Search

The El Arraiche mud volcano field is situated on the Atlantic Moroccan margin. About 8 small to giant mud volcanoes are clustered around two sub-parallel thrust ridges, the Vernadsky and Renard ridges, with steep fault escarpments. The ridges rise up in water depths of about 700 m and stretch to the shelf edge. Most mud volcanoes occur on top of the Renard ridge (Lazarillo de Tormes mv, Gemini mv, Don Quichote mv and Fiúza mv). Isolated mud volcanoes occur between the ridges (Adamastor mv, Mercator mv, Al Idrissi mv). The largest mud volcano, Al Idrissi, is situated at the shelf edge and is almost 250 m high, 5.3 km wide at the base and 1.4 km at the top.The mud volcano cluster was discovered during a R/V Belgica cruise in May 2002 and surveyed again with the R/V Logachev in July 2002. The surveys yielded detailed swath bathymetry over the entire area, dense grids of high-resolution seismic data, a few very high-resolution deep-tow sub bottom profiles, side scan sonar mosaics over the major structures, selected TV-lines, TV-grabs, dredge samples and gravity cores. Integration of the data set allows to reconstruct the structure of active mud volcanoes in detail, and moreover, it allows to zoom at selected places from the regional structures gradually down to microscopic scale.In the study area small coral banks and carbonate crusts were found at the Pen Duick escarpment at the southern flank of the Renard Ridge.The Pen Duick escarpment is a fault scarp about 4.5 km long, 100 m high, and the waterdepth at the top is 525 m. The eastern part of the platform is characterized by a hummocky topography, to the west the pattern changes to parallel elongated ridges. On basis of the TV lines, TV guided grab samples were taken from dead coral banks and from a fault zone with carbonate slabs. The coral bank consisted of a dead coral framework with terrigeneous mud matrix and few living corals at the top. It is indicative of a more favourable coral habitat in the past, probably related to strong currents in this area. The carbonate slabs consist of carbonate cement with shells, corals and debris.During this presentation the first results from the geophysical surveys, geochemical analysis on carbonate slabs, and the biological sampling will be presented.

Lake Baikal in Siberia, one of the world’s largest rift lakes, is known to be the only fresh-water lake with gas hydrates in the subsurface. Seismic data clearly image the base of the hydrate layer in the Central and Southern Baikal Basins, at both sides of the Selenga delta. Gas seeps and short-lived mud volcanoes were discovered in Lake Baikal’s South Basin at places where the gas hydrate layer shows anomalous thickness variations, which was attributed to localized heat flow anomalies along active fault segments. The gas seeps were interpreted as the result of localized destabilization of gas hydrates by injected thermal water along fault segments; it is probably the inevitable consequence of gas hydrate accumulation in an active rift basin.New data from Lake Baikal’s Central basin, acquired during the summer of 2002, show that the four seeps in the South Basin are not isolated cases. More seeps were discovered, all situated in Baikal’s hydrate accumulation area, all near active fault segments, and all associated with anomalous thickness variations of the underlying hydrate layer. This poster gives an overview of the occurrence of gas seeps in Lake Baikal in relation with thickness anomalies of the gas hydrate layer. The gas seeps in Lake Baikal were found in three areas: 1. Posolsky fault area. Four methane seeps were encountered in the footwall of a small antithetic fault of the Posolsky fault zone. The seeps occur as blow-out craters and conical mud volcanoes. 2. Olkhon fault splay area. Two gas seeps, St.Petersburg and Novosibirsk, were discovered in 2002 at the footwall of a splay of the large Olkhon border fault. The seeps appear to be conical mud volcanoes. 3. The Kukuyu canyon area. The Kukuyu canyon is a large, probably fault-related, canyon, at the northern slope of the Selenga delta. Gas seeps are documented on side scan sonar data and subbottom acoustic profiles.The newly discovered seeps support the interpretation that gas seeps and mud volcanoes in Lake Baikal are caused by the localized dissociation of gas hydrates by thermal input at the base of the hydrate layer. Seepage is probably intense but short-lived, and sometimes accompanied by mud extrusion at the lake floor. They are a rare example where hydrates are the source for intense methane venting, and not vice versa.

The Porcupine basin is a north-south trending graben of Jurassic age filled with Mesozoic and Tertiary sediments lying unconformably on upper Palaeozoic rocks. The basin formed as a result of an early abortive northeasterly arm of the Mid-Atlantic spreading ridge. It continues to be a subsiding basin at present time. The Connemara field is located in the northern part of the basin. Structurally, it lies at the confluence of two principal fault systems: a N-S fault regime characteristic of the main Porcupine basin, and the E-W and NE-SW trends of reactivated Caledonian faults. It can be imaged as a broad, tilted block faulted structure trending northeast-southwest, bounded by major faults to the east and west. Between these faults appears a structure broadly synclinal in section, dipping southwest, with an axis parallel to the trend of the major faults, filled of mesozoic sediments, and partly closed by the base of the Cretaceous unconformity. Hydrocarbons were found at the end of the 70sŠ in three reservoirs within the middle Jurassic in the synclinal structure. Tight 2D and 3D seismic coverage over the Connemara field showed the presence of vertical paths of fluid and/or gas migrating upward in the formation. Most of these chimneys stop at the Plio-Pleistocene boundary, about 0.1 msec TWT below the seafloor. None of them reaches the present-day surface. They are mostly located above the structural high corresponding to the tilted fault block (horst), and are found along a south-north trend starting above the N-S fault bounding the horst to the west. Some isolated chimneys were also detected in the western part of the field. Detailed interpretation of chosen seismic cross-sections throughout the field, constrained by well-logs and core analysis allowed to propose a scenario for the genesis of these features. Some of the oil generated in the middle Jurassic source rocks, further south of the Porcupine basin, migrated updip to the north where it has either become entrapped within Jurassic reservoir layers, or percolated up fault planes to shallower layers where fluid and gas could escape through the more permeable upper Cretaceous and Tertiary sandstones. Another interesting feature is the abundance of pockmarks throughout the entire field. More than 1100 were mapped from 2D survey seismic profiles. They are known to form through the eruption of methane gas trapped in the sediments. They are proposed to be the expression at the surface of the fluid and gas that migrated from deeper parts of the basin, first through fault planes, then through the formation using more or less energetic ways (shock waves, percolation, diffusion).

In this paper, we present new seismic and heat-flow data that show the base of the hydrate stability zone (BHSZ) in Lake Baikal to be locally characterized by abnormal variations in depth, with distinct regions of deeper-than-normal and regions of shallower-than-normal BHSZ. These variations are related to strong lateral variations in heat flow, and occur in close association with important rift-basin faults. Areas of shallow BHSZ are also characterized by the presence of several methane seeps and mud volcanoes at the lake floor. We infer that the seeps are the surface expression of escape pathways for overpressured fluids generated by the dissociation of pre-existing hydrates, in response to a thermal pulse caused by an upward flow of hydrothermal fluids towards the BHSZ. It thus seems that present-day hydrate dissociation in Lake Baikal is modulated by the tectonic activity in the rift rather than by – climatically controlled – changes in lake level or water temperature.

The sedimentary fill of Lake Annecy (northwestern Alps) - related to the last glacial/post-glacial episode - was investigated through high resolution (sparker) and very high resolution (2.5 kHz) seismic-reflection surveys. A seismostratigraphic approach led to subdivision of a 150 m-thick pile (maximum thickness in axial part) into five units. Basal units (1 and 2) represent an imbrication of subglacial and glacio-lacustrine deposits, close to the grounding line of the glaciers'' fronts (respectively at the northern and southern terminations of the lake). The first acoustically well-stratified unit (3) developed during a fast retreat of the glaciers fronts far from the lake basin, and a progradational alluvial regime, with abundant underflows, in a lake larger than the present one. Unit 4 represents the progressive decrease of this clastic input mixed with the progressive development of in situ bio-induced production. As in many other alpine lakes, a topmost unit (5), relatively thin (about 8-10 m) and with a conspicuous drape configuration, is the signature of the Holocene interglacial climatic conditions with a sedimentation rate of about 1 mm/yr. On the lacustrine basin slopes, slumps and debris flow occurred mainly within Unit 3; they may be due to, either climate-induced high rate terrigenous sedimentation, or/and to a period of increased seismo-tectonic activity.

Multi-channel seismic profiling and deep drilling have evidenced the presence of gas hydrates in Lake Baikal, Siberia. They occur in the deep basins around the large Selenga River Delta. The presence of the hydrates is evident on seismic records by virtue of a distinct high-amplitude, reversed-polarity, cross-cutting BSR. Locally, however, the BSR shows a very anomalous behaviour. In the vicinity of some of the main, active, intra-basin faults, its depth strongly fluctuates, with undulations (positive as well as negative compared to background positions) and vertical displacements of several hundreds of ms TWT. Locally, the BSR is even entirely disrupted by vertical ‘chimneys’ that reach up to the lake bottom.High-resolution deep-tow side-scan sonar mosaics over one of such areas of deformed and disrupted BSR show a cluster of morphological irregularities on the lake floor, in contrast to areas above a regular BSR where the lake floor is absolutely regular and flat. Four large irregularities - aligned parallel to the fault – were discovered, one of them coinciding with one of the ‘chimneys’. They were mapped in detail by bathymetric sounding and proved to be either elevations (mud volcanoes ?) or depressions (craters) at the lake floor. Echosounding has also shown venting associated with these features, which is evidenced by an acoustically non-transparent plume, reaching 10-25 m above the bottom (in other places in a similar context plumes were observed of > 200 m of height). CTD-profiling, which shows very little change in bottom-water temperature at the venting sites, suggests that the plumes represent cold seeps.Heat-flow values measured over the area show a good correlation with changes in BSR depth: values vary between 50-60 mW/m² to 80-90 mW/m². In the craters, heat-flow values are highest, but they do not exceed 165 mW/m². Our observations suggest that the Baikal hydrates are locally - along particular segments (about 15 km long) of active faults - destabilizing by tectonically controlled upward flow of fluid and heat, and that this results in active venting of gasses and/or fluids at the lake floor.

Four cold seeps are studied in Lake Baikal’s South Basin using side scan sonar in combination with detailed bathymetry, measurements of near-bottom water properties, and selected seismic profiles. The cold seeps occur at the crest of a fault block in the hanging wall of a large normal fault (the Posolsky fault), at a waterdepth of 1340 m to 1420 m. They were named Bolshoy (large), Stari (old), Malenki (small), and Malyutka (very small). The seeps at the lake floor are identified on side scan sonar and echosounding data and interpreted as flat-topped (craters, e.g. Malenki) and/or conical-shaped mud volcanoes (e.g. Bolshoy). Each vent is characterized by a near-bottom acoustic anomaly in the water column. Measurements of near-bottom water properties indicate a negative oxygen anomaly and a small but consistent positive temperature anomaly but no difference in salinity. The expelled fluids appear to be not much different from the lake water and most likely originate from shallow depth, probably from gas hydrate dissociation between 300 m and 150 m below the lake floor.Combination with subsurface data from seismic profiles shows that the seeps occur where the base of gas hydrate layer (BGH) is irregular and shallowing rapidly towards the vent sites. Coincident with the shallowing BGH, positive heat flow anomalies occur. Local dissociation of gas hydrates from the base to the top is attributed to a pulse of hydrothermal heat flow along an active fault segment. Volumetric expansion associated to hydrate dissociation probably caused a high-pressure compartment bounded at the top by the elevated BGH. From the top of the domeshaped BGH a vertical feeder pipe extends to the mud volcano at the lake floor, disrupting the horizontally stratified seismic facies. The mud volcanism seems to be shortlived, as is illustrated by the Bolshoy mud volcano that seems to be in a final phase of mud extrusion despite its recent initiation. The mud volcanoes and cold seeps are interpreted to be an example of a vigorous gas and fluid expulsion caused by gas hydrate dissociation due to a hydrothermal event. The duration of fluid and mud expulsion is limited by the amount of hydrate, by the duration of the hydrothermal pulse, or by the time needed for a new equilibrium to be installed. This mechanism may be an explanation for traces of short-lived mud volcanism in other gas hydrate areas in a tectonically active setting.

Gas hydrates have been found in the near-bottom sediments within gas venting structures in the southern basin of Lake Baikal. The Baikal basin consists of three subbasins, separated by accommodation zones: the Academician Ridge between the north and central basins, the Selenga Delta Accommodation Zone between the central and south basins. The venting structures are located south of the Posolskiy Bank, which is a basement ridge with a thick sequence of sediments on top, trending NE, oblique to the main border fault. The Posolskiy Bank belongs to the Selenga Delta Accommodation Zone. It is delimited at its south-eastern side by the Posolskiy Fault with a displacement of more than 1000 metres. The venting structures are located close to an ENE trending fault, parallel to the Posolskiy Fault. Besides the fault which is delimiting the venting area at its northern side, and which has a vertical offset of its northern footwall of 25-30 metres, a fault with a vertical offset of its eastern footwall is not more than 5 metres. The venting area corresponds to a zone of local elevation of the lake floor, which is delimited at its northern side by the ENE trending fault and which is gently dipping towards SW in its southern part. It consequently appears as a zone of updoming probably related to a fluid and gas flow from depth, finally disrupted along its northern and eastern sides. The updoming is interpreted as the consequence of the disruption of the deep gas hydrate layer resulting from fluid overpressure. This interpretation is favoured by the regional pattern of the BSR, which shows strong variations in depth over this part of the south Baikal basin: it reaches minimum depths in the venting area and drops abruptly down to significantly larger depths north of the fault delimiting the venting area. The upward migration of the BSR is interpreted as a consequence of a regional fluid flow at depth, resulting in heat transport, consequent upward migration of the gas hydrates stability zone, updoming of the area and finally disruption along a fault zone. The origin of the fluid flow is discussed. Folding structures in the lake sediments along a NW-SE to WNW-ESE trend have been described in different parts of the central and southern basins. These folding structures are assumed to be related to left-lateral strike-slip movements along the eastern side of the basin. In this assumption, the NE trending fault associated with the venting structures and the spatial alignment of the structures themselves would correspond to the direction of extension associated with the left-lateral strike-slip, whereas the NW oriented direction of compression would correspond to the fluid flow.

An example of a Late Miocene expulsion rollover syncline related to mobile shale is described from 3D seismic data offshore Brunei. Prodelta shales accumulated in the subsiding syncline were altered after deep burial by overpressure increase and hydraulic fracturing. Consequently seismic facies was modified from a continuous divergent reflection pattern to a low-amplitude, disrupted to chaotic seismic facies bounded by a high-amplitude event that cross-cuts original stratification. Without the resolution provided by 3D data, the high-amplitude event would be mistaken for early growth faults and the subdued country-rock reflections for mobile shale, typical features at the base of large deltas. This study shows the existence of expulsion rollovers related to mobile shale, which have not been described before, and demonstrates that not all bright reflections at chaotic-coherent facies boundaries are actually fault planes. It also suggests that expulsion synclines, even during early phases of delta development may be compaction-related features rather than ones that drive the shale mass out of the area of progradation.

Natural gas hydrates store a substantial portion of the Earth’s organic carbon, although their occurrence is restricted by thermobaric boundaries and the availability of methane-rich fluids. The complexity of geological systems and the multiphase flow processes promoting hydrate formation can result in a mismatch between the predicted and the observed hydrate distribution. The purpose of this research is to achieve a better comprehension of the factors that influence the distribution of gas hydrates and the mechanism of fluid movements beneath and across the gas hydrate stability zone (GHSZ). Therefore, this study integrates seismic, petrophysical and geochemical data from different gas hydrate provinces. This work provides evidence that hydrates can occur below bottom-simulating reflectors, in the presence of sourcing thermogenic hydrocarbons. The relationship between fluid-escape pipes and gas hydrates is further explored, and pipe-like features are suggested to host a significant volume of hydrates. The host lithology also represents a critical factor influencing hydrate and free gas distribution and, in evaluating a natural gas hydrate system, needs to be considered in conjunction with the spatial variability in the methane supply. The three-dimensional distribution of gas hydrate deposits in coarse-grained sediments, representing the current target for hydrate exploration, is shown to be correlated with that of the underlying free gas zone, reflecting sourcing mechanisms dominated by a long-range advection. In such systems, the free gas invasion into the GHSZ appears controlled by the competition between overpressure and sealing capacity of the gas hydrate-bearing sediments. Globally, the thickness of the free gas zones is regulated by the methane supply and by different multi-phase flow processes, including fracturing, capillary invasion and possibly diffusion. In conclusion, this research indicates that geological, fluid flow and stability factors interweave at multiple scales in natural gas hydrate systems.