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Contribution to atmospheric methane by natural seepages on the Bulgarian continental shelf
Abstract
A regional estimation of the contribution to atmospheric methane by natural gas seepages on the UK continental shelf was undertaken by Judd et al. (Mar. Geol. 137(1/2) (1997) 165). This paper is the second in the series, and provides an estimation of the atmospheric methane flux from Bulgarian Black Sea continental shelf.Potential gas source rocks include Holocene gas-charged sediments, Quaternary peats and sapropels, and deep-lying Palaeocene and Neogene clays, Cretaceous coals, and other sediments of late Jurassic to early Cretaceous age. These cover almost the whole continental shelf and slope and, together with irregularly developed seal rocks and widespread active and conducting faults, provide good conditions for upward gas migration.A total of 5100 line kilometers of shallow seismic (boomer) and echo-sounder records acquired during the Institute of Oceanology's regional surveys, and several detailed side-scan sonar lines, have been reviewed for water column targets. Four hundred and eighty-two targets were assigned as gas seepage plumes. It is estimated that a total of 19,735 individual seeps exists on the open shelf. The number of seeps in coastal waters was estimated to be 6020; this is based on available public-domain data, specific research, and results of a specially made questionnaire which was distributed to a range of “seamen”.More than 150 measurements of the seabed flux rates were made in the “Golden sands” and “Zelenka” seepage areas between 1976 and 1991. Indirect estimations of flux rates from video and photo materials, and a review of published data have also been undertaken. Based on these data, three types of seepages were identified as the most representative of Bulgarian coastal waters. These have flux rates of 0.4, 1.8, and 3.5l/min.The contribution to atmospheric methane is calculated by multiplying the flux rates with the number of seepages, and entering corrections for methane concentration and the survival of gas bubbles as they ascend through seawater of the corresponding water depth. The estimation indicates that between 45,100,000 (0.03Tg) and 210,650,000m3 (0.15Tg)methaneyr−1 come from an area of 12,100km2.
... Then, it is almost constant from 200 m below sea level to sea bottom [Railsback, 2010]. CH 4 seepage is extremely intense on the shelf and on the slope of the Black Sea [Sozansky, 1997;Vassilev and Dimitrov, 2003;Dimitrov, 2002;Küçük et al., 2015]. In Fig. 2, natural gas seepage along the Bulgarian Black Sea Coast [Dimitrov, 2002] is shown. ...
... CH 4 seepage is extremely intense on the shelf and on the slope of the Black Sea [Sozansky, 1997;Vassilev and Dimitrov, 2003;Dimitrov, 2002;Küçük et al., 2015]. In Fig. 2, natural gas seepage along the Bulgarian Black Sea Coast [Dimitrov, 2002] is shown. The gas composition includes 92e96.3% ...
... l/min; (c) a strong seep with more than 80 bps, 2.5e3.8 l/min [Dimitrov, 2002]. [Vassilev and Dimitrov, 2000]. ...
Gas hydrate deposits which are found in deep ocean sediments and in permafrost regions are supposed to be a fossil fuel reserve for the future. The Black Sea is also considered rich in terms of gas hydrates. It abundantly contains gas hydrates as methane (CH4∼80–99.9%) source. In this study, by using the literature seismic and other data of the Black Sea such as salinity, porosity of the sediments, common gas type, temperature distribution and pressure gradient, it was estimated that up to 71.8 (median) standard trillion cubic meters (tcm) of CH4 can be available in the Black Sea. Due to biogenic and thermogenic gas potential of the Black Sea, the composition of natural
gas may also include ethane(C2H6), propane(C3H8) and other impurities. This is an indication of sI and sII types of hydrate potential in the Black Sea. Moreover, according to the seismic data, single and multiple bottom-simulating reflector (BSR) lines were observed in the literature. Therefore, there is a high potential of Class 1 hydrates (stable hydrate layer and an underlying free gas zone) in the Black Sea. In this study by using HydrateResSim numerical simulator, gas production potentials from a hypothetical Class 1 hydrate reservoir in the Black Sea conditions by depressurization (at different production pressures) and depressurization combined with wellbore heating were simulated. When the depressurization (production) pressure is lower, much more gas is produced but until certain value. If the depressurization pressure is very low, there is a risk of hydrate reformation and ice formation along the wellbore and/or inside the reservoir. Moreover, it was shown that wellbore heating might be necessary in order to avoid any hydrate reformation along the wellbore during the production. The effect of intrinsic permeability on gas production was also investigated. It was observed that until 400 mD, there is no important effect of intrinsic permeability on gas production but below 400 mD, the gas production is quite low because of very low effective permeability with 65% hydrate saturation.
... Then, it is almost constant from 200 m below sea level to sea bottom [Railsback, 2010]. CH 4 seepage is extremely intense on the shelf and on the slope of the Black Sea [Sozansky, 1997;Vassilev and Dimitrov, 2003;Dimitrov, 2002;Küçük et al., 2015]. In Fig. 2, natural gas seepage along the Bulgarian Black Sea Coast [Dimitrov, 2002] is shown. ...
... CH 4 seepage is extremely intense on the shelf and on the slope of the Black Sea [Sozansky, 1997;Vassilev and Dimitrov, 2003;Dimitrov, 2002;Küçük et al., 2015]. In Fig. 2, natural gas seepage along the Bulgarian Black Sea Coast [Dimitrov, 2002] is shown. The gas composition includes 92e96.3% ...
... l/min; (c) a strong seep with more than 80 bps, 2.5e3.8 l/min [Dimitrov, 2002]. [Vassilev and Dimitrov, 2000]. ...
Gas hydrate deposits which are found in ocean and in permafrost region are supposed
to be a fossil fuel reserve for the future. The Black Sea is also considered as a unique energyrich sea, especially in terms of gas hydrates. It abundantly contains gas hydrates as methane
(CH4~80 to 99.9 %) source. In this study, by using the literature seismic data of the Black Sea,
it was calculated that the amount of methane in the gas hydrate of the Black Sea ranges from
1.726 to 297.4 (as a mean 71.8) standard trillion cubic meters. Due to biogenic and thermogenic
gas potential of the Black Sea, the composition of natural gas can consists of ethane, propane
and other impurities. This is an indication of sI and sII hydrate potential of the Black Sea.
Moreover, according to the seismic data, single and multiple bottom-simulating reflectors
(BSR) lines were observed. Therefore, there is a high potential of Class 1 hydrates (stable
hydrate layer and an underlying free gas zone). In this study by using HydrateResSim
Numerical Simulator, gas production potentials from Class 1 hydrate reservoir in the Black Sea
conditions by depressurization (at different production pressures), thermal stimulation (at
different temperatures) and their combinations were evaluated.
... Then, it is almost constant from 200 m below sea level to sea bottom [Railsback, 2010]. CH 4 seepage is extremely intense on the shelf and on the slope of the Black Sea [Sozansky, 1997;Vassilev and Dimitrov, 2003;Dimitrov, 2002;Küçük et al., 2015]. In Fig. 2, natural gas seepage along the Bulgarian Black Sea Coast [Dimitrov, 2002] is shown. ...
... CH 4 seepage is extremely intense on the shelf and on the slope of the Black Sea [Sozansky, 1997;Vassilev and Dimitrov, 2003;Dimitrov, 2002;Küçük et al., 2015]. In Fig. 2, natural gas seepage along the Bulgarian Black Sea Coast [Dimitrov, 2002] is shown. The gas composition includes 92e96.3% ...
... l/min; (c) a strong seep with more than 80 bps, 2.5e3.8 l/min [Dimitrov, 2002]. [Vassilev and Dimitrov, 2000]. ...
... Naturally occurring methane seepage from petroleum systems has been quantified using direct measurements at actively seeping locations and indirectly via gas leakage appraisals based on surface data (Dimitrov, 2002;Hornafius et al., 1999;Judd, 2004). This approach has allowed the relevance of geological sources of 0012-821X/$ -see front matter © 2013 Elsevier B.V. All rights reserved. ...
... A clear picture of the need for a focusing mechanism can be gained by comparing the continuous and uniformly distributed leakage that our model of the giant WCSB predicts, against seepage measured in several smaller basins in Europe and the United States. Even if absence of methanotrophic degradation in soils is assumed, our WCSB model predicts leakage rates (in Tg/yr) that are ten to hundreds of times smaller than seepage documented in e.g. the Bulgarian Black Sea, the Black Sea Continental Shelf, or Coal Oil Point California (Dimitrov, 2002;Hornafius et al., 1999;Judd, 2004). At these locations, the high methane fluxes have been attributed to leaking gas reservoirs (Clarke and Cleverly, 1991;Dimitrov, 2002;Hornafius et al., 1999) where faults provide not only a focused migration route, but also conduits where methane can migrate fast enough to survive microbial consumption during upward migration (Etiope and Klusman, 2002;Etiope and Martinelli, 2002;Brown, 2000). ...
... Even if absence of methanotrophic degradation in soils is assumed, our WCSB model predicts leakage rates (in Tg/yr) that are ten to hundreds of times smaller than seepage documented in e.g. the Bulgarian Black Sea, the Black Sea Continental Shelf, or Coal Oil Point California (Dimitrov, 2002;Hornafius et al., 1999;Judd, 2004). At these locations, the high methane fluxes have been attributed to leaking gas reservoirs (Clarke and Cleverly, 1991;Dimitrov, 2002;Hornafius et al., 1999) where faults provide not only a focused migration route, but also conduits where methane can migrate fast enough to survive microbial consumption during upward migration (Etiope and Klusman, 2002;Etiope and Martinelli, 2002;Brown, 2000). To illustrate the point even better, the modeled kitchen area in our model of the WCSB is more than 20 times larger than the Central Graben's. ...
The immense mass of organic carbon contained in sedimentary systems, currently estimated at 1.56×1010 Tg1.56×1010 Tg (Des Marais et al., 1992), bears the potential of affecting global climate through the release of thermally or biologically generated methane to the atmosphere. Here we investigate the potential of naturally-occurring gas leakage, controlled by petroleum generation and degradation as a forcing mechanism for climate at geologic time scales. We addressed the potential methane contributions to the atmosphere during the evolution of petroleum systems in two different, petroliferous geological settings: the Western Canada Sedimentary Basin (WCSB) and the Central Graben area of the North Sea. Besides 3D numerical simulation, different types of mass balance and theoretical approaches were applied depending on the data available and the processes taking place in each basin. In the case of the WCSB, we estimate maximum thermogenic methane leakage rates in the order of 10−2–10−3 Tg/yr10−2–10−3 Tg/yr, and maximum biogenic methane generation rates of 10−2 Tg/yr10−2 Tg/yr. In the case of the Central Graben, maximum estimates for thermogenic methane leakage are in the order in 10−3 Tg/yr10−3 Tg/yr. Extrapolation of our results to a global scale suggests that, at least as a single process, thermal gas generation in hydrocarbon kitchen areas would not be able to influence climate, although it may contribute to a positive feedback. Conversely, only the sudden release of subsurface methane accumulations, formed over geologic timescales, can possibly allow for petroleum systems to exert an effect on climate.
... Naturally occurring methane seepage from petroleum systems has been quantified using direct measurements at actively seeping locations and indirectly via gas leakage appraisals based on surface data (Dimitrov, 2002;Hornafius et al., 1999;Judd, 2004). This approach has allowed the relevance of geological sources of 0012-821X/$ -see front matter © 2013 Elsevier B.V. All rights reserved. ...
... A clear picture of the need for a focusing mechanism can be gained by comparing the continuous and uniformly distributed leakage that our model of the giant WCSB predicts, against seepage measured in several smaller basins in Europe and the United States. Even if absence of methanotrophic degradation in soils is assumed, our WCSB model predicts leakage rates (in Tg/yr) that are ten to hundreds of times smaller than seepage documented in e.g. the Bulgarian Black Sea, the Black Sea Continental Shelf, or Coal Oil Point California (Dimitrov, 2002;Hornafius et al., 1999;Judd, 2004). At these locations, the high methane fluxes have been attributed to leaking gas reservoirs (Clarke and Cleverly, 1991;Dimitrov, 2002;Hornafius et al., 1999) where faults provide not only a focused migration route, but also conduits where methane can migrate fast enough to survive microbial consumption during upward migration (Etiope and Klusman, 2002;Etiope and Martinelli, 2002;Brown, 2000). ...
... Even if absence of methanotrophic degradation in soils is assumed, our WCSB model predicts leakage rates (in Tg/yr) that are ten to hundreds of times smaller than seepage documented in e.g. the Bulgarian Black Sea, the Black Sea Continental Shelf, or Coal Oil Point California (Dimitrov, 2002;Hornafius et al., 1999;Judd, 2004). At these locations, the high methane fluxes have been attributed to leaking gas reservoirs (Clarke and Cleverly, 1991;Dimitrov, 2002;Hornafius et al., 1999) where faults provide not only a focused migration route, but also conduits where methane can migrate fast enough to survive microbial consumption during upward migration (Etiope and Klusman, 2002;Etiope and Martinelli, 2002;Brown, 2000). To illustrate the point even better, the modeled kitchen area in our model of the WCSB is more than 20 times larger than the Central Graben's. ...
Around 500 to 600 Tg (1 Tg = 1012 g) of methane enter the atmosphere
every year, mainly as product of microbial processes and combustion of
fossil fuels and burning biomass. The importance of another source, the
geologic emissions of methane, is up to now only loosely constrained. In
this study, we addressed the potential methane emissions during the
geological evolution of the Western Canada sedimentary basin (WCSB),
which holds the largest oil sand accumulations in the world, and the
Central Graben area of the North Sea. In the case of the WCSB,
thermogenic gas generation and leakage at the sediment surface were
addressed through 3D petroleum systems modeling. In this basin, the
accumulated oil experienced intense biodegradation that resulted in
large masses of biogenic methane. We quantified this latter mass though
a two-step mass balance approach. Firstly, we estimated the rate of
petroleum degradation and the magnitude of petroleum loss. After this,
we calculated the mass of biogenic methane generated using a model that
assumes hexadecane (C16H34) as representative of the saturated compounds
(Zengler et al., 1999). Our 3D model suggests that 90000-150000 Tg of
dry gas (mostly methane) could have leaked during the interval from 80
Ma to 60 Ma. Therefore, uniform leakage rates would have been in the
order of 10-3-10-2 Tg yr-1. Biogenic methane generation could have taken
place at rates of 10-4 to 10-2 Tg yr-1. However, the effective mass of
thermogenic and biogenic methane reaching the atmosphere might have been
up to 90% lower than calculated here due to methanotrophic consumption
in soils (Etiope and Klusman, 2002). We addressed the thermogenic gas
generation and leakage in the Central Graben through two different
methods. The first is based on a previous 3D petroleum system modeling
of the region (Neumann, 2006). The second consisted of calculating the
mass of generated petroleum based on source rock extension and
properties (Schmoker, 1994), and then estimating the gas mass available
for leakage based on the concept of petroleum systems and the
proportions among its constituents (Hunt, 1995). We propose that a
maximum of 10-4-10-3 Tg of thermogenic gas (mostly methane) could have
leaked annually from the sediment surface. The portion of this gas that
reached the atmosphere is unconstrained, and it would depend on the
extent of oxidation through the water column. The maximum rate of
thermogenic gas generation in the WCSB is in the order of 10-2 Tg yr-1
(10-8 Tg yr-1Km-2, when normalized to area of kitchen). In the case of
the Central Graben, the maximum would be in the order of 10-3Tg yr-1
(10-8 Tg yr-1 Km-2). These results suggest that thermal maturation alone
would not be able to promote leakage rates as high as those reported for
some single sedimentary basins at present-day, these last reaching up to
3.5 Tg yr-1 (Judd, 2004). Mechanisms promoting the release of previously
accumulated gas masses in a short time span are thus a basic requisite
for petroleum systems to exert an impact on climate.
... Still, there is uncertainty in the estimates of its natural sources and sinks and how its variations can affect the growth rate of atmospheric CH 4 (Borges et al. 2016). Although some studies estimate the CH 4 flux from coastal gas-charged sediments to the atmosphere via immediate water plumes and pockmarks (Judd et al. 1997;Dimitrov 2002;Garcia-Gil et al. 2002), there is insufficient knowledge about methane dynamics within unconsolidated sediments. ...
... The CH 4 seepage to the water column and eventually to the atmosphere is little known, as the current works are restricted to recognizing gas plumes in the water column or pockmarks (Judd et al. 1997;Dimitrov 2002;Garcia-Gil et al. 2002). Borges et al. (2016) reported high CH 4 concentrations in surface waters of the Belgian coastal zone associated with the presence of shallow gas in sediments. ...
High-resolution seismic surveys were carried out at the Paranaguá Estuarine Complex (Southern Brazil) to map the intrasedimentary shallow gas. The seismic signatures representing gas accumulation were separated according to the upper gas boundary characteristics in acoustic blanking with sharp top, acoustic blanking with diffuse top, turbidity pinnacles, and black shadows (gas accumulation at the water/sediment boundary). The main source of the gas has been recognized here as Pre-Holocene continental deposits. These deposits were capped by a seismic unit interpreted as a regressive mud deposited over the last 5000 years. This seismic unit is quite heterogeneous, the gas being trapped in its different internal layers. Each gas signature represents the efficiency of the sealing layer and has specific locations and burial depths. The results point to different phases of gas migration along with the sedimentary layers. Thus, we proposed a gas migration and accumulation model based on acoustic data and sedimentary inferences within the Paranaguá Estuarine Complex.
... Akustik yöntemler kullan larak Ye il rmak deltas civar ndaki s gaz birikimleri haritalanm r (Çifçi ve di ., 2002). Karadeniz baseni etraf nda metan ç lar oldukça s k görülmektedir (Limonov ve di er., 1997;Okyar ve Ediger, 1999;Amouroux ve di er., 2002;Dimitrov, 2002). Do u Karadeniz elf alan nda s sulardaki gaz birikimleri üzerine çal lm ve akustik veri üzerinde metan üretim zonunun taban gösterilmi tir (Okyar ve Ediger, 1999). ...
... Karadeniz baseni etraf nda ve s elf alanlar nda s gaz birikimleri oldukça yayg nd r (Limonov ve di er., 1997;Okyar ve Ediger, 1999;Amouroux ve di er., 2002;Dimitrov, 2002). Yüksek tortula ma oran na sahip elf ve yamaç bölgeleri metan kaynaklar olarak dikkate al nmakta olup (Reeburgh ve di er., 1991), derin basende de çamur volkanlar ile birlikte parlak noktalar eklinde metan birikim alanlar gözlenmektedir (Çifçi, Dondurur ve Ergün, 2003;Limonov, Woodside ve Ivanov, 1994;Limonov ve di er., 1997;Ivanov, Limonov ve vanWeering, 1996). ...
... It abundantly contains gas hydrates and H 2 S as CH 4 and hydrogen source, respectively (Demirbas, 2009;Dondurur and Cifci, 2009). CH 4 seepage is extremely intense on the shelf and on the slope of the Black Sea (Kruglyakova et al., 2004;Demirbas, 2009;Sozansky, 1997;Dimitrov, 2002;Heeschen et al., 2011;Xing, 2013;Küçük et al., 2015). ...
... (Vassilev and Dimitrov, 2003). Although it is considered that 99% of all gas hydrate reservoirs in the world includes almost 100% CH 4 (Demirbas, 2010;Kenyon et al., 2001;Kvenvolden, 2002;Johnson, 2011;Collett et al., 2015), this percentage could be lower in the Black Sea because many samples (Sozanksy, 1997;Dimitrov, 2002 2001) include thermogenic gases such as ethane, propane, nbutane, i-pentane etc. These gases as impurities in CH 4 change hydrate type of CH 4 from sI to sII hydrate. ...
Many gas seepages, temperature, pressure, salinity, anoxic environment and high source gas potential of the Black Sea indicates that the Black Sea might have huge potentials for biogenic and thermogenic gas hydrates. However, the last important parameter to consider gas hydrate as an energy source is the type of sediments. Coarse marine sands are considered as good hydrate reservoirs because of high porosity and high permeability. Only very limited data is available related to the types of lithology of the Black Sea sediments. Hence, in this study, the literature data (especially the drilling and coring data of DSDP Leg 42B program) about gas seepages, temperature gradient, pressure gradient, salinity, anoxic environment and high source gas potential, and the types of the sediments in the Black Sea were investigated and analyzed. Although gas seepages, temperature gradient, pressure gradient, salinity, anoxic environment and high source gas potential of the Black Sea are appropriate for producible gas hydrate reservoirs, the sediments of the Black Sea appear to be generally fine grained with high clay content. Sandy-silt and silty sand layers in turbidites of the Black Sea might be potential producible hydrate reservoirs but these sediments are fine. As well as turbidites, separate thin sand layers might be potential gas hydrate reservoirs as an energy source in the Black Sea.
... Methane and other products within the Black Sea, and originating from numerous cold-water seeps, mud volcanoes, and methane clathrates are not dissipated quickly, but rather remain in the water column and become heavily oxidised and fractionated . At present, methane is being added to the Black Sea water column approximately as fast as it is being oxidized, a proportion of which consists of old radiocarbon (Gulin et al., 2003;Reeburgh et al., 2006), and originates in source rocks of a variety of ages (Slack et al., 1998;Dimitrov, 2002;Kessler et al., 2006;Naudts et al., 2008;Stadnitskaia et al., 2008). 14 CeCH 4 from seeps in the Black Sea was found to have radiocarbon ages between 18.2 ka 14 C BP (Gulin et al., 2003) and 24 ka 14 C BP (Kessler et al., 2006). ...
... Molluscs that grow adjacent to seeps at a number of locations within the Black Sea basin (e.g. Mytilus galloprovincialis growing at Zelenka gas seepage, 200e250 m from Cape Kaliakra, Bulgaria, Dimitrov, 2002) incorporate a portion of old 14 C (Hovland and Risk, 2003;Reeburgh, 2007). This is likely to lead to a spatial variation in reservoir age across the Black Sea between taxa that inhabit seep communities and those that do not, and complicates attempts to estimate modern reservoir ages for the Black Sea (Siani et al., 2000;Kwiecien et al., 2008). ...
The restricted environment of the Black Sea is particularly sensitive to
climatic and oceanographic fluctuations, owing to its connection with
the Mediterranean Sea via the narrow Bosphorus Strait. The exact
mechanism and timing of the most recent connection between these water
bodies is controversial with debate on the post-glacial history of the
Black Sea being dependent on radiocarbon dating for numerical ages. Here
we present new 23 accelerator mass spectrometer (AMS) radiocarbon ages
on peat and bivalve molluscs, supported by the first amino acid
racemization (AAR) dating of bivalve molluscs (n = 66) in the
Black Sea. These data indicate infilling of the Black Sea during the
early Holocene from an initial depth 107 m below sea-level, and
72 m below that of the Bosphorus Sill. These data combined with a
review of previous radiocarbon ages has enabled a unique perspective on
the post-glacial Black Sea. A sea-level curve based on conventional and
AMS radiocarbon ages on peat and AMS-based ages on Dreissena sp. shells
indicate the water-level in the earlier lake phase continued, until the
early Holocene, to be lower than the Bosphorus Sill after the Younger
Dryas ended. However, the absence of AMS-dated mollusc ages from the
shelves of this basin older than the Younger Dryas is suggestive of
sub-aerial exposure of the shelves, and comparatively lower water-levels
when the Younger Dryas began. Thus post-glacial outflow from the Black
Sea occurred through a lowered or open Bosphorus seaway. Basin-wide
radiocarbon ages on peat indicate a prompt increase in water-level from
that of the pre-existing and unconnected palaeo-lake during the earliest
Holocene (9600-9200 cal a BP). Mass colonisation of the Black Sea
by Mediterranean taxa did not occur until salinity had risen
sufficiently, a process which took 1000 a or more from the initial
transgressive event. This gradual change in salinity contrasts with the
prompt transgression which would have taken ˜400 a to occur.
... Часть ученых объясняет эти изменения в атмосфере воздействием антропогенной деятельности человечества, другие -поступлением холодных и горячих флюидов из недр [Сывороткин, 2002]. Их выводы о геологическом воздействии под-тверждаются изотопным составом углерода метана [Dimitrov, 2002]. Поступление газов холодной дегазации сложно проследить на суше, но их можно исследовать в сипах и грязевых вулканах на дне морей и океанов. ...
Seep fluids of the underwater coastal slope of Laspi Bay are composed of CH4, N2, O2, He, C2H4, C2H6, C3H8 and are accompanied by aqueous solutions. The formation of beachfront bacterial build-ups occurred due to the processing of hydrocarbon fluids by Archaea and bacteria. Their carbonate cement has light isotopic carbon composition (-20,93; -20,83‰). The structures of beachfront bacterial build-ups have all the signs of genetic relation with carbonates of hydrocarbon seepage. The different appearance of the bacterial build-ups of Laspi Bay with other build-ups of cold seeps is explained by the physical and geographical conditions of their formation.
... Areas with methane seepage are also considered promising areas for gas mining and producing gas fuel [3]. Active seeps in the Black Sea region have been recorded along the coast of Crimea [1], Bulgaria [4] and the Caucasus [5], as well as the Sea of Azov [6]. Researchers use various methods to estimate the amount of gas released, including video recording [7] and the trap method [8], as well as active [9] and passive hydroacoustics [10,11]. ...
This work is devoted to studies of the shallow methane seepages temporal variability in the coastal area of Laspi Bay and high-frequency monitoring of the hydrological parameters daily dynamics in this area. The paper presents results obtained during the summer months of 2016, 2018-2021, and early February 2023. Using the passive acoustic method, it is shown that the intensity and periodicity of bubble gas emission by individual point sources can vary during the day and from season to season. It was obtained that a decreased content of dissolved oxygen and its saturation is observed above the active site of the seepages as compared to the distant (background) site. Moments of sharp decrease in oxygen content not accompanied by changes in temperature and other hydrological parameters were noted.
... Faults may also influence shallow gas migration, as observed in other coastal regions (Dimitrov, 2002;Garcia-Gil et al., 2002;Schroot and Schuttenhelm, 2003). Faults have been observed in the Pearl River Estuary, as well as in the southern part of the estuary near the entrance (Fig. 1c) (Li et al., 2006;Shi et al., 2013). ...
High-resolution seismic and side-scan surveys were conducted to investigate shallow gas accumulation and seepage in the Holocene sediment of the Pearl River Estuary. Extensive shallow gas accumulation and active seepage were detected in the Holocene sediments of the estuary and adjacent shelf regions. Laterally continuous and extremely shallow gas fronts were observed in areas with relatively higher sedimentation rates, whereas shallow gas was absent or discontinuous in regions with the sediment with lower sedimentation rates. Gas accumulation was present at extremely shallow depths in the West Shoal, which is the recent depocentre location. The co-occurrence of shallow gas distributions and variations in the sedimentation rate was related to the role of sediment accumulation in organic carbon burial and preservation, both of which are required for gas generation and accumulation in sediment. In addition, the consistency between the distribution of highwater column methane concentrations in the West Shoal, the super shallow gas accumulation pattern in the sediment profile, and active seepage indicate that shallow gas in the Holocene sediment is a methane source in the Pearl River Estuary. Further, considerable evidence of recent or relic seepage is present in areas that have experienced active anthropogenic disturbances, including sand mining, dredging, and bridge building. The disrupted and discontinuous gas fronts at these sites suggest that active shallow gas releases were influenced by anthropogenic activity.
... Високата геодинамична активност на региона се доказва и от известните ~50 полигона Фиг. 1. а -карта на района на нос Калиакра и местоположението на метанови извори Зеленка (Google Earth, earth.google. com/web/); подводни снимки от 10.09.2009, 36 дни след земетресение M 4.7 (фото Гериловски); б -газови извори на морското дъно; в -неактивни микрократери с подводни, предимно метанови газови извори (Dimitrov, 2002), привързани към активни разломи. Първото документирано наблюдение (Palii et al., 1951) е от Аладжа банка. ...
Aim of this work is to show evidences for pre-earthquake processes and to set up earthquake forecasting experiments, based on two hypotheses: 1) extreme change of the activity of underwater methane seeps appears days before earthquakes; 2) effects from increase of shallow water seeps emissions are visible on satellite images.
... According to Riboulot et al. (2018), gas hydrates are currently stable in water depths larger than 660 m though this depth may increase to 720 m in the future (5000 years) due to salinization of sediments. Within the Black Sea, the existence of methane in the form of free gas or as solid methane hydrate has been documented by direct observations of methane seeps, gas flares, and mud volcanism (Limonov et al., 1997;Dimitrov, 2002;Greinert et al., 2006;Naudts et al., 2006;Nikolovska et al., 2008;Egorov et al., 2011) by acoustic investigations using side-scan sonar (Klaucke et al., 2006) and by direct sampling in gravity cores and drillings (Kruglyakova et al., 2004;Heeschen et al., 2011;Riedel et al., 2020). The latter yield direct information about the deeper structure of the seafloor down to 30 m in gravity cores (e.g. ...
The Black Sea is known to have extensive direct and indirect indicators of methane hydrates. Since the resistivity of the seafloor increases significantly in the presence of gas hydrates or free gas, marine controlled-source electromagnetics (CSEM) is a suitable method for the investigation of hydrates. We have collected CSEM data in a channel-levee system of the Danube paleo-delta (Bulgarian sector) at water depths of about 1500 m. The working area is within the gas hydrates stability zone and seismics suggests the presence of gas hydrates. The CSEM data were acquired with stationary receivers and a novel mobile, dual polarization transmitter system, and interpreted in terms of rotational invariants by means of 1-D inversions at common midpoints (CMP) to generate pseudo 2D resistivity sections. The inversion results reveal two resistive layers at shallow depths of 60–120 mbsf and greater depths of 270–400 mbsf. A comparison with seismics shows a good correlation of the shallow layer with high amplitude reflections and a velocity anomaly. The deeper layer can be tied to the current bottom simulating reflector (BSR). The comparison to a second CSEM experiment, which was conducted by the BGR (Federal Institute for Geosciences and Natural Resources, Germany) shows a good agreement of the derived sections down to a depth of about 350 m. Based on salinity and porosity models derived from boreholes in the Black Sea, we apply Archie's law to estimate potential gas hydrate saturations of up to 23% for the shallow resistor and up to 7% for the deeper layer. Differences are evident at depths greater than 450 m, where we see a conductive layer not evident in the BGR section. This deeper conductor could be evidence for increasing salinities of pore fluids at greater depths, which were previously found in DSDP drilling cores.
... Considering the presence and activity of the transtensional faults below the Dikilitash Formation, we suggest that the freshwater was likely sourced from an aquifer below the Dikilitash Formation (e.g. the Beloslav Formation or deeper; Sinclair et al., 1998), which advected fluids from onshore resulting in episodic submarine groundwater discharge offshore. A similar coupled groundwater-methane discharge system is currently active along the eastern Bulgarian coastal areas documented by ongoing venting of methane (Dimitrov, 2002) and elevated Radon isotope ratios (Moore & Falkner, 1999). In order to ...
Increased greenhouse gas emissions entering the atmosphere and hydrosphere are causing changes to global climate. Geological carbon sequestration is a proven technology, used to reduce anthropogenic emissions from the atmosphere. However, there are concerns about the unintended migration of CO2 from sub-surface storage reservoirs. Fluid-escape structures, which act as conduits for pressure-driven fluids, are observed in sedimentary basins globally. These structures can extend over 500 m across and intrude vertically through kilometres of sedimentary overburden. The quantitative assessment and nature of fluid-escape conduits are currently poorly constrained. Here we sample and characterise analogous onshore field outcrop analogues in Panoche Hills, California and Varna, Bulgaria to complement the study of active structures in the Witch Ground Basin, Central North Sea. A key aim is to quantify permeability and determine the process mechanisms of fluid flow through focused fluid conduits. Here we generate an accurate, repeatable and upscalable 3D X-ray micro-computed tomography (XCT) image-based methodology workflow to calculate porosity, effective porosity, and permeability of fluid-escape conduit samples. During fieldwork and sampling, the geometry, distribution, physical interaction with host-rocks and 3D properties are determined. Given the large scale of the structures (>150 m wide), porosity and permeability transects are performed across the intrusions and their host sediments, to characterise natural variability and identify preferential fluid flow pathways. Studies of sand intrusions in the Panoche Hills reveal permeability heterogeneity is largely controlled by silica cementation processes linked to the drainage of pore waters from silica-rich host rock sediments during intrusion formation. Sub-vertically orientated intrusions have reduced permeability due to the incorporation of clay and silt host rock sediments into the matrix of intrusions, and subsequent dewatering, grain compaction and silica Opal-A to Opal-CT transformation. Further, the investigation of fluid-escape structures in Varna and Panoche Hills highlights a three orders of magnitude reduction in permeability of unconsolidated host rock sediments caused by the transformation of methane gas by anaerobic oxidation to carbonates. The mapping of carbonate pipes revealed that focused fluid flow process mechanisms are not restricted to fine-grained host rock strata, and can occur in unconsolidated sediments. High-resolution 2D and 3D seismic reflection data, integrated with sediment core data, are used to characterise the Scanner Pockmark Complex, Central North Sea. The study has revealed that gas flows vertically upwards through chimneys, directly observed as a series of interconnected fractures. Previous studies interpret seismic chimneys as vertical conduits in hydraulic connection to a single, deeper zone of overpressured fluid. Here we observe that fluid overpressure generation, leading to chimney and pockmark genesis, represents a complex system with gas-bearing zones at multiple depth intervals. When integrated into a multi-disciplinary approach, these findings can improve our understanding of focused fluid flow within the shallow overburden, for applications to shallow geohazard assessment and carbon sequestration.
... At numerous sites, oil and gas migrate 85 towards the sediment surface and escape into the water body. Seafloor seepage of light hydrocarbons and partially of oil is widely distributed on Black Sea continental shelves and slopes (e.g., Artemov et al., 2007;Bohrmann et al., 2003Bohrmann et al., , 2007Bohrmann et al., , 2011Dembicki, 2020;Dimitrov, 2002;Greinert et al., 2006;Klaucke et al., 2006;Körber et al., 2014;Kruglyakova et al., 2004;Michaelis et al., 2002;Naudts et al., 2006;Nikolovska et al., 2008;Reitz et al., 90 2011;Riboulot et al., 2018;Römer et al., 2012Römer et al., , 2020Sahling et al., 2009;Schmale et al., 2005;Zander et al., 2020, this issue). At seepage sites located within the gas hydrate stability zone (GHSZ), which at current water salinities is located in waters deeper than about 720 m below sealevel (bsl) (Naudts et al., 2006;Pape et al., 2010), concentrations of methane and other light hydrocarbons exceeding solubility lead to gas hydrate formation in the sediment 95 (Vassilev and Dimitrov, 2002). ...
Numerous hydrocarbon seep sites at the continental shelf, slope, and in the deep water basin are known to feed the Black Sea water reservoir of dissolved methane. In this study, we identified the likely sources of gas and oil that are emitted at four sites located on the continental slope offshore Georgia in the Eastern Black Sea at 830 to 1,140 m water depth – an area with gas seepage only (Batumi seep area) and three areas of joint gas and oil seepage (Iberia Mound, Colkheti Seep, and Pechori Mound). The geochemistry of bulk parameters, organic fractions and individual hydrocarbon biomarkers in near-surface sediments and of gas/oil expelled from the seafloor was analyzed and jointly interpreted to assign most likely hydrocarbon source rocks in the studied region. Presence of oleanane in shallow oil-impregnated sediments and oil slicks attests that the source rock at all sites is younger than Mid Cretaceous in age. We conclude that hydrocarbons ascending at all the four seepage areas originate from the Eocene Kuma Formation and/or the Oligocene–Lower Miocene Maikop Group, which are considered the principal hydrocarbon sources in the Eastern Black Sea region. Distributions of crude oil biomarkers in shallow sediments suggests moderate to heavy biodegradation. C1/C2+ ratios (10 to 4,163) along with stable C and H isotopic ratios (δ¹³C-CH4 ‒46.3 to ‒53.1.3‰ V-PDB; δ²H-CH4 ‒159 to ‒178‰ SMOW) indicate gas mixtures of oil-associated thermogenic and secondary microbial light hydrocarbons that are discharged from the four seep sites. Light hydrocarbons discharged at the Batumi Seep area are characterized by significant enrichments of methane, but almost similar δ¹³C-CH4 values if compared to the other study sites. Such methane enrichments likely result from a comparably higher degree of petroleum degradation and associated formation of secondary microbial methane.
... About 50 areas with underwater gas seeps are identified near the Bulgarian Black Sea coast (Dimitrov, 2002). The gas is mainly methane (Dimitrov et al., 1979). ...
We propose a new possible earthquake precursor, a coastal water jet originating from shallow water gas seeps and colored by sediments, lifted from increased gas seeps emissions, preceding some earthquakes with offshore epicenters along the Bulgarian Black Sea coast and M ≥ 3. The new possible earthquake precursor is site specific and may be observed in shallow coastal water regions where active faults are accompanied by hydrocarbon gas seeps. We point out to a region where it can be easily detected by direct observation and satellite remote sensing, Zelenka methane seeps (ZMS), near the Bulgarian Black Sea coast. The ZMS activation is related to the tectonic stress and fracturing preceding and accompanying the earthquakes in the Shabla-Kaliakra-Balchik region along the northern part of the Bulgarian Black Sea coast. We also propose an earthquake forecasting experiment, based on the following four hypotheses: 1) The change of the methane emission of shallow horizons is related to crustal strain changes preceding earthquakes in the region; 2) extreme change of the activity of underwater methane seepages appears immediately before nearby earthquakes; 3) shallow water methane seepages activity can be monitored by remote sensing; 4) satellites can register effects from an extreme increase in their emissions. The proposed earthquake forecasting experiment is based on the monitoring of methane seepages activity in the ZMS area by direct observation and remote sensing which may provide indication for preparatory earthquake activity preceding offshore earthquakes in the region.
... In the Black Sea region seeps were found along the coast of Crimea (Egorov et al. 2011), Bulgaria (Dimitrov 2002), the Caucasus (Tkeshelashvili et al. 1997) and in the Sea of Azov (Pasynkov et al. 2009). Over the past decade, many new areas of bubble seep emission have been discovered in the Crimea coastal waters (Egorov et al. 2012;Malakhova et al. 2015). ...
... Considering the presence and activity of the transtensional faults below the Dikilitash Formation, we suggest that the fresh or brine water was likely sourced from an aquifer below the Dikilitash Formation (e.g. the Beloslav Formation or deeper; Sinclair et al., 1997), which advected fluids from onshore resulting in episodic submarine groundwater discharge (SGD). A similar coupled groundwatermethane discharge system is currently active along the eastern Bulgarian coastal areas documented by ongoing venting of methane (Dimitrov, 2002) and elevated Radon isotope ratios (Moore & Falkner, 1999). In order to create topography-driven groundwater flow far out into the shelf (Hughes et al., 2009;Morrissey et al., 2010;, tectonic compression in the onshore realm during the Paleogene (Sinclair et al., 1997) likely provided the necessary hydraulic head (Figure 6.7). ...
Fluids are an important agent in nearly all geologic processes that shape the planet Earth. Fluid abundance and composition are governed by flow along permeable beds or natural and anthropogenic structures in the subsurface including faults, wells, and chimneys/pipes. Spatial and temporal variations in fluid flow activity modify total fluxes between geosphere, cryosphere, hydrosphere, and atmosphere. These fluxes have broad implications for geological processes including the formation of natural resources or the occurrence of geohazards including landslides, earthquakes and blowouts. They further play a crucial role for the global carbon cycles and the climate system. A qualitative and quantitative understanding of fluid flow in the subsurface is therefore important to assess the role of fluids in the Earth system and to quantify fluxes from the geosphere into the hydro- and atmosphere. In this Ph.D. thesis I use an integrated, interdisciplinary approach to study natural and anthropogenic fluid migration pathways in marine sediments in the North Sea, the convergent Hikurangi margin, and a section of the ancient Tethys margin which is now exposed near Varna, Bulgaria. The applied methods include conventional 3D seismic, high-resolution 3D seismic, and 2D seismic data as well as hydroacoustic, sedimentological, unmanned aerial vehicle-based photogrammetric and geochemical data. In each of the studied systems, natural and/or anthropogenic fluid migration pathways allow the transport of significant amounts of fluids through marine sediments towards the seafloor. Often the co-existence of multiple pathways enables the fluids to bypass permeability barriers within the Earth’s crust resulting in the formation of structurally complex flow systems. Focused fluid flow along normal faults in the Hikurangi margin likely plays an active role in the subduction drainage system, influences the slope stability and the morphotectonic evolution of the margin. Results from the Eocene Tethys margin show that focused fluid flow in marine sediments is possible in unconsolidated sands if seepage is focused at the top of faulted units and the flux rate is high enough. This stands in contrast to the general assumption that focused fluid flow in marine sediments is limited to low-permeable sediments. In the marine environment the term fluid flow is often used to exclusively refer to the flow of hydrocarbons. However, geochemical data from the North Sea and the Tethys margin indicate that the involved fluids are of different origin including compaction-related dehydration and submarine groundwater discharge. In each of the investigated cases, the temporal and spatial evolution of fluid flow is not fully addressed yet, especially with regard to vertical fluid conduits or the safety of subsurface drilling and storage operations. The results of my thesis highlight that the investigation of fluid migration pathways requires an interdisciplinary approach which may indicate the origin of the fluids, help understand the fluxes of fluids from the geosphere into the hydrosphere and atmosphere of the past, present and future and reveal the resulting consequences for the global carbon cycles and the climate system.
... In the Black Sea region seeps were found along the coast of Crimea (Egorov et al. 2011), Bulgaria (Dimitrov 2002), the Caucasus (Tkeshelashvili et al. 1997) and in the Sea of Azov (Pasynkov et al. 2009). Over the past decade, many new areas of bubble seep emission have been discovered in the Crimea coastal waters (Egorov et al. 2012;Malakhova et al. 2015). ...
This article proposes a method for extracting hot water from a well drilled until the point where temperature exceeds the boiling point of water at a given depth. Core of this method lies on the physical mechanism of the eruption of a natural geyser. It bases on the cold water erupting from the well after entering inside from the surface of the earth due to the gas liquid imbalance (hereafter GLI). This column of water rises up due to the mutual movement between contacting liquid and gas by the pressure of the gas applied on it. The paper researches the physical conditions under which spontaneous outflow of hot water from the well to the surface begins. Described method of extraction of underground heat allows to eliminate the cost of energy required for the operation of circulating pumps of traditional geothermal power plants, since the heated water will be thrown out to the outside, obeying the GLI mechanism. Thus, this method can be considered as an environmentally friendly and significantly reduces the cost of extracting heated water from the well.
... In the last two decades, several studies have been performed in the Black Sea indicating shallow gas accumulations, oil and gas seeps, and existence of mud volcanoes Limonov et al., 1997;Okyar and Ediger, 1999;Amouroux et al., 2002;Dimitrov, 2002;Ergün et al., 2002;Bohrmann et al., 2003;Lüdmann et al., 2004;Klaucke et al., 2006;Sahling et al., 2009;Naudts et al., 2009;Pape et al., 2011;Römer et al., 2012;Dondurur et al., 2013;Körber et al., 2014). Menlikli et al. (2009) showed the potential hydrocarbon resources of the Turkish Black Sea. ...
Multichannel seismic, 3.5-kHz Chirp subbottom profiler and multibeam bathymetric data were collected along the western Black Sea margin, offshore Sakarya River, to investigate the bottom-simulating reflections (BSRs), free gas accumulations, and mud volcanoes. Geometries from the seismic data indicate widespread BSRs along the continental rise between 750 and 1950 m water depths, 70 to 350 ms below the seafloor. Seismic attribute analyses have been applied to the seismic data to reveal the acoustic properties of the gas hydrates. According to the results from such analyses, we conclude that there are acoustically transparent zones beneath most of the BSRs in the area, which are interpreted as free gas accumulations, and the gas hydrate-bearing sediments are acting as seals for the free gas in the underlying sediments. Stability analysis of the gas hydrates from different BSR zones in the area suggests that the gas composition in the gas hydrates may change locally. As we do not have ground truth data from BSR zones, the exact composition of the gas forming the gas hydrates is unknown. However, hydrocarbon productivity of the area, chromatography results of the shallow sediment samples nearby, and stability analysis of the gas hydrates indicate the possible existence of a thermogenic gas component in the gas hydrate composition, resulting in a mixture of gas hydrates.
... Areas with methane seepage are also considered promising areas for gas mining and producing gas fuel [3]. Active seeps in the Black Sea region have been recorded along the coast of Crimea [1], Bulgaria [4] and the Caucasus [5], as well as the Sea of Azov [6]. Researchers use various methods to estimate the amount of gas released, including video recording [7] and the trap method [8], as well as active [9] and passive hydroacoustics [10,11]. ...
A passive acoustic method for estimating the flow of underwater bubble gas emissions for shallow water areas has been proposed and tested. The method is based on the connection between the frequency of the acoustic signal produced by a gas bubble when it separates from the underwater outlet channel and the size of the bubble. In the conducted laboratory experiments acoustic signals with frequencies in the range from 2.7 to 0.4 kHz were recorded during generation of bubbles with a size from 2 to 15 mm within the fluid. The analysis of acoustic recordings made near existing shallow-water seeps in Laspi Bay showed a series of short audio signals produced by the released methane bubbles of 0.5-2 s duration, grouped into packages containing approximately ten pulses. For the two investigated seeps, the frequency peaked at 1 and 1.4 kHz. According to a theoretical estimate, the bubbles generating such a signal are 7 and 5 mm in diameter, respectively. Taking the intensity of the bubble discharge into account, the calculated gas flows were 40 and 6 liters per day, respectively.
... Количество таких выходов на суше и в океанах не поддаётся подсчету. Только в прибрежной части Болгарии обнаружено 19735 выходов сипов, которые выделяют от 45.1 до 210.6 млн м 3 метана в год [47]. Выбросы этих газов влияют на состав атмосферы и гидросферы, а также на динамические процессы в них [43]. ...
Background. In this paper, we provide new data on the morphology and geochemistry of heraclites, which are represented by specific products formed by degradation of the carbonate paleostructures of prokaryotes around the degassing zones of the Miocene age. The heraclites exhibiting siltstone and sandstone structures are referred to as micrometanolites. An analysis of literature data showed that heraclites had formed in the upper sedimentary strata under the conditions of increased methane levels in the affinity of the stream exits.Methods. The geochemical examination of heraclites of a different colour and morphology was carried out using ICP-MS at the facilities of the Geoanalytik centre, UB RAS Institute of Geology and Geochemistry, Ekaterinburg.Results. According to lithological characteristics and morphology, the following heraclite types were distinguished: slag-like, banded, angular and сementation slabs. Morphologically, the heraclites under study were characterized by high concentrations of Sr, Ba, Co, Ni, Bi, Ce, Nd and Yb, which significantly exceed those in carbonate rocks. The concentrations of Rb, Cs, Nb, Th, Cu, Tl, As, Sc, Sb, Eu, Gd, Tb, Dy and Er in some morphological differences were above the Clarke values, while others demonstrated their deficit. The samples under study showed low concentrations of some heavy REE. A high content of lithophilic, chalcophile, siderophilic and REE elements indicate paleo-fluids of a deep mantle-derived character. The data on the oxide environment during construction material formation was obtained, which is confirmed by low concentrations of U (0.579 to 2.096 ppm) and Bi (0.014-0.084 ppm), as well as the ratios of U/Th (0.4-2.1), V/Cr (0.3-0.9) and Mo/Mn (less than 0.0014). Such conditions existed near or within the cellular organism of prokaryotes. Low concentrations of U, Ti, Mn and Zr confirm a significant growth rate of heraclite carbonate substances. Minor differences in the concentrations of chemical elements in heraclites of a different morphology are believed to have resulted from the physic-geographical and chemical conditions of their formation across the areas of regional faults with an active tectonic regime.Conclusions. Our results have confirmed the assumption that the carbonate material of heraclites formed by processing of deep fluids. In the South-Western Crimea, heraclites constitute a geological phenomenon of the Miocene hydrocarbon paleo-degassing, which was associated with the neotectonic stage of the uplift of the Crimean Mountains. The presence of methane, ethane, propane and petroleum products in the composition of heraclites allows an assumption to be made about the presence of oil and gas deposits in the Sevastopol region.
... In regions of intense benthic methanogenesis, gas bubbles supersaturated with CH 4 episodically rise through the water column to the surface (e.g. Dimitrov, 2002;Kitidis et al., 2007). This process of ebullition will result in CH 4 emissions that are not quantified using air-sea flux calculations based on seawater CH 4 concentration (see below). ...
Air–water CH4 and CO2 fluxes were directly measured using the eddy covariance technique at the Penlee Point Atmospheric Observatory on the southwest coast of the United Kingdom from September 2015 to August 2016. The high-frequency, year-long measurements provide unprecedented detail on the variability of these greenhouse gas fluxes from seasonal to diurnal and to semi-diurnal (tidal) timescales. Depending on the wind sector, fluxes measured at this site are indicative of air–water exchange in coastal seas as well as in an outer estuary. For the open-water sector when winds were off the Atlantic Ocean, CH4 flux was almost always positive (annual mean of ∼0.05 mmol m⁻² d⁻¹) except in December and January, when CH4 flux was near zero. At times of high rainfall and river flow rate, CH4 emission from the estuarine-influenced Plymouth Sound sector was several times higher than emission from the open-water sector. The implied CH4 saturation (derived from the measured fluxes and a wind-speed-dependent gas transfer velocity parameterization) of over 1000 % in the Plymouth Sound is within range of in situ dissolved CH4 measurements near the mouth of the river Tamar. CO2 flux from the open-water sector was generally from sea to air in autumn and winter and from air to sea in late spring and summer, with an annual mean flux of near zero. A diurnal signal in CO2 flux and implied partial pressure of CO2 in water (pCO2) are clearly observed for the Plymouth Sound sector and also evident for the open-water sector during biologically productive periods. These observations suggest that coastal CO2 efflux may be underestimated if sampling strategies are limited to daytime only. Combining the flux data with seawater pCO2 measurements made in situ within the flux footprint allows us to estimate the CO2 transfer velocity. The gas transfer velocity and wind speed relationship at this coastal location agrees reasonably well with previous open-water parameterizations in the mean but demonstrates considerable variability. We discuss the influences of biological productivity, bottom-driven turbulence and rainfall on coastal air–water gas exchange.
... Nevertheless, a considerable portion of the methane formed in shallow marine bottom sediments escapes into the atmosphere as gas seepages (Hovland et al., 1993;Judd, 2004). Such methane gas jets (predominantly of biogenic origin) along the Black Sea northern and western coasts (Crimean, Caucasian, and Bulgarian) are widespread and exert a noticeable influence on the biogeochemical processes in the Black Sea region (Tkeshelashvili et al., 1997;Amouroux et al., 2002;Dimitrov et al., 2002;Мichaelis et al., 2002;Egorov et al., 2012). However, discharge of thermogenic methane can not be ruled out in shallow seepages near the Southern Coast of Crimea (Lysenko and Shik, 2013). ...
The Black Sea is the largest meromictic basin, in the bottom sediments of which a powerful biogenic process of sulfide production occurs. The goal of the present work was to obtain data on phylogenetic diversity of the sulfur cycle microorganisms (sulfate-reducing and sulfur-oxidizing bacteria) in the Black Sea coastal gas-saturated bottom sediments. The samples were collected in the Chersonesus (Blue) Bay near Sevastopol
from whitish bacterial mats of sulfurettes, and from the upper layer of the nearby seabed. Using DNA isolated from the native samples and obtained enrichment cultures, PCR analysis was performed with oligonucleotide primers specific to the fragments of the 16S rRNA genes of the main subgroups of sulfatereducing bacteria (SRB) and to the fragments of the dsrB gene (both reductive and oxidative types), encoding
the β-subunit of dissimilatory (bi)sulfite reductase, the key enzyme in the sulfur cycle, inherent in both sulfate-reducing and sulfur-oxidizing microorganisms. The presence of 16S rRNA gene fragments specific to the genera Desulfobacterium, Desulfobacter, Desulfococcus–Desulfonema–Desulfosarcina, and Desulfovibrio–Desulfomicrobium was detected in the DNA samples isolated from coastal bottom bacterial mats. Usage of denaturing gradient gel electrophoresis (DGGE) with subsequent sequencing of reamplified dsrB gene fragments revealed that according to deduced amino acid sequences encoded by the dsrB gene (reductive type), SRB from the coastal gas-saturated bottom sediments of the Black Sea had the highest homology (92-99%)
with the dsrB gene of cultured SRB belonging to the genera Desulfovibrio, Desulfatitalea, Desulfobacter, and Desulfobacterium, as well as with uncultured SRB strains from various marine habitats, such as bottom sediments of the Northern and Japanese Seas. Deduced amino acid sequences encoded by the oxidative dsrB gene had the highest homology (90-99%) with the relevant sequences of the genera Thiocapsa, Thiobaca, Thioflavicoccus, and Thiorhodococcus.
... The Black Sea is considered as the world's most isolated sea, the largest anoxic water body on the planet and a unique energy-rich sea (Overmann and Manske, 2006). CH4 seepage is extremely intense on the shelf and on the slope of the Black Sea (Dimitrov, 2002;Mert Küçük et al., 2015;Vassilev and Dimitrov, 2003), especially along the Ukrainian Naudts et al., 2006;Naudts et al., 2008;Naudts et al., 2009) and Romanian margins (Popescu et al., 2007). Black Sea sediment abundantly contains GHs and H2S as CH4 and hydrogen source, respectively (Demirbas, 2009). ...
The Romanian sector of the Black Sea deserves attention because the Danube deep-sea fan is one of the largest sediment depositional systems worldwide and is considered the world's most isolated sea, the largest anoxic water body on the planet and a unique energy-rich sea. Due to the high sediment accumulation rate, presence of organic matter and anoxic conditions, the Black sea sediments offshore the Danube delta is rich in gas and thus show Bottom Simulating Reflectors (BSR). The cartography of the BSR over the last 20 years, exhibits its widespread occurrence, indicative of extensive development of hydrate accumulations and a huge gas hydrate potential. By combining old and new datasets acquired in 2015 during the GHASS expedition, we performed a geomorphological analysis of the continental slope north-east of the Danube canyon compared with the spatial distribution of gas seeps in the water column and the predicted extent of the gas hydrate stability zone. This analysis provides new evidence of the role of geomorphological setting and gas hydrate extent in controlling the location of the observed gas expulsions and gas flares in the water column. Gas flares are today considered an important source of the carbon budget of the oceans and, potentially, of the atmosphere.
... 2. The anoxic environment of the Black Sea preserves organic materials during sedimentation and ease the formation of hydrocarbons and also TOC of the Black Sea sediments are quite high (more than 0.5% generally) (Küçük, 2016;Merey and Sinayuc, 2016a). 3. CH 4 and gas mixtures (including CH 4 mostly but other impurities such as C 2 H 6 , C 3 H 8 , i-C 4 H 10 , CO 2 and H 2 S) seepages are extremely intense on the shelf and on the slope of the Black Sea so there are both sI and sII gas hydrate potential in the Black Sea (Sozansky, 1997;Vassilev and Dimitrov, 2003;Dimitrov, 2002;Hester and Brewer, 2009;Küçük et al., 2015;Merey and Sinayuc, 2016a). 4. In the Black Sea, many mud volcanoes, and pockmarks were detected, and also many gas hydrate samples from sea floor were collected (Ergun and Cifci, 1999;Küçük, 2016). ...
Gas hydrates are considered as a promising energy source and the Black Sea has a high potential of gas hydrates. The Danube Delta of the Black Sea is the most well-known prospect in the Black Sea after many geological and geophysical studies such as bottom-simulation reflectors (BSR) and electromagnetic surveys. In this study, gas production simulations from two gas hydrate layers (6 m thick hydrate layer at 60 mbsf and 30 m-thick hydrate layer at 140 mbsf above BSR at 350 mbsf) at the same locations with approximately 50% hydrate saturation in the Danube Fan of the Black Sea were run with depressurization method separately at 2 MPa, 3 MPa, 4 MPa, 5 MPa, and 6 MPa by using HydrateResSim numerical simulators. Moreover, different production tests strategies were suggested in this region.
... Recent hydroacoustic investigations have shown that active seep sites releasing gas bubbles (consisting mainly of methane) into the water column are widely distributed along the coast, the shelf, shelf edge, and upper slope of the Black Sea ( Fig. 1; Dimitrov, 2002;Naudts et al., 2006;Nikolovska et al., 2008). Methane is also emitted from submarine mud volcanoes (MVs; Fig. 1). ...
A steady state box model was developed to estimate the methane input into the Black Sea water column at various water depths. Our model results reveal a total input of methane of 4.7 Tg yr<sup>−1</sup>. The model predicts that the input of methane is largest at water depths between 600 and 700 m (7% of the total input), suggesting that the dissociation of methane gas hydrates at water depths equivalent to their upper stability limit may represent an important source of methane into the water column. In addition we discuss the effects of massive short-term methane inputs (e.g. through eruptions of deep-water mud volcanoes or submarine landslides at intermediate water depths) on the water column methane distribution and the resulting methane emission to the atmosphere. Our non-steady state simulations predict that these inputs will be effectively buffered by intense microbial methane consumption and that the upward flux of methane is strongly hampered by the pronounced density stratification of the Black Sea water column. For instance, an assumed input of methane of 179 Tg CH<sub>4</sub> d<sup>−1</sup> (equivalent to the amount of methane released by 1000 mud volcano eruptions) at a water depth of 700 m will only marginally influence the sea/air methane flux increasing it by only 3%.
... Global CH 4 emissions from coastal regions are poorly quantified and may be influenced by processes such as riverine outflow and tidal circulations. In shallow waters, ebullition (bubbles rising from the sediment) represents an additional pathway for CH 4 transfer (Dimitrov, 2002;Kitidis et al., 2007). Some bubbles are not fully dissolved in seawater before surfacing and this transfer to the atmosphere is not accounted for in bulk flux calculations based on aqueous CH 4 concentrations. ...
We present air–sea fluxes of carbon dioxide (CO2), methane (CH4), momentum, and sensible heat measured by the eddy covariance method from the recently established Penlee Point Atmospheric Observatory (PPAO) on the south-west coast of the United Kingdom. Measurements from the south-westerly direction (open water sector) were made at three different sampling heights (approximately 15, 18, and 27 m above mean sea level, a.m.s.l.), each from a different period during 2014–2015. At sampling heights ≥ 18 m a.m.s.l., measured fluxes of momentum and sensible heat demonstrate reasonable ( ≤ ±20 % in the mean) agreement with transfer rates over the open ocean. This confirms the suitability of PPAO for air–sea exchange measurements in shelf regions. Covariance air–sea CO2 fluxes demonstrate high temporal variability. Air-to-sea transport of CO2 declined from spring to summer in both years, coinciding with the breakdown of the spring phytoplankton bloom. We report, to the best of our knowledge, the first successful eddy covariance measurements of CH4 emissions from a marine environment. Higher sea-to-air CH4 fluxes were observed during rising tides (20 ± 3; 38 ± 3; 29 ± 6 µmole m−2 d−1 at 15, 18, 27 m a.m.s.l.) than during falling tides (14 ± 2; 22 ± 2; 21 ± 5 µmole m−2 d−1), consistent with an elevated CH4 source from an estuarine outflow driven by local tidal circulation. These fluxes are a few times higher than the predicted CH4 emissions over the open ocean and are significantly lower than estimates from other aquatic CH4 hotspots (e.g. polar regions, freshwater). Finally, we found the detection limit of the air–sea CH4 flux by eddy covariance to be 20 µmole m−2 d−1 over hourly timescales (4 µmole m−2 d−1 over 24 h).
... The best investigated regions in this respect are the Santa Barbara Basin, where at Coal Oil Point an emission of 28 g CH 4 m À2 year À1 was estimated (Hornafius et al., 1999), and the Black Sea, where more than 2,000 seeps were detected in the area west of the Crimean Peninsula alone. The yearly atmospheric methane contribution from seeps in the Black Sea is estimated to range between 0.03 and 0.15 Tg (Dimitrov, 2002). ...
... Global CH 4 emissions from coastal regions are poorly quantified and may be influenced by processes such as riverine outflow and tidal circulations. In shallow waters, ebullition 20 (bubbles rising from the sediment) represents an additional and a potentially significant source of CH 4 (Dimitrov 2002;Kitidis et al. 2007). Some bubbles are not fully dissolved in seawater before surfacing and this transfer to the atmosphere is not accounted for in bulk flux calculations that use aqueous CH 4 concentrations. ...
We present air-sea fluxes of carbon dioxide (CO2), methane (CH4), momentum, and sensible heat measured by the eddy covariance method from the recently established Penlee Point Atmospheric Observatory (PPAO) on the South West coast of the United Kingdom. Measurements from the southwest direction (background marine air) at three different sampling heights (approximately 15, 18, 27 m above mean sea level, AMSL) in three different periods during 2014–2015 are shown. At sampling heights ≥ 18 m AMSL, measured fluxes of momentum and sensible heat demonstrate reasonable agreement with their expected transfer rates over the open ocean. This confirms the suitability of PPAO for air-sea exchange measurements. We observed reductions in the air-to-sea fluxes of CO2 from spring to summer in both years, which coincided with the breakdown of the spring phytoplankton bloom. At all sampling heights, mean CH4 fluxes were positive, suggesting marine emissions. Higher CH4 fluxes were observed during rising tides (20±3; 29±6; 38±3 μmole m−2 d−1 at 15, 27, 18 m AMSL) than during falling tides (14±2; 21±5; 22±2 μmole m−2 d−1, respectively), consistent with an elevated CH4 source from an estuarine outflow driven by local tidal circulation. Based on observations at PPAO, we also estimate the detection limit of the eddy covariance CH4 flux measurement to be ~20 μmole m−2 d−1 over hourly timescales (~4 μmole m−2 d−1 over 24 hours).
... Black Sea shallow water jets were originally found on the Bulgarian coast (Dimitrov et al., 1979). They have been since observed throughout the Azov-Black Sea coast: on Bulgarian (Dimitrov, 2002), Caucasian (Tkeshelashvili et al., 1997;Kruglyakova et al., 2009), and Crimean coasts (Egorov et al., 2011), as well as in the Sea of Azov (Pasynkov et al., 2009). Isotopic anal ysis revealed that methane of these jet emissions was often of biogenic origin (Egorov et al., 2012. ...
Hydroacoustic techniques were used for detection and mapping of gas jet areas in the coastal regions of the Crimean Peninsula. Gas jet areas in the bays Laspi, Khersones, and Kazach’ya were chosen for detailed microbiological investigation. The first type of gas jets, observed in the Laspi Bay, was probably associated with discharge of deep thermogenic methane along the faults. Methane isotopic composition was characterized by δ13C of–35.3‰. While elevated rates of aerobic methane oxidation were revealed in the sandy sediments adjacent to the methane release site, no evidence of bacterial mats was found. The second type of gas emission, observed in the Khersones Bay, was accompanied by formation of bacterial biofilms of the “Thiodendron” microbial community type, predominated by filamentous, spirochete-like organisms, in the areas of gas seepage. The isotopic composition of methane was considerably lower there (–60.4‰), indicating a considerable contribution of modern microbial methane to the gas bubbles discharged in this bay. Activity of the third type of gas emission, the jets of the Kazach’ya Bay, probably depended directly on modern microbial processes of organic matter (OM) degradation in the upper sediment layers. The rates of sulfate reduction and methanogenesis were 260 and 34 μmol dm–3 day–1, respectively. Our results indicate different mechanisms responsible for formation of methane jets in the Laspi Bay and in the coastal areas of the Heracles Peninsula, where the bays Kazach’ya and Khersones are located.
... It is believed that the ecological role of methane seeps could be understood better at the cost of increasing the factual data pool representing the local and regional features of seepage manifestations, such as: number, spatial distribution and dimensions of gas outlets, the relation between the sea-floor morphology and seep occurrences, intensity of ebullition from the sea floor, physicalchemical characteristics of bubbles in gas plumes and others. Earlier the rate of atmospheric methane emission from marine seeps has been documented for very limited regions, mainly shallower than 50 -70 m: in southern California on shelf of northern Santa Barbara Channel [18,31,64], on the UK continental shelf [38] and in the offshore area of Bulgarian Black Sea Zone [25]. ...
... Methane enters the oceans via a number of different sources including coastal runoff and rivers [17,79], diffusion from organic-rich anoxic sediments, seeps, vents, and mud volcanoes [71]. These seeps supply methane to a number of regions including the Black Sea, which is the world's largest surface water reservoir of dissolved methane [78] contributing between 0.03 and 0.15 Tg of methane per year [20]. The oceans are also a large reservoir of methane, most of which is contained within methane clathrate hydrates. ...
Given the recent increases in natural gas reserves and associated drawbacks of current gas-to-liquids technologies, the development of a bioconversion process to directly convert methane to liquid fuels would generate considerable industrial interest. Several clades of anaerobic methanotrophic archaea (ANME) are capable of performing anaerobic oxidation of methane (AOM). AOM carried out by ANME offers carbon efficiency advantages over aerobic oxidation by conserving the entire carbon flux without losing one out of three carbon atoms to carbon dioxide. This review highlights the recent advances in understanding the key enzymes involved in AOM (i.e., methyl-coenzyme M reductase), the ecological niches of a number of ANME, the putative metabolic pathways for AOM, and the syntrophic consortia that they typically form.
... The seepage of natural gas is known to be widespread in both land and marine environments (Hovland & Judd, 1988). Gas seeps are known to be associated with leakage from gas reservoirs and shallow gas accumulations, and from gas hydrates; consequently, they occur in all the oceanic environments: coastal environments of deposition (bays, estuaries etc.); major deltas; hydrocarbon-bearing sedimentary basins on the continental shelf and slope etc. 50 long existing gas seepage areas are described in Bulgarian coastal waters with more than 6,000 individual seeps and 482 water column targets representing gas seeps are identified offshore Bulgaria (Dimitrov, 2002a). Some ten thousand of seepage are reported to exist within the Georgian shelf (Tkelashvili et al., 1998). ...
Data on shallow gas, mud volcanoes, gas seepage and related seabed
features suggest that the Black Sea may be is one of the most prolific
areas in the World: 50 long existing gas seepage areas are described in
Bulgarian coastal waters with more than 6,000 individual seeps; Some
ten thousand of seepage venting between 0.5 and 15 (?!) billion cubic
meters of methane per year are reported to exist within the Georgian
shelf; Several areas of active gas venting are also known in Romanian
(Danube delta preferably), Ukrainian and Turkish waters as well as oil
seeps; 482 water column targets representing gas seeps are identified
offshore Bulgaria; More than 200 gas plumes are documented along the
shelf break of the North-Western part of the Black Sea; Two large
pockmark areas are situated along the Bulgarian shelf break and another
one on the uppermost eastern Turkish continental slope; Some areas with
number of methane derived carbonate buildups are discovered on the upper
parts of north and north-western continental slopes; There are 46 mud
volcanoes on the easternmost part of Kerch peninsula, 42 on the Taman
peninsula and more than 50 in adjacent shallow waters of Azov and Black
Seas; Nine large mud volcanoes are documented in the central abyssal
plain and eight in the Sorokin Trough, south-east of Kerch Peninsula;
Several mud volcanoes have been found on the continental slope in the
north-west Caucasian margin, southern Bulgaria part and in eastern
Turkish continental slope, … etc.. Most of these examples will be
presented and the contribution to atmospheric methane of the Black Sea
gas seepage and venting structures will be discussed.
... Dickens ve diğerleri, (1995) Paleosen sonundaki hızlı küresel sıcaklık artışlarından; Nisbet (1990), daha yakın dönemde Kuvaterner'deki son buzul çağının sona ermesinden gaz hidratlarda oluşan bu çözülmeyi sorumlu tutmaktadırlar. Dimitrov (2002), Schmale ve diğerleri (2005), gaz hidratların katastrofik bir şekilde çözülmesine bağlı olarak açığa çıkan metan gazının, sığ ve oksijenli sularda, bu suların bariyer etkisine bağlı olarak yüzeye ve oradan da atmosfere kadar yükselemeyeceğini vurgulayarak, gaz hidratların iklimsel süreçlerdeki olumsuz etkilerine karşı görüşlerini belirtmişlerdir. ...
... It has been shown that bubbles start to form at CH 4 concentrations well below its solubility (~1 mM) and that these may contaiñ 40-100 % CH 4 (Chanton et al. 1989). This results in a rapid, episodic release of CH 4 enriched bubbles to the water column (ebullition) and potentially directly to the troposphere with minimal oxidation (Dimitrov 2002). CH 4 ebullition may typically exceed the diffusional sediment CH 4 flux by more than an order of magnitude (Ostrovsky 2003;Barnes et al. 2006;Nirmal Rajkumar et al. 2008). ...
Understanding and quantifying ocean–atmosphere exchanges of the long-lived
greenhouse gases carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4)
are important for understanding the global biogeochemical cycles of carbon and
nitrogen in the context of ongoing global climate change. In this chapter we
summarise our current state of knowledge regarding the oceanic distributions,
formation and consumption pathways, and oceanic uptake and emissions of CO2,
N2O and CH4, with a particular emphasis on the upper ocean. We specifically
consider the role of the ocean in regulating the tropospheric content of these
important radiative gases in a world in which their tropospheric content is rapidly
increasing and estimate the impact of global change on their present and future
oceanic uptake and/or emission. Finally, we evaluate the various uncertainties
associated with the most commonly used methods for estimating uptake and
emission and identify future research needs.
... One type is found in shallow waters on the shelf and upper continental slope. Thousands of seeps discharging predominantly biogenic methane have been discovered at the Dnepr and Danube paleodeltas offshore Ukraine and Romania (Michaelis et al., 2002;Naudts et al., 2006;Schmale et al., 2010) as well as at the shelf and upper slope offshore Bulgaria (Dimitrov, 2002), Turkey (Çifçi et al., 2003;Ergün et al., 2002), and Georgia (Egorov et al., 2003). A second type is located within the gas hydrate stability zone (GHSZ) below 725 m below sea level (bsl) (Naudts et al., 2006;Pape et al., 2010). ...
Analysis of Advanced Synthetic Aperture Radar satellite images in combination with water column and seafloor investigations documented natural oil seepage from Pechori Mound and Colkheti Seep in 1,000 – 1,200 m water depth in the eastern Black Sea offshore Georgia. Hydroacoustic imaging of the water column using multibeam echosounder evidenced numerous gas emissions from both structures. Gas bubbles rose as high as 45 m below sea surface. It is proposed that oil coatings around gas bubbles hamper their dissolution allowing them to reach the sea surface where widespread oil slicks are formed. Slow rise velocities (14 ± 1 cm s-1) of bubbles with radii of 2.6 ± 0.3 mm (n = 101) derived from video data obtained with the remotely operated vehicle MARUM QUEST4000, corroborate the assumption that bubbles are oil-coated. High resolution seafloor mapping by autonomous underwater vehicle MARUM SEAL5000 at Colkheti Seep revealed a crater-strewn morphology whose formation is explained by frequent rafting of shallow gas hydrate deposits. Satellite imaging of oil slicks on the sea surface above both sites indicates that oil seepage is rather persistent since 2003. An order-of-magnitude estimation of minimum oil seepage rates suggests discharge rates of ∼40 liters per hour from both sites. The data presented are the first comprehensive description of oil seepage in the Black Sea.
During the period from 2019 to 2021, complex studies of new shallow-water methane bubble gas emission sites were carried out in the coastal zone near Cape Fiolent (Southwest Coast of Crimea). The studies included determining the hydrocarbon and isotopic composition of bubble gas, measuring the concentration of methane and nutrients in the water in the areas of gas emissions, estimating the value of bubble flows, and measuring hydrophysical parameters over the sip sites compared to background areas. The seasonal type of Cape Fiolent methane seeps was noted, its active phases of gas emissions differed in duration in different years. The increased pore water silica concentration at the seep sites and their localization in the vicinity of freshwater slope springs may indicate its association with submarine freshwater discharge in the area. However, no significant desalination of both pore water and the bottom water layer above the siphons was recorded. Dissolved methane concentrations in pore water at seep sites were two orders of magnitude higher compared to background areas and reached 448 μmol/L. Also high values were obtained for surface water directly above the bubble gas emission points (maximum 353 nmol/L). Multi-hour monitoring of hydrophysical parameters above the active seeps showed a dissolved oxygen decrease compared to the background sites. The maximum difference in O2 concentrations was 3 mg/l. The carbon isotopic composition of bubble gas methane δ13C-CH4 (–62.84…38.27‰) and сarbon dioxide δ13C-CO2 (–16.83…–10.17‰) was corresponded to a mixture of isotopically heavy gas and near-surface isotopically light gas of microbial origin. The question remains open: what are the reasons for the change in the summer active and the cold season passive gas emission phases?
Marine sediments are the world's largest known reservoir of methane. In many coastal regions, methane is trapped in sediments buried at depths ranging from centimeters to hundreds of meters below the seafloor, in the forms of gas pockets, dispersed gas bubbles and dissolved gas, also known as shallow gas (methane-dominated gas mixture). The existence of shallow gas affects the engineering geological environment and threatens the safety of artificial facilities. The escape of shallow gas from sediments into the atmosphere can even threaten ecosystem security and affect global climate change. However, until now, shallow gas has remained a mystery to the scientific community. For example, how it is generated, how it distributes and migrates in sediments, and what are the factors that influence these processes that are still unclear. In the context of increasingly intense offshore development and global warming, there is a huge gap between existing scientific understanding of shallow gas and the need to develop scientific solutions for related problems. Based on this, this paper systematically collects the information on all aspects of shallow gas mentioned above, comprehensively summarizes the current scientific understanding, and analyzes the existing shortcomings, which will provide systematic references for the research on environmental disaster prevention, engineering technology, climate change, and other fields.
Focused fluid flow shapes the evolution of marine sedimentary basins by transferring fluids and pressure across geological formations. Vertical fluid conduits may form where localized overpressure breaches a cap rock (permeability barrier) and thereby transports overpressured fluids towards shallower reservoirs or the surface. Field outcrops of an Eocene fluid flow system at Pobiti Kamani and Beloslav Quarry (ca 15 km west of Varna, Bulgaria) reveal large carbonate‐cemented conduits, which formed in highly permeable, unconsolidated, marine sands of the northern Tethys Margin. An uncrewed aerial vehicle with an RGB sensor camera produces ortho‐rectified image mosaics, digital elevation models and point clouds of the two kilometre‐scale outcrop areas. Based on these data, geological field observations and petrological analysis of rock/core samples; fractures and vertical fluid conduits were mapped and analyzed with centimetre accuracy. The results show that both outcrops comprise several hundred carbonate‐cemented fluid conduits (pipes), oriented perpendicular to bedding, and at least seven bedding‐parallel calcite cemented interbeds which differ from the hosting sand formation only by their increased amount of cementation. The observations show that carbonate precipitation likely initiated around areas of focused fluid flow, where methane entered the formation from the underlying fractured subsurface. These first carbonates formed the outer walls of the pipes and continued to grow inward, leading to self‐sustaining and self‐reinforcing focused fluid flow. The results, supported by literature‐based carbon and oxygen isotope analyses of the carbonates, indicate that ambient seawater and advected fresh/brackish water were involved in the carbonate precipitation by microbial methane oxidation. Similar structures may also form in modern settings where focused fluid flow advects fluids into overlying sand‐dominated formations, which has wide implications for the understanding of how focusing of fluids works in sedimentary basins with broad consequences for the migration of water, oil and gas.
The applicability of a passive acoustic method for monitoring bubble gas emissions in shallow water is being investigated. Experiments on the registration of audio signal generated by single gas bubbles with a diameter of 3–4 mm were conducted under laboratory and field conditions. Under laboratory conditions, one maximum in the frequency spectrum was observed, occurring at about 2500 Hz for bubbles of small diameter and 1570 Hz for larger bubbles. In situ experiments were carried out in Laspi Bay (the Black Sea). Several peaks in the frequency bubbles spectrum were shown: at 1550, 1100 and 600 Hz. The highest frequency recorded by hydrophone qualitatively coincides with the resonant frequency of a 4 mm diameter bubble calculated from the Minnaert equation.
This chapter provides an overview of other types of degassing in the Black Sea. In addition to mud volcanoes, there are numerous areas where gas bubbles escape from the Black Sea floor; these are called gas seeps, acoustic (i.e., obtained using echo sounders or sonars and visible on echograms) plumes, gas torches (due to their cone-like shape on eco sounder images), or gas flares; these are all referred to in the chapter as gas seeps. They are localized on the periphery of the Black Sea, particularly in its northwestern part, on the Bulgarian and Kerch-Taman shelf (including the Kerch Strait), and along the coast of the Caucasus. Quite often, they are confined to paleo-deltas of the largest Black Sea rivers, the Danube, Dnieper, Dniester, and Don, and within the canyons formed by them. The deepest seeps are associated with faults and mud volcanoes in the central Black Sea basin.
Deepwater hydrocarbon exploration drilling only began in the Black Sea less than 20 years ago, primarily because of the economical/technological challenges associated with mobilizing suitable rigs through the Bosporus. However, to date (end 2017), c. 20 deepwater wells have now been drilled, targeting a large variety of plays in this underexplored basin. The deepwater wells drilled to date are categorized by their main play objectives, within either the sag/post-rift or syn-rift basin fill of the Black Sea. The sag/post-rift play types have proven to be more successful, finding either biogenic gas in Miocene to Pliocene reservoirs associated with the Paleo-Danube and Paleo-Dnieper/Dniester or oil in Oligocene deepwater siliciclastic systems. Syn-rift or early postrift plays, in contrast, assumed mostly shallow water carbonate reservoir targets. Only one well targeted pre-rift stratigraphy. Most of the exploration failures to date are directly related to the lack of reservoir at the targeted stratigraphic levels. However, the recent discoveries have underlined the presence of at least two active and effective petroleum systems that cover large parts of the deepwater Black Sea Basin.
Gas hydrates, a solid established by water and gas molecules, are widespread along the continental margins of the world. Their dynamics have mainly been regarded through the lens of temperature-pressure conditions. A fluctuation in one of these parameters may cause destabilization of gas hydrate-bearing sediments below the seafloor with implications in ocean acidification and eventually in global warming. Here we show throughout an example of the Black Sea, the world's most isolated sea, evidence that extensive gas hydrate dissociation may occur in the future due to recent salinity changes of the sea water. Recent and forthcoming salt diffusion within the sediment will destabilize gas hydrates by reducing the extension and thickness of their thermodynamic stability zone in a region covering at least 2800 square kilometers which focus seepages at the observed sites. We suspect this process to occur in other world regions (e.g., Caspian Sea, Sea of Marmara).
The numerous, mostly untested deepwater Black Sea play types can be subdivided into syn-rift and post-rift plays. The largest targets are synrift fault blocks, such as the Andrusov and Tetyaev highs in Turkey and Ukraine, respectively. Although their internal stratigraphy is poorly constrained (i.e. proportion of pre-rift and syn-rift versus basement) translating to not only reservoir risk but also to reservoir quality risk, the trap sizes are very large. Also, the assumed lateral charge from the Miocene-Oligocene Maikop Formation and perhaps even from Eocene sources makes these structures extremely attractive. The overall structure of the Shatsky Ridge is not as clear as it has elements of an extremely large carbonate platform on top. The Polshkov High is unique in the sense that it represents a large rotated syn-rift fault block along the lower plate edge of the Western Black Sea in Bulgaria. On the conjugate upper plate margin, very large inverted syn-rift structures, such as the Kozlu Anticline, are recognized in the Turkish sector.
On top of most of the syn-rift highs described above, various typical carbonate geometries can be interpreted on seismic data such as backreef pinnacles, lowstand buildups, raised rims, backstepping, aggradation and prograding clinoforms. These syn-rift to post-rift carbonate platforms tend to grow on footwall blocks of syn-rift faults and can reach more than 800 meters in thickness. The age of these carbonate features is poorly constrained at present as to the exact opening age of the Black Sea basins, i.e. anywhere between Jurassic to Eocene.
Several intra-Tertiary reservoirs could be targeted in the compactional anticlines above the large syn-rift highs. Another play associated with Tertiary sands is that of the deepwater extension of the Subbotina discovery in Ukraine. The Subbotina structure is a compressional anticline situated in a dominantly Miocene, south-vergent folded belt offshore Kerch Peninsula. Similar folded belts are also known in the Russian, Georgian, Turkish and Bulgarian sectors of the Black Sea. Also, pure stratigraphic traps may exist in a widely recognized Eocene low-stand wedge along the basin margins. However, reservoir quality is a definite risk for the Tertiary reservoir intervals in certain segments of the Black Sea as the function of the provenance area(s).
The surface expressions of natural gas seepage can be classified on the basis of spatial dimension, visibility, and fluid typology. Macro-seeps (or seeps) are “channelled” flows of gas, typically related to fault systems. They include gas and oil seeps, mud volcanoes and gas-bearing springs. Gas flux is expressed in terms of mass/time (e.g., kg/day or tons/year). Microseepage is the pervasive, widespread exhalation of gas throughout relatively large areas, conceptually independent from seeps, even if also enhanced along faults. Gas flux is expressed in terms of mass/area/time (for methane it is usually in mg m−2 day−1). Sometimes the term “micro-seeps” is used in the scientific literature, especially in the marine environment to define relatively smaller seeps, not observable, for example, by hydroacoustic methods. However, the term can be misleading as it may be confused with microseepage. This classification is, in theory, valid for either subaerial (land-based) or underwater (marine and lake) environments. The marine environment can have specific gas-seepage structures.
Y sont reflétés les résultats des explorations sur certaines sources naturelles à gaz et des manifestations de gaz unitaires dans l’aquatoire dela baie de Ваlchik.
Des études détaillées sur le chimisme sont les déductions et le débit de gaz ont été effectuées sur les manifestations de gaz Zelenka et Zlatni piassaci.
Sur le plan structural les sources de gaz maritimes sont disposées dans le prolongement marin de la dépression de Varna.
En partant des résultats obtenus concernant le chimisme du gaz, la structure géologique de la région et la disposition en ligne stricte des sources de gaz, faites suivantes:
Les sources de gaz naturelles dans la partie d’haut fond du plateau conlinental sont une migration de gaz naturel par des failles des gisements à gaz d'une origine profonde.
Les structures pétrolifères et gazéiféres de la terre continuent dans le plateau continental.
Les manifestations de gaz de ce type peuvent egalement servir de caractére immediat pour le levé des structures pétrolifères et gazéifères du plateau continental. [The results of studies of some natural gas sources and single gas manifestations in the water area of the Balchik Bay are described. The chemistry and flow rate of gas in the Zelenka and Golden Sands gas shows have been studied in detail. Structurally, offshore gas sources are located in the offshore extension of the Varna depression. Based on the results obtained on the chemistry of gas, the geological structure of the region, as well as the strict location of gas sources in the line, the following conclusions were drawn:
1) natural gas sources in the shallow part of the shelf represent the migration of natural gas along the faults of gas deposits of deep origin;
2) the oil and gas bearing structures of the land continue in the shelf.
3) gas manifestations of this type should be considered a direct sign for mapping the oil and gas bearing structures of the shelf.]
sounder and shallow seismic data analyses of investigation s conducted before and after 1988, allow the main pockmark area to be enlarged and given more details. A total of 305 pockmarks have been located within an area of more than 100 km2 (2-5 km wide and 41 km long). The bottom of the area is covered with Holocene soft silty clay. The following trends are defined when moving from the northern part of the pockmark area to the southern part: 1) increase in seabed sediments thickness from 2.5 m to more than 3.2 m and decrease in mean sediment grain size; 2) increase in general water depth from 160-300 m to 230-340 m in areas where pockmarks occur; 3) increase in average individual pockmark size from 86 to 132 m in diameter and from 2 to 4 m in depth; 4) decrease of pockmark density from 8-9 p.m./knv2 to 3-4 p.m./km2. Similar trends are observed seaward where the pockmark profile shape changes from a smooth U-shaped to a sharp V-shaped form, with pockmark wall slope angle reaching more than 10°. A variety of acoustic anomalies such as acoustic turbidity, bright spots, enhanced reflections etc., are recorded in many places within the pockmark area, indicating the presence of gas in the sediments. Water column anomalies have also been recorded. The paper discusses the origin of the gas (most likely to be a mixture of thermogenic and biogenic gas) and die time of pockmark formation.
Active has vents and methane hydrate deposits were found in two areas of the Okhotsk Sea in October, 1991 onboard the R. V Antropov, a Russian research vessel operated by Dalmorgeologia from the Far-East Region of Russia. Gas vents were identified in water depths of 700 to 800 in using a 20 khz fish finding echo sounder. Sediment and hydrate samples were recovered using a 3 in long gravity corer. Some cores contained 30 to 40% by volume methane hydrate in lenses, layers and veins. A small fraction of the venting gas is captured in a methane hydrate phase, while a majority of the vented gas is released and dispersed in the overlying seawater. Small gas bubbles were observed at the sea surface, indicating that some of the venting gas reaches the atmosphere directly. Preliminary hydrocarbon gas and methane isotopic composition suggest that the venting gas is biogenic in origin. Based on the flux of organic carbon reaching the seafloor and pore water ammonia results, recent biogenic processes are not sufficient to support the production of the venting methane. It is concluded that biogenic methane from subsurface sediment layers is migrating upwards alone preferential pathways associated with faulting.
The effects of methane gas seepage were studied at an intertidal/shallow subtidal site on the
Kattegat coast of Denmark. A 30 m wide zone, containing approximately 65 gas seeps, extended over 70 m seawards from the shore. This was characterised by the presence of slabs, pavement and small pillars of carbonate-cemented sandstone which formed a partially buried reef. The escaping gas contained 91-100% methane with some carbon dioxide, 0.6-0.9%, and hydrogen sulphide. The hydrogen sulphide concentration varied over time and between individual seeps. Gas flow rates of individual seeps ranged up to 21 litre h-1 and the estimated total flow was 110 litre h-1. Seeps were often stopped by sand movement, but the overall gas flow from the site appeared to be constant. The escaping gas generated an interstitial water circulation and drew overlying seawater into the sediment. Water pumped out by the seeps was enriched in phosphate and ammonia. Sulphate reduction rates in the seep area were 1.1-17.1 m moles sulphate reduced and aerobic methane oxidation rates were 0.2 - 5.5 m moles methane consumed m-2 day-1. The composition of the flora and fauna surrounding the seeps was affected by the presence of hard substrate (the cemented sandstone). Epifauna was more abundant in the seep zone than elsewhere, whereas the macrobenthic infauna was reduced in the seep zone, possibly due to the cementation. The sediment was almost devoid of meiobenthic organisms, except nematodes. Nematode species numbers, abundance and biomass were lower at the seeps than 5-20 cm away.
The nematode fauna penetrated deeper into the sediment close to the seeps than at the seeps themselves. This is explained by the interstitial water circulation at and close to the seeps. I4C measurements showed that little methane carbon was entering the food web surrounding the seeps.
The seepage of methane through the seabed of the world's continental shelves is considered. Methane may be generated by both bacterial and thermogenic processes acting on the complex organic matter stored in the seabed. Because these processes are ubiquitous, occurrences of gas and gas seepages are probably widespread. Although numerous observations of methane seepages from shallow marine sources have been reported, there are few documented flux measurements. Whereas a prolific 400 g CH4 m−2 year−1 has been calculated for an 18 km2 area in the Santa Barbara Channel, California, a more modest 26 g CH4 m−2 year−1 has been estimated for a site in the North Sea. Descriptions of these and other areas of gas seepage, from a variety of geological and oceanographic environments, are presented.A rough estimation of the global flux suggests that between 8 and 65 Tg of methane passes through the seabed of the continental shelves annually. If a significant proportion of this passes through the water column and into the atmosphere, the oceans may represent a more significant source of methane than has been suggested previously. As much of this methane is 14C-depleted, it may at least partially account for the shortfall of fossil methane in current atmospheric methane budgets.
It is suspected that most shallow reservoirs of natural gas vent to the surface to some degree. This seeping may be through diffusion of dissolved gas or by a flow of gas bubbles which entrain interstitial water during the rise through the sediments to the surface. Methane bubbles dissolved other gases, notably hydrogen sulphide and carbon dioxide, during their ascent. Under suitable temperature-pressure conditions gas hydrates may be formed close to or at the seabedBlack suphide-rich sediments and mats of sulphur oxidizing bacteria are frequently observed close to the sediments surface at seep sites, including a sharp oxic/anoxic boundary. Animal species associated with these gas seeps include both species which obtain nutrition from symbiotic methane-oxidizing bacteria and species with symbolic sulphur-oxidizing bacteria. It is suspected that at some microseepage an enhanced biomass of meiofauna and macrofauna is supported by a food chain based on free-living and symbiotic sulphur-oxidizing and methane-oxidizing bacteria.The most common seep-related features of sea floor topography are local depressions including pockmark craters. Winnowing of the sediment during their creation leads to an accumulation of larger detritis in the depressions. Where the deprssions overlies salt diapirs they may be filled with hypersaline solutions. In some areas dome-shaped features are associated with seepage and these may be colonized by coral reefs. Other reefs, “hard-grounds”, columnar and disc-shaped protrusions, all formed of carbonate-cemented sediments, are common on the sea floor in seep areas. Much of the carbonate appears to be derived from carbon dioxide formed as a result of methane oxidation. The resulting hard-bottoms on the sea floor are often colonized by species not found on the neighboring soft-bottoms. As a result seep areas may be characterized by the presence of a rich epifauna.
This chapter deals with the biogenic sources of methane in the gulf of Mexico. Factors which influence the concentration of methane are exchange of water with the atmosphere, injection of petroleum-related gas due to man's activities, microbial production from natural organic matter, and microbial oxidation to carbon dioxide. Microbial generation is by far the greatest source of methane to the Gulf, whereas atmospheric exchange is the predominant methane sink. Methane is produced in environments depleted in sulfate by anaerobic micro-organisms using organic acids and carbon dioxide. Methane is most extensively produced in deltaic, estuarine and near-shore sediments, which are rich in these labile organic compounds. Methane produced in shallow sediments can exceed saturation concentrations, form bubbles, and escape from the sediments by rising through the shallow overlying waters. Methane which dissolved during bubble ascent or diffuses directly into the water from surficial sediments is advected into surrounding waters, is slowly diluted by normal Gulf mixed water, and finally escapes to the atmosphere by exchange. Highest dissolved methane concentrations in the Gulf are found in the Mississippi delta region, where dissolved methane concentrations are frequently orders of magnitude greater than atmospheric equilibrium values.
Rates of petroleum seepage must ultimately be related to the rate of supply from below. This could be either from a leaking accumulation or directly from the source rock itself. However, many things can happen during migration to the surface, such that the relationship between trap leakage rates and surface seep rates remains obscure. We calculate the potential flux rates of gas leakage across seals and compare these with measurements of fluxes for three seepage sites on the European continental shelf. We conclude that seepage flow rates can be modeled effectively by assuming Poiseuille flow through the matrix of mudstone seals. Row rates calculated in this way are about 0.4-1.0 m3 gas/m2/year, consistent with field observations.
This chapter deals with those sources that are commonly regarded as making a minor contribution to the total atmospheric concentration of methane. The definition of “minor” is somewhat arbitrary; that is, they are sources not covered elsewhere in this volume or identified in the other chapters as being of only “minor significance.” In isolation the majority of these sources may be regarded as inconsequential; however, in combination they provide a significant proportion of atmospheric methane. In certain cases it is possible that the true significance of their contribution is yet to be realized, and some may have been overlooked hitherto. More detailed discussions of these sources are provided by Hovland et al. (1993), Lacroix (1993), Lambert and Schmidt (1993), Rouland et al. (1993), Smith et al. (1993), and Khalil et al. (1993).
Natural gas seepages occur on the United Kingdom's continental shelf and although published reports suggest that they are very rare, the petroleum industry has identified, but not publicly reported, many more. There is also very little data on the flux of gas from seabed seepages, and even less on the contribution of seepages to atmospheric concentrations of gases such as methane. Potential gas source rocks include Quaternary peats as well as petroliferous source rocks such as the Carboniferous Coal Measures and the Upper Jurassic Kimmeridge Clays. There are also other organic-rich sediments which are potential source rocks. Together these cover a considerable part of the U.K. continental shelf. Analogue seismic reflection (pinger) profiles acquired during the British Geological Survey's regional mapping programme have been reviewed to identify water column targets including fish and plumes of gas bubbles. The ability to distinguish targets is critical to an assessment of the distribution of gas seepages. Both theoretical predictions of target identity and the habits of shoaling fish have been investigated in order to identify a method of distinction. Data from seabed seepages and measurements of seepage rates have been used to establish likely ranges of gas flux rates and the sizes of gas bubbles. The likelihood that a rising bubble will survive and escape into the atmosphere is determined primarily by bubble size and water depth; methane, the principal constituent of seepage gas, is relatively unreactive and sparingly soluble. The studies have enabled a new estimate of the distribution of gas seepages on the U.K. continental shelf, and of the contribution to atmospheric methane levels. The results suggest that natural gas seepages are significantly more important as a source of methane than had hitherto been established. It is estimated that between 120,000 and 3.5 mtonnes of methane per year come from a continental shelf area of about 600,000 km(2). This represents between 2% and 40% of the total United Kingdom methane emission. It is suggested that similar contributions arise from other continental shelf areas worldwide, and that geological sources of atmospheric methane are more significant than is generally acknowledged.
More than 99% of the diffusive methane flux from marine sediments appears to be consumed by microbial oxidation when dissolved sulfate is present. At 3 sites, methane bubble vents were found. Two of the vents, located in the Okhotsk Sea, appear to result from gas being released along faults. The third plume site was located in a shallow harbour where a large amount of organic carbon is deposited in shallow water. The global diffusive and advective (bubbles) release of methane to the atmosphere from marine sediments is estimated to fall into the range of 1 - 10 TgC a"1 (1 Tg = 10 12 g). Researchers have speculated that marine gas hydrate deposits hold vast quantitites of methane that may melt and be released as a result of global warming. A warmer atmosphere could directly heat the ocean and/or change ocean current systems, which could bring warmer water to some areas. Hydrate samples were recovered from two sites in the Okhotsk Sea. These deposits, in 700 to 800 m of water, would require water temperatures to increase by 8°C in order to melt the hydrate. Quantitative estimates of hydrate reservoirs near the minimum pressure stability zone are needed to remove the uncertainty whether this will be a significant positive feedback loop for global warming.
Gas seeps occurring on tidal flats on the northern shore of the inner Firth of Forth are described. The principal gas is methane, which is considered to come from the coal-bearing rocks of the Lower Limestone Series (Carboniferous); either naturally or from abandoned coal workings. Seep activity has been known at the site for several years, and it is suggested that the presence of white filamentous bacteria (Beggiatoa sp.) and a carbonate precipitate are indicative of long-term seepage. Comparative studies at the seep and at a control site revealed that the seeps have only a marginal effect on the intertidal fauna.Migration of gas through the thin (
The crystalline Stockholm Archipelago contains several deep seated tectonic lineaments of mainly Svecofennian age. Along parts of these lineaments, gas with a thermogenic signature rises to the surface creating pockmarks in the seabed. Ground water is known also to leak from intersecting submarine eskers in the area, but this phenomenon has so far not been proved to develop distinct pockmarks. The east-west direction is regarded to be the youngest out of four main lineament directions in the Archipelago. Investigations have been carried out along the eastwest Strömma lineament at Stavsnäs with respect to pockmark structures, location of main fractures within the lineament, and the gas content in clay samples along a line perpendicular to the lineament. Chemical analyses show that anomalous amounts of gas with thermogenic signature are contained in samples collected from within the lineament as compared to samples collected in the adjacent areas to the north and south. Gas eruptions are reported to occur frequently nearshore along the islands within the lineament at Stavsnäs.
Gas seeps were located, by echo sounding, SCUBA divers and ROV observations, at hydrothermal sites around the island of Milos, in the Hellenic Volcanic Arc. Samples were collected by SCUBA divers and by a ROV from water depths between 3 and 110 m. Fifty-six flow rates from 39 individual seeps were measured and these ranged from 0.2 to 18.51 h−1 at the depth of collection. The major component, 54.9–91.9% of the gas, was carbon dioxide. Hydrogen (≤3%), methane (≤9.7%) and hydrogen sulphide (≤8.1%) were also measured. Hydrothermal free gas fluxes from the submarine hydrothermal areas around Milos were estimated to be greater than 1010 moles y−1. It was concluded that submarine gas seeps along volcanic island arcs may be an important carbon dioxide source.
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