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Seismic evidence for gas accumulation related to the area of mud volcanism in the deep Black Sea
Abstract
Bright spots were observed on seismic time sections acquired in 1991 and 1993 during cruises carried out within the UNESCO-ESF
Training-through-Research (TTR) program in the deep Black Sea mud volcanic area. We demonstrate the results of detailed study
of the anomalous reflections, which indicate that gas is responsible for the bright spots in the study area. Most of the bright
spots are characterized by inverted polarity and enlarged elastic energy attenuation.
... Sea mud volcanoes has very high gas saturation, with gas content of 80-99 per cent methane (Ivanov et al. 1996;Dimitrov 2002). Observations from seismic reflection profiles indicate that the roots of mud volcanoes in the Black Sea can be traced to ∼6 km depth and into the Maikop formation (Ivanov et al. 1996;Gaynanov et al. 1998). Clasts in the mud breccia brought up by these volcanoes also indicate the source is at least as deep as the Maikop formation (Ivanov et al. 1996;Gaynanov et al. 1998;Dimitrov 2002;Krastel et al. 2003). ...
... Observations from seismic reflection profiles indicate that the roots of mud volcanoes in the Black Sea can be traced to ∼6 km depth and into the Maikop formation (Ivanov et al. 1996;Gaynanov et al. 1998). Clasts in the mud breccia brought up by these volcanoes also indicate the source is at least as deep as the Maikop formation (Ivanov et al. 1996;Gaynanov et al. 1998;Dimitrov 2002;Krastel et al. 2003). The Maikop formation extends throughout the Black Sea with a uniform thickness, but mud volcanism only occurs in a few locations. ...
... The final velocity models (Fig. 5) reveal a deep sedimentary basin, dominated by a thick LVZ at 6-8 km depth within the centre of the basin, and above the MBSH at 5-6 km depth. The LVZ indicates widespread overpressure linked to the Maikop formation, a result that is supported by the numerous mud volcanoes and gas/oil seeps located in the basin, which are shown to have origins in the Maikop (Ivanov et al. 1996;Gaynanov et al. 1998). Using wideangle seismic data, in conjunction with normal incidence traveltime data, well-constrained seismic velocities for the entire thickness of the sedimentary column were determined. ...
... Sea mud volcanoes has very high gas saturation, with gas content of 80-99 per cent methane (Ivanov et al. 1996;Dimitrov 2002). Observations from seismic reflection profiles indicate that the roots of mud volcanoes in the Black Sea can be traced to ∼6 km depth and into the Maikop formation (Ivanov et al. 1996;Gaynanov et al. 1998). Clasts in the mud breccia brought up by these volcanoes also indicate the source is at least as deep as the Maikop formation (Ivanov et al. 1996;Gaynanov et al. 1998;Dimitrov 2002;Krastel et al. 2003). ...
... Observations from seismic reflection profiles indicate that the roots of mud volcanoes in the Black Sea can be traced to ∼6 km depth and into the Maikop formation (Ivanov et al. 1996;Gaynanov et al. 1998). Clasts in the mud breccia brought up by these volcanoes also indicate the source is at least as deep as the Maikop formation (Ivanov et al. 1996;Gaynanov et al. 1998;Dimitrov 2002;Krastel et al. 2003). The Maikop formation extends throughout the Black Sea with a uniform thickness, but mud volcanism only occurs in a few locations. ...
... The final velocity models (Fig. 5) reveal a deep sedimentary basin, dominated by a thick LVZ at 6-8 km depth within the centre of the basin, and above the MBSH at 5-6 km depth. The LVZ indicates widespread overpressure linked to the Maikop formation, a result that is supported by the numerous mud volcanoes and gas/oil seeps located in the basin, which are shown to have origins in the Maikop (Ivanov et al. 1996;Gaynanov et al. 1998). Using wideangle seismic data, in conjunction with normal incidence traveltime data, well-constrained seismic velocities for the entire thickness of the sedimentary column were determined. ...
Mud volcanism associated with the degassing of deeply buried sediments has been identified in the Black Sea and Caspian Sea. Sampling of mud volcano breccia and seismic imaging of the "roots" suggest the origin lies in the Maykop formation, an organic-rich shale deposited in the Oligocene/Miocene that constitutes the major hydrocarbon source rock in the area. High sedimentation rates, leading to rapid burial and undercompaction, combined with hydrocarbon generation are possible mechanisms for generating overpressure in this layer. To our knowledge the magnitude of this overpressure in the Black Sea has not been quantified. The excess pore- pressure can be estimated from seismic velocities, however the Maykop formation is too deep (4 - 7 km below the seabed) for accurate velocities to be obtained from conventional multichannel seismic data. In spring 2005, four new wide-angle seismic reflection/refraction profiles were collected across the Eastern Black Sea Basin that provide better constraints on the sediment and crustal velocity structure. The data are of high quality allowing multiple sedimentary phases to be identified and linked with coincident reflection data. We have used the seismic tomography code JIVE, which performs simultaneous inversion of reflections and refractions from multiple layers. The model shows a thick sedimentary package, ~9 km in the centre of the basin. A low-velocity zone that may be associated with overpressure occurs near the base of the sediments. It is ~4 km thick in the centre of the basin, with anomalous velocities of ~2.6-3 kms-1 within the zone and ~3.6 kms-1 above and below. The low velocity zone is fairly continuous across the eastern basin and can also be identified on a profile that crosses the Mid-Black Sea High into the western basin. Overpressure can be expressed in terms of effective stress (lambda); the ratio of excess pore pressure to lithostatic pressure. Using our velocity model, we estimate values of lambda of approximately 0.77±0.02, corresponding to pore pressures of ~45 MPa within the low velocity zone. If overpressure exists throughout the Maykop formation, as suggested by our data, it may be one of the largest overpressured regions in the world.
... Sea mud volcanoes has very high gas saturation, with gas content of 80-99 per cent methane (Ivanov et al. 1996;Dimitrov 2002). Observations from seismic reflection profiles indicate that the roots of mud volcanoes in the Black Sea can be traced to ∼6 km depth and into the Maikop formation (Ivanov et al. 1996;Gaynanov et al. 1998). Clasts in the mud breccia brought up by these volcanoes also indicate the source is at least as deep as the Maikop formation (Ivanov et al. 1996;Gaynanov et al. 1998;Dimitrov 2002;Krastel et al. 2003). ...
... Observations from seismic reflection profiles indicate that the roots of mud volcanoes in the Black Sea can be traced to ∼6 km depth and into the Maikop formation (Ivanov et al. 1996;Gaynanov et al. 1998). Clasts in the mud breccia brought up by these volcanoes also indicate the source is at least as deep as the Maikop formation (Ivanov et al. 1996;Gaynanov et al. 1998;Dimitrov 2002;Krastel et al. 2003). The Maikop formation extends throughout the Black Sea with a uniform thickness, but mud volcanism only occurs in a few locations. ...
... The final velocity models (Fig. 5) reveal a deep sedimentary basin, dominated by a thick LVZ at 6-8 km depth within the centre of the basin, and above the MBSH at 5-6 km depth. The LVZ indicates widespread overpressure linked to the Maikop formation, a result that is supported by the numerous mud volcanoes and gas/oil seeps located in the basin, which are shown to have origins in the Maikop (Ivanov et al. 1996;Gaynanov et al. 1998). Using wideangle seismic data, in conjunction with normal incidence traveltime data, well-constrained seismic velocities for the entire thickness of the sedimentary column were determined. ...
Although the Black Sea is one large depositional structure today, previous studies have shown that the basin can be divided into two sub-basins, which have different tectonic histories. There is a general consensus on the formation history of the Western Black Sea Basin but there is little agreement on the history of the Eastern Black Sea. In March 2005 we collected a series of onshore-offshore, wide-angle seismic refraction/reflection profiles throughout the basin to sample the crustal structure. Four lines were surveyed; Line 1 was a 470 km profile, shot along strike through the centre of the basin, Lines 3 and 4 were dip lines shot across the Mid-Black Sea High and one dip line (Line 2) was shot further to the east. Data quality is very good and deep arrivals can be typically detected to 100 km offset. These data will be combined with co-incident industry reflection profiles to constrain the extensional evolution of the Eastern Black Sea Basin. We present data from Line 1, which has been modelled to investigate the velocity structure of the basin. Line 1 has 34 ocean bottom seismometers, spaced at ~14 km, deployed along the profile with an additional seven seismometers deployed on land. The seismic source was an air-gun array with a total source volume of 3140 cu.in and the line was shot at a spacing of 60 s, equivalent to a spatial interval of about 120 m. A combination of ray-tracing and first-arrival seismic tomography has been used to create a 2D velocity model through the crust. The model shows that the basin has a thick sedimentary package (up to ~10 km thick) with velocities ranging from 1.6 - 4.2 km/s. These overlay a crystalline crust with velocities of 4.8 - 7.2 km/s, which thins to ~8 km thick in the deepest part of the basin. The model indicates the presence of a low velocity zone near the base of the sediment package. This anomalous layer stretches across most of the profile, with velocities as low as 3.1 +/- 0.12 km/s compared with sediment velocities of 3.9 km/s above. The most likely explanation for these low velocities is over pressurisation of pore-fluids leading to under-compaction of the sediments.
... Sea mud volcanoes has very high gas saturation, with gas content of 80-99 per cent methane (Ivanov et al. 1996;Dimitrov 2002). Observations from seismic reflection profiles indicate that the roots of mud volcanoes in the Black Sea can be traced to ∼6 km depth and into the Maikop formation (Ivanov et al. 1996;Gaynanov et al. 1998). Clasts in the mud breccia brought up by these volcanoes also indicate the source is at least as deep as the Maikop formation (Ivanov et al. 1996;Gaynanov et al. 1998;Dimitrov 2002;Krastel et al. 2003). ...
... Observations from seismic reflection profiles indicate that the roots of mud volcanoes in the Black Sea can be traced to ∼6 km depth and into the Maikop formation (Ivanov et al. 1996;Gaynanov et al. 1998). Clasts in the mud breccia brought up by these volcanoes also indicate the source is at least as deep as the Maikop formation (Ivanov et al. 1996;Gaynanov et al. 1998;Dimitrov 2002;Krastel et al. 2003). The Maikop formation extends throughout the Black Sea with a uniform thickness, but mud volcanism only occurs in a few locations. ...
... The final velocity models (Fig. 5) reveal a deep sedimentary basin, dominated by a thick LVZ at 6-8 km depth within the centre of the basin, and above the MBSH at 5-6 km depth. The LVZ indicates widespread overpressure linked to the Maikop formation, a result that is supported by the numerous mud volcanoes and gas/oil seeps located in the basin, which are shown to have origins in the Maikop (Ivanov et al. 1996;Gaynanov et al. 1998). Using wideangle seismic data, in conjunction with normal incidence traveltime data, well-constrained seismic velocities for the entire thickness of the sedimentary column were determined. ...
Wide-angle seismic data from the Eastern Black Sea have been used to determine the geological structure of the sediments, the entire crust, and upper mantle. Data were acquired using a combination of ocean-bottom seismometers (OBS), land seismometers, and a marine air-gun source, providing refracted and reflected energy recorded to offsets in excess of 100 km.
... Sea mud volcanoes has very high gas saturation, with gas content of 80-99 per cent methane (Ivanov et al. 1996;Dimitrov 2002). Observations from seismic reflection profiles indicate that the roots of mud volcanoes in the Black Sea can be traced to ∼6 km depth and into the Maikop formation (Ivanov et al. 1996;Gaynanov et al. 1998). Clasts in the mud breccia brought up by these volcanoes also indicate the source is at least as deep as the Maikop formation (Ivanov et al. 1996;Gaynanov et al. 1998;Dimitrov 2002;Krastel et al. 2003). ...
... Observations from seismic reflection profiles indicate that the roots of mud volcanoes in the Black Sea can be traced to ∼6 km depth and into the Maikop formation (Ivanov et al. 1996;Gaynanov et al. 1998). Clasts in the mud breccia brought up by these volcanoes also indicate the source is at least as deep as the Maikop formation (Ivanov et al. 1996;Gaynanov et al. 1998;Dimitrov 2002;Krastel et al. 2003). The Maikop formation extends throughout the Black Sea with a uniform thickness, but mud volcanism only occurs in a few locations. ...
... The final velocity models (Fig. 5) reveal a deep sedimentary basin, dominated by a thick LVZ at 6-8 km depth within the centre of the basin, and above the MBSH at 5-6 km depth. The LVZ indicates widespread overpressure linked to the Maikop formation, a result that is supported by the numerous mud volcanoes and gas/oil seeps located in the basin, which are shown to have origins in the Maikop (Ivanov et al. 1996;Gaynanov et al. 1998). Using wideangle seismic data, in conjunction with normal incidence traveltime data, well-constrained seismic velocities for the entire thickness of the sedimentary column were determined. ...
We present new data that explores the link between pore pressure and
seismic velocity to estimate the magnitude of the overpressure within
the deep sediments of the Eastern Black Sea basin. New wide-angle
seismic data, combined with coincident reflection data, have been
modelled simultaneously using the seismic tomography code, Jive3D, to
provide a well-constrained seismic velocity model of the sediments. Our
models reveal a widespread low-velocity zone at the depth of 5.5-8.5 km,
which is characterized by a velocity decrease from 3.5 to ~2.5 km
s-1. Using two separate methods that relate changes in
seismic velocity to changes in effective stress, we estimate pore
pressures of at least 160 MPa within the low-velocity zone. These pore
pressures give λ* values of 0.8-0.9 within the centre
of the basin and above the Mid-Black Sea High. The low-velocity zone
occurs within the Maikop formation, an organic-rich mud layer identified
as the source of mud volcanism in the Black Sea and South Caspian Sea.
... Most recent studies of mud volcanoes in the Black Sea concentrate on the central part of the Black Sea. Nine large mud volcanoes were identified west of the Crimea fault (Ivanov et al. 1996;Limonov et al. 1997;Gaynanov et al. 1998). The Sorokin Trough is the second main area with abundant mud volcanoes (Ginsburg et al. 1990;Soloviev and Ginsburg 1994;Woodside et al. 1997). ...
... The geology of the Black Sea has been studied for many years (e.g., Ross et al. 1974;Finetti 1988;Okay et al. 1994). The Black Sea is generally considered to be a result of back-arc extension associated with northward subduction of the African and Arabian plates. ...
... This is supported by Milkov (2000) who indicates that most mud volcanoes are associated with diapirs. In terms of size and depth of the feeder channel, the Kazakov mud volcano is comparable to the mud volcanoes in the central Black Sea (Ivanov et al. 1996;Limonov et al. 1997;Gaynanov et al. 1998). ...
The Sorokin Trough (Black Sea) is characterized by diapiric structures formed in a compressional tectonic regime that facilitate fluid migration to the seafloor. We present acoustic data in order to image details of mud volcanoes associated with the diapirs. Three types of mud volcanoes were distinguished: cone-shaped, flat-topped, and collapsed structures. All mud volcanoes, except for the Kazakov mud volcano, are located above shallow mud diapirs and diapiric ridges. Beyond the known near-surface occurrence of gas hydrates, bottom simulating reflectors are not seen on our seismic records, but pronounced lateral amplitude variations and bright spots may indicate the presence of gas hydrates and free gas.
... Most recent studies of mud volcanoes in the Black Sea concentrate on the central part of the Black Sea. Nine large mud volcanoes were identified west of the Crimea fault (Ivanov et al. 1996;Limonov et al. 1997;Gaynanov et al. 1998). The Sorokin Trough is the second main area with abundant mud volcanoes (Ginsburg et al. 1990;Soloviev and Ginsburg 1994;Woodside et al. 1997). ...
... The geology of the Black Sea has been studied for many years (e.g., Ross et al. 1974;Finetti 1988;Okay et al. 1994). The Black Sea is generally considered to be a result of back-arc extension associated with northward subduction of the African and Arabian plates. ...
... This is supported by Milkov (2000) who indicates that most mud volcanoes are associated with diapirs. In terms of size and depth of the feeder channel, the Kazakov mud volcano is comparable to the mud volcanoes in the central Black Sea (Ivanov et al. 1996;Limonov et al. 1997;Gaynanov et al. 1998). ...
Detailed morphological data collected from the submarine flanks of the Canary Islands have revealed numerous submarine canyons down to water depths of >3,000 m. These canyons are interpreted to have formed by submarine erosion. We postulate formation of proto-canyons by downslope-eroding mass flows which originate on land, enter the sea, and continue below sea level for several tens of kilometers. Once proto-canyons have been formed, they become deepened by further erosion and failures of the canyon walls and/or floor. Large amounts of sediments, funnelled through the canyons from the islands into the adjacent deep-ocean sedimentary basins, play an important role in the evolution of volcanic aprons surrounding ocean islands. Some major canyon systems appear to have persisted for at least 14 million years.
... The high gas content of sediments has been inferred from observations of intense gas escape from the seafloor (Polikarpov et al., 1989; Luth et al., 1999; Kutas et al., 2002; Dimitrov, 2002; Egorov et al., 2003; Shnyukov et al., 2003; Naudts et al., 2006), mud volcanism (Ivanov et al., 1996; Limonov at al., 1997; Bohrmann et al., 2003), or gas hydrate sampling (Ginsburg and Soloviev, 1998; Vassilev and Dimitrov, 2002; Mazzini et al., 2004). Geophysical studies have described the acoustic signature of free gas in the sediment pore space (Gaynanov et al., 1998; Ergün et al., 2002; Ion et al., 2002; Kutas et al., 2002), as well as Bottom-Simulating Reflections (BSRs) suggesting the presence of gas hydrates (Ginsburg and Soloviev, 1998, and references therein; du Fornel, 1999; Ion et al., 2002; Lüdmann et al., 2004; Popescu et al., 2006). Here we present a synthesis of the main gas features in recent sediments at the scale of the western Black Sea basin, based on investigation of an extensive seismic dataset (Fig. 1), integrated with published data and results. ...
... Dotted grey line shows the minimum theoretical water depth for methane hydrate stability in the Black Sea. We also indicate the location of gas seeps (including information from Egorov et al., 2003; Shnyukov et al., 2003; Naudts et al., 2006), gas hydrate samples (after Vassilev and Dimitrov, 2002; Mazzini et al., 2004), mud volcanoes (after Gaynanov et al., 1998) and oil and gas exploration fields (after Robinson et al., 1996; Dinu et al., 2005). Thin dashed line around the BSR area in the northern basin shows BSR occurrence after Lüdmann et al. (2004). ...
This study is a synthesis of gas-related features in recent sediments across the western Black Sea basin. The investigation is based on an extensive seismic dataset, and integrates published information from previous local studies. Our data reveal widespread occurrences of seismic facies indicating free gas in sediments and gas escape in the water column. The presence of gas hydrates is inferred from bottom-simulating reflections (BSRs). The distribution of the gas facies shows (1) major gas accumulations close to the seafloor in the coastal area and along the shelfbreak, (2) ubiquitous gas migration from the deeper subsurface on the shelf and (3) gas hydrate occurrences on the lower slope (below 750 m water depth). The coastal and shelfbreak shallow gas areas correspond to the highstand and lowstand depocentres, respectively. Gas in these areas most likely results from in situ degradation of biogenic methane, probably with a contribution of deep gas in the shelfbreak accumulation. On the western shelf, vertical gas migration appears to originate from a source of Eocene age or older and, in some cases, it is clearly related to known deep oil and gas fields. Gas release at the seafloor is abundant at water depths shallower than 725 m, which corresponds to the minimum theoretical depth for methane hydrate stability, but occurs only exceptionally at water depths where hydrates can form. As such, gas entering the hydrate stability field appears to form hydrates, acting as a buffer for gas migration towards the seafloor and subsequent escape.
... Seismic sections also indi cate that gas does not form in situ but migrates upward through deep faults in high porosity strata. A similar model is described in [14] for the central part of the Black Sea. According to this model, normal faults reach the seafloor due to the uplift of the Gas sips and shallow accumulations of gas on the shelf of Turkey in the Black sea Fig. 11. ...
It has been suggested that shelf and slope sediments of high deposition rate are methane sources, whereas the deep basin is methane sink. The methane production and migration in sediments may cause massive slope failures so methane is geologically important. Methane production is also economically important as methane seeps may indicate the presence at depth of hydrocarbon reservoirs, and methane hydrate may be an important source of energy.
Recent studies in marine geology indicate potential geo-resources in the Turkish coast of Black Sea. The Black Sea sediments are rich in calcite and organic carbon, the latter showing a high degree of preservation due to anoxia in the waters below 100-150 m. Different marine geophysical surveys at different times were carried out in order to understand the sedimentary features of gas-saturated sediments in the Black Sea. Multibeam, side scan sonar, sub-bottom profiler and multi-channel seismic data were collected to make both high-resolution bathymetric and reflectivity maps of the seafloor. In some cruises, deep-tow combined side scan sonar and subbottom profiler was used to obtain acoustic images of both the seafloor surface and subbottom sediments. Several different structures were observed in the Black Sea basin as slumps, pockmarks, faults, gas chimneys, shallow gas accumulations and dome-like structures. Structures, which contain gas hydrates, are present on the seismic sections as strong acoustic reflections.
... Seabed sampling of flare regions shows diverse chemosynthetic communities and shallow gas hydrates within 2-3 mbsf. associated with cold seeps are reported in the vicinity of intrusive structures like a shale/salt diapir (Hovland and Curzi, 1989;Hovland, 1990;Lüdmann and Wong, 2003;Römer et al., 2012) or mud volcanoes Gaynanov et al., 1998;Greinert et al., 2006;Loher et al., 2018;Loncke and Mascle, 2004;Milkov et al., 2003;Pape et al., 2011;Paull et al., 2008). Drilling/coring data acquired during NGHP-Exp-01 in the KG basin shows that overburden sediments comprise of low-permeability clay. ...
Drilling/coring activities during NGHP-Expeditions 01 and 02 discovered significant gas hydrate deposits in the Krishna-Godavari (KG) basin, a proven petroliferous basin located along the eastern continental margin of India. Active and paleo-cold seeps are also reported from deep waters (>1000 m), indicating methane venting episodes in the KG basin. In the present study, we analyzed geophysical data acquired onboard RV Sindhu Sadhana to study the spatial distribution of gas flares in the KG offshore basin. Twenty-two gas flares were observed in water-column images at seafloor depths ranging from 638 m to 1960 m. All flares except one are detected within the gas hydrate stability zone (GHSZ), which occurs beyond 700–720 m water depths. Most gas flares seem to terminate at these depths, indicating that the bubbles might be coated with a thin methane hydrate skin. Seabed sampling of flare regions shows the presence of chemosynthetic communities and shallow gas hydrates within 2–4 mbsf.
The spatial distribution of gas flares is not random and tends to align with the toe-thrust zones and shale diapiric mounds. These compressive structures formed due to shale tectonism in the KG basin have provided an environment conducive for the formation of gas hydrate deposits and gas flares/cold seeps. The analysis of high-resolution seismic data across the flares shows the presence of seismic chimneys and faults, which facilitated migration of deep-seated fluid/gas through the GHSZ. Focused fluid flow along the fault zone and the perturbation of methane + seawater phase curve due to salinity increase owing to the continuous formation of methane hydrate are the most likely mechanisms to explain methane gas migration.
... Robinson et al. (1996) classified several sedimentary units in the basin, including the clay-rich Maikop Formation (Late Oligocene to Early Miocene), which is the most significant hydrocarbon source rock in the Black Sea and adjacent areas, e.g. the Caspian region. Generally, about 2.5 km of Plio-Quaternary sediments overlie the Maikopian Formation (Gaynanov et al., 1998;Bohrmann et al., 2007). Additionally, carbonates and volcanogenic sedimentary rocks were identified at the Cretaceous to Cenozoic boundary (Zonenshain and Lepichon, 1986;Robinson et al., 1996). ...
Four seep sites located within an $20 km 2 area offshore Georgia (Batumi seep area, Pechori Mound, Iberia Mound, and Colkheti Seep) show characteristic differences with respect to element concentrations, and oxygen, hydrogen, strontium, and chlorine isotope signatures in pore waters, as well as impregnation of sediments with petroleum and hydrocarbon potential. All seep sites have active gas seepage, near surface authigenic carbonates and gas hydrates. Cokheti Seep, Iberia Mound, and Pechori Mound are characterized by oil-stained sediments and gas seepage decoupled from deep fluid advection and bottom water intrusion induced by gas bubble release. Pechori Mound is further characterized by deep fluid advection of lower salin-ity pore fluids. The Pechori Mound pore fluids are altered by mineral/water reactions at elevated temperatures (between 60 and 110 °C) indicated by heavier oxygen and lighter chlorine isotope values, distinct Li and B enrichment, and K depletion. Strontium isotope ratios indicate that fluids originate from late Oligocene strata. This finding is supported by the occurrence of hydrocarbon impregnations within the sediments. Furthermore, light hydrocarbons and high molecular weight impregnates indicate a predominant thermogenic origin for the gas and oil at Pechori Mound, Iberia Mound, and Colkheti Seep. C 15+ hydrocarbons at the oil seeps are allochtonous, whereas those at the Batumi seep area are autochthonous. The presence of oleanane, an angiosperm biomarker, suggests that the hydrocarbon source rocks belong to the Maikopian Formation. In summary, all investigated seep sites show a high hydrocarbon potential and hydrocarbons of Iberia Mound, Colkheti Seep, and Pechori Mound are predominantly of thermogenic origin. However, only at the latter seep site advection of deep pore fluids is indicated.
... Small gas plumes in very limited areas may result from the decomposition of Quaternary organic matter through the action of bacteria (Shnyukov and Kutnyi 2003). Kutas et al. 2002) A possible role of deep faults in the formation of gas seeps in the Black Sea is as follows (Gaynanov et al. 1998;Kobolev et al. 1999;Kutas et al. 2002). Gases are generated in deep layers, including the upper mantle. ...
A new heat flow map has been compiled for the Black Sea. Marine segments of interregional deep faults and marine regional deep faults have been traced in detail in the northern Black Sea. Gas seeps are situated in zones of deep faults. The potential role of these faults in the formation of gas leakage is evaluated. For the first time the spatial fault coincidence with gas release is clearly interpreted as being directly interrelated. As the gas is largely of deep origin, the seeps may be indicators of subsurface hydrocarbon accumulation.
Воссоздана наиболее протяженная сложнопостроенная система рифтогенного типа: г. Котлас – кряж
Ветреный Пояс – Лехта – Имандра-Варзуга – Паанаярви – Карасйок-Печенга и далее к Гренландии в Лаврентию, протяженностью более 1500 км. В каждый из мегаэтапов развития этой системы на территории С.Америки и С.Европы формировались крупные магматические провинции [5, 6], в пре-
делах которых сохранились следы океанической коры, что требует дополнительных масштабных исследований.
The principal objective of this paper is to understand the gas migration process and the structures of mud volcanoes in the central Black Sea. Multichannel seismic lines were acquired across 6 mud volcanoes in the central Black Sea: MSU, Yuzhmorgeologiya, Malyshev, Kornev, Goncharov and Vassoevitch. Based on a high resolution seismic processing, we analyzed acoustic anomalies and studied near-surface sediment structures of these mud volcanoes. Four types of pathways for gas and fluid migration and three types of gas reservoirs were recognized. A regional "Bottom Simulating Reflection" (BSR) seems to be absent in most parts of the study area, however a clear BSR was observed in one of the seismic profiles. The free gas migrating upwards along these pathways were sealed by gas hydrates or fine-grained sediments in the gas hydrate stability zone. Four seismic units were separated according to a suggested age model and identified seismic facies. Combining with the possible sedimentary processes of the central Black Sea, we try to reveal the active mechanism of these mud volcanoes. These mud volcanoes reveal two to three major active stages. These active stages might be related to distinct sea level falls, which seem to be one of the main trigger factors of the mud volcano eruptions in the central Black Sea. A structural model for the MSU mud volcano was presented with respect to mechanisms of gas migration and the origin of the gas.
The Sorokin Trough (Black Sea) is characterized by diapiric structures and compressional tectonics that facilitate fluid migration to the seafloor. Abundant mud volcanoes and near surface gas hydrate occurrences were identified in this area. We present acoustic images of the mud volcanoes in the Sorokin Trough, which we collected recently with a variety of different seismic and acoustic imaging systems. The internal structure of the mud volcanoes and the surrounding sedimentary deposits were studied with multichannel seismics and the Parasound sediment echosounder. The Parasound system allows to resolve the upper tens of meters with a sub-meter resolution, while the multichannel seismic data reveals information of the deeper subsurface and the feeder channels of the mud volcanoes. Morphological information of the mud volcanoes was gathered by means of deep-tow side-scan sonar and swath sounder systems. Several mud diapirs and diapiric ridges mainly striking in a WSW-NEN direction were identified on the seismic sections. Mud volcanoes are located above or on the edges of these near surface diapirs and diapiric ridges. Their feeder channels, imaged as transparent zones on the seismic records, originate from the mud diapirs. Three different types of mud volcanoes can be distinguished: cone-shaped, flat-topped, and collapsed. The mud volcanoes have diameters of up to 2.5 km and heights of up to 120 m above the surrounding sea floor. Most of the acoustic images will be from Dvurechenskii mud volcano, a flat-topped and very active mud volcano in the Sorokin Trough. Mudflows imaged by the side-scan sonar are visible on the flanks of this and other mud volcanoes. Despite the known near-surface occurrences of gas hydrates, bottom simulating reflectors were not present in our acoustic images, but bright spots - indicating free gas - were identified in the approximate depth of the base of the gas hydrate stability zone. Pronounced lateral amplitude variations in the Parasound data may indicate the occurrence of near surface gas hydrates and/or carbonate crusts.
The possibilities for the application of dynamic parameters of acoustic records (amplitude, reflection polarity, and frequency
composition) for identifying the lithology of deposits and their physical properties, as well as their gas saturation, are
discussed. Using examples of different field measurements it is shown that application of the dynamic parameters of acoustic
records during interpretation allowed us in some cases to understand in detail the physical nature of visually recognized
acoustic anomalies, while in other cases it provided grounds for a different interpretation of the data, and in some cases
even permitted correction of errors of visual interpretation of time sections.
Following the investigations of the 8th Training Through Research (TTR8) cruise (1998), the Vøring Plateau/Storegga Slide area was revisited during the TTR10 expedition of RV Professor Logachev in August 2000 using a single-channel seismic system. Bottom simulating reflections (BSRs) were observed in undisturbed sediments of the Vøring Plateau and below the deposits of the Storegga Slide. The BSR is expressed on the seismic records either as a strong reflection, often crosscutting local stratigraphy, or as a facies change between high-amplitude (enhanced) reflections below and normal amplitude reflections above it. Analysis of the seismic signature of the BSRs and adjacent enhanced reflections showed that the anomalous reflections demonstrate reversed polarity and cause amplitude and frequency shadows below, indicating the presence of free gas trapped below hydrate-bearing sediments. For each of the lines where BSR was observed, the theoretical base of the local gas hydrate stability field (GHSF) was calculated. The resulting calculated depths of the theoretical GHSF fit well with observed BSRs, and confirm the gas-hydrate nature of the reflections. On most of the profiles the BSR is ‘split’ into several isolated zones. The reflection appears at specific stratigraphic layers, between strata where the BSR is not observed. Four ‘BSR-demonstrating’ layers were identified, continuing upslope (below the GHSF) as enhanced reflections. These layers must possess some physical or petrographical property, that makes them most favourable for gas hydrate formation and/or accumulation of free gas. All these layers lie within the thick (>1 s TWT) upper Pliocene–Quaternary glacial/interglacial sedimentary sequence. It is assumed that they correspond to some strata of coarser terrigenic material, which would feature higher permeability relative to surrounding sediments and/or to undercompacted layers with increased water content.
Both the anaerobic and the aerobic oxidation of methane are fundamental microbial processes with far reaching influences on global element cycles and, consequently, on the physico-chemical nature of the hydro- and atmosphere. These processes lead to substantial removal of the radiatively active gas methane and are powerful factors in controlling the composition of ecosystems and the distribution of authigenic deposits at methane-rich sites. For instance, the sulfate-dependent anaerobic oxidation of methane (AOM) mediated by methanotrophic Archaea and sulfate-reducing Bacteria yields hydrogen sulfide and bicarbonate ions, which subsequently react with ions derived from pore waters and the water column to form sulfidic and carbonaceous minerals. The aerobic oxidation of methane, performed by obligate aerobic Bacteria particularly effective at oxic-anoxic boundaries, leads to the generation of carbon dioxide, which is a less radiative gas than methane but has higher residence times in the atmosphere.
Hydrocarbon gases were determined in sediments from three mud volcanoes in the Sorokin Trough. In comparison to a reference station outside the mud volcano area, the deposits are characterized by an enrichment of high-molecular hydrocarbons (C2–C4), an absence of unsaturated homologues, a predominance of iso-butane in comparison with n-butane, and the presence of gas hydrate. The molecular composition of the hydrocarbon gases suggests their deep sources and thermogenic origin. In the pelagic sediments at the reference station, the methane concentration is relatively low (up to 49ml/l); maximum concentrations are reached in deposits of the Dvurechenskii mud volcano (up to 400ml/l). It was the first time that gas hydrate was sampled at the Dvurechenskii mud volcano. The gas extracted by dissociation of hydrate samples was dominated by methane (99.5%) with low amounts of ethane and propane (less than 0.5%). The isotopic composition of the methane varies between –62 and –66 PDB in 13C, and between –185 and –209 SMOW in D, indicating a mainly biogenic origin with an admixture of thermogenic gas.
Seismic techniques for direct hydrocarbon indication rely on the changes in seismic character which accompany changes in pore fluid content. Interpretation and modeling of direct hydrocarbon indicators (DHIs) require an understanding of the factors affecting seismic velocities and reflection coefficients which are, in turn, dependent on pore fluid properties, rock frame properties, and fluid substitution within a given rock frame. Recent developments in rock physics have greatly increased our general understanding in these areas but have also revealed complications that require further research.
The available Landsat MSS images were studied using photogeologic interpretation techniques to produce a synthetic map of linear features in the area surrounding the Black Sea. A classification of linears was made according to their length and continuity. On a regional scale the main characteristics of the deformative mechanism were analyzed. Imagery proved very useful to correlate the geological information gathered by various authors and using different criteria. In particular, it helped to understand the evolution of the structural domains and their relationships with the present setting of the Black Sea, which appears to represent the residual of an original sequence of basins formed during the Mesozoic by crustal extension. -from Authors
Describes two mud volcanoes discovered in the central part of the Black Sea at depths of more than 2000 m. This confirmed an earlier hypothesis that such volcanoes could exist in this part of the sea. Both volcanoes were studied using variously orientated seismoacoustic profiles, and samples of material were taken using a single-pass pipe. -translated by P.Cooke
Seismic wave attenuation in rocks was studied experimentally, with particular attention focused on frictional sliding and fluid flow mechanisms. Sandstone bars were resonated at frequencies from 500 to 9000 Hz, and the effects of confining pressure, pore pressure, degree of saturation, strain amplitude, and frequency were studied. Observed changes in attenuation and velocity with strain amplitude are interpreted as evidence for frictional sliding at grain contacts. Since this amplitude dependence disappears at strains and confining pressures typical of seismic wave propagation in the earth, we infer that frictional sliding is not a significant source of seismic attenuation in situ. Partial water saturation significantly increases the attenuation of both compressional (P) and shear (S) waves relative to that in dry rock, resulting in greater P‐wave than S‐wave attenuation. Complete saturation maximizes S‐wave attenuation but causes a reduction in P‐wave attenuation. These effects can be interpreted in term...
The mechanical properties of crystalline rock have been studied in the laboratory as a function of temperature and pressure at the low frequencies and the strain amplitudes characteristic of seismic waves. The inelasticity observed arises at interfaces in the rock structure and is insensitive to temperature but highly pressure dependent. It cannot be described in terms of viscoelasticity but causes the rock to display static hysteresis. The internal friction ϕ of the rock is accompanied by a large modulus defect, δM/M; both ϕ and δM/M are shown to be independent of frequency and strain amplitude throughout a very wide range. A model based on a network of cracks in an elastic medium accounts for these properties. The presence of a fluid phase in the rock does not in itself significantly increase the internal friction in the range of frequencies of seismic waves, but the presence of a small amount of intergranular fluid can make interface inelasticity persist in the presence of a large confining pressure. Interface inelasticity can occur at substantial depths in the earth, resulting in low seismic velocities and low Q. The low velocity zones in certain areas are interpreted in this way as arising from the presence of a fluid phase.
Seismic amplitudes are affected by both geometric and lithologic features of reflecting layers. Observation of 'bright spots' is a function of not only pore fluid saturations, but also of layer thickness and surrounding rock type. The numerical model developed uses a finite sum of reflected and mode-converted waves to evaluate the application of wave analysis in determining pore fluid types. If a priori knowledge of the lithology or thickness of the structure is available, the amplitude-phase-converted wave algorithm can be applied to other than bright spot data to characterize the structure. This indicates there are important applications of the amplitude-phase-converted wave algorithm to field development and to bright spot analysis for exploration.-from Authors
Morphological and structural characteristics of the Black Sea and Mediterranean Ridge mud volcanoes, studied during several UNESCO-ESF Cruises of the R/V Gelendzhik (1991–1994), are compared on the basis of single- and multi-channel seismic and sidescan sonar observations. Morphologically, three types of mud volcanoes were distinguished in the Black Sea, and six types in the Mediterranean. They differ in size, shape, and presence of the mud flows. Some types from both localities are very similar to each other, but in any case, their deep structure is very distinctive. This difference in deep structure is explained by their development in different geological conditions: under an extensional regime with high terrigenous input in the Black Sea and under strong lateral compression on the Mediterranean Ridge.