphy. Results show a variety of fluid- or gas-related features such as seafloor and subsurface pockmarks, volumes of acoustic blanking, bright spots, conic pinnacle mounds, gas chimneys and high sweetness zones that represent possible secondary reservoirs. It is suggested that gas/fluid migrate upwards along lithological conduits such as falling-stage systems tracts and sequence boundaries during both highstands and lowstands. In all, 13 mid-late Pleistocene sequence boundaries are accompanied by independent evidence of 13 eustatic sea-level drops. Whether this connection is coincidental or not requires further research. These findings fill gaps between previously reported sporadic appearances throughout the Levant Basin and margin and throughout geological time from the Messinian until the present day, and create a unified framework for understanding the system as a whole. Repetitive appearance of these features suggests that their role in the morphodynamics of continental margins is more important than previously thought and thus may constitute one of the key elements of continental margin development.