Spatial and quantitative evaluation of the Black Sea gas hydrates

Abstract

The magnitude and spatial distribution of potential Black Sea methane hydrate reservoirs has been estimated on the basis of 6′ x 6′ longitude-lattitude data grid. The general input includes bathymetry; bottom temperatures; heat flow (487 quoted measurements are considered); temperature gradients; thermal conductivity of the sediments; pressure-temperature hydrate phase relations; organic carbon content as a function of depth; sediment porosity-depth curves; percentage of hydrate occupying the hydrate stability zone; and volumetric gas expansion factor. The estimations are based on the two main theories of gas hydrate formation - in situ bacterial production and pore fluid expulsion models. The spatial evaluation of the most probable gas hydrate distribution is also discussed. The calculations show that average water depth from which methane hydrate starts to forms in the Black Sea runs from 620 to 700 m, embracing a prone area of 288,100 km2, i.e., 91 % of the deep Black Sea basin. The average thickness of the MHSZ is 303 m with a bulk of sediment running from 85,310 to 100,280 km3. The evaluations show the hydrate content of 77-90 to 350·109 m3, i.e., about 10 to 50·1012 m3 of gas methane are trapped within the Black Sea sediments in the form of hydrate.

Full text

Russian Geology Geologiya
and Geophysics i Geofizika
Vol. 43, No. 7, pp. 672-684, 2002 UDC 550.42:552.578.1(262.5)
SPATIAL AND QUANTITY EVALUATION
OF THE BLACK SEA GAS HYDRATES
A. Vassilev and L. Dimitrov
Institute of Oceanology, Bulgarian Academy of Sciences, POB 152, Varna 9000, Bulgaria
An estimation of the ma
g
nitude and spatial distribution of potential Black Sea methane
hydrate reservoirs have been made based on 6×6 minute (in longitude and lattitude) data
grid. The general input includes bathymetry; bottom temperatures; heat flow (487 quoted
in situ measurements are considered); temperature gradients; thermal conductivity of the
sediments; pressure-temperature hydrate phase relations; organic carbon content as
function of depth; sediment porosity-depth curves; percentage of hydrate occupying the
hydrate stability zone; and volumetric gas expansion factor.
The estimations are based on the two main theories of gas hydrate formation — in situ
bacterial production and pore fluid expulsion models. The spatial evaluation of the most
probable gas hydrate distribution is also discussed.
The calculations show that average water depth from which methane hydrate start to forms
in the Black Sea ranges from 620 m to 700 m embracing a prone area of 288,100 km2,
representing 91% of the deep Black Sea basin. The average thickness of the MHSZ is 303 m
with a bulk of sediment ranging from 85,310 to 100,280 km3. The evaluations set the hydrate
content on 77–90 to 350⋅109 m3 or about 10 to 50⋅1012 m3 of gas methane are trapped within
the Black Sea sediments in the form of hydrate.
Gas hydrates, methane, Black Sea, thermal field, temperature gradients, heat flow, temperature,
prone area, stability zone, hydrate quantification
INTRODUCTION
The discovery of natural gas hydrates in marine sediments happened fortuitously in 1979: in the Caspian Sea
during core sampling, and in the Pacific, during deep sea drilling. After the realization that they are globally
widespread and could contain a huge amount of world gas reserves the science community starts to evaluate their
quantity. Now it can be distinguished three stages in these efforts: initial, very optimistic estimations of
(156−760)⋅1016 m
3 [1, 2] were followed by less optimistic ones of (0.3−13.9)⋅1016 m
3 [3–8], and nowadays the
estimation of their quantity varies from 1.0⋅1015 m
3 to 1.0⋅1014 m
3 [9, 10], but still remains speculative and
uncertain.
During this time period, it goes on continuous data collection of gas hydrates geographical distribution
worldwide, their properties, composition, mode of formation etc., as well as for geological environments in which
they occur. All this knowledge helps now to create a sensible assessment for the role of gas hydrates in the past
and the future.
There are several publications describing Black Sea gas hydrate findings, seismic and other evidences for
their presence in the sediments. The only estimation made sets their gas quantity within the range of
(40−50)⋅1012 m
3 — half of them recoverable [11]. Though the calculation used a decreasing area coefficient of
0.3 this amount seems to be too high, even the total Black Sea resources of conventional hydrocarbons are lower
by an order of magnitude — about 3.5⋅1012 m3 [12]. Kutas and coworkers [13, 14] have presented a very useful
discussion of the geothermal conditions in the Black Sea basin in respect of hydrate formation in a few publications.
©2002 UIGGM, SIBERIAN BRANCH OF THE RAS
637

The main result is that the deep Black Sea basin is marked by appreciable low heat flow and is extremely favorable
for gas hydrate formation below 750–800 m water depths.
This paper presents results of quantity evaluation of the Black Sea gas hydrates based on 6′×6′ data grid.
This starts with bathymetry (pressure respectively) and thermal field at the sea bottom (both in the seawater and
in sediments), and gradually the model is complicating adding different parameters to provide an estimation tending
to reality. The spatial evaluation of the most probable gas hydrate distribution is also discussed.
BLACK SEA GAS HYDRATE OCCURRENCES
The very first sampling of gas hydrates in the Black Sea was made in the 1970s when many small crystals
like “hoarfrost” were observed in a number of “very gassy” cores taken from the seabed. These aggregates were
virtually ignored because they were thought to be simple ice, carbonic acid crystals, whatever else but gas hydrates.
Such marginal evidences have been mentioned in many places of sediment sampling on the Black Sea continental
slopes, in the Bulgarian sector (Prof. P. Dimitrov, IO-BAS — oral communication), in 860 m water depth in front
of Georgia [12] etc.
It could be accepted that the first documented discovery of gas hydrates is their sampling during the cruise
of R/V “Moscow University” in 1972 [15, 16]. Gas hydrate were found within a core taken in the apron of Danube
fan at a depth of 1,950 m (Fig. 1; Table 1) and described as “small, white, fast disappearing crystals” occurring
in large gas cavities at 6.4 m below sea floor. Another sample was taken somewhere in the eastern part of Danube
fan (“about 20 nautical miles west of the area with anomalous acoustic features” as reported by the authors) during
the cruise of R/V “Akademik Vernadsky” in 1992 [17]. It is possible that this finding is the same as reported by
Kruglyakova [oral communication] and have coordinates as described in Table 1.
In 1988, during the 21st cruise of R/V “Evpatoriya” seven cores with gas hydrate were sampled south of
Crimea in the Feodosiya mud volcano [18, 23]. The corers are situated within a circle 100 m in diameter and
water depth of about 2050 m. The six hydrate occurrences are from silt clay sediments and one representing the
best example of 10 cm long monocrystal hydrate from mud volcano breccias. All occurrences are situated from
0.4 to 2.2 m below seafloor. The content of hydrates varies from 3% up to 10% of the total volume of the
sediments.
During several MSU training and research cruises organizing and supervised by M. Ivanov and A. Limonov
starting from 1988–89 onboard of the R/V “Feodosiya” [20] followed by the UNESCO Training Trough Research
(TTR) cruises in 1993 and 1994 (R/V “Gelendzhik”), gas hydrates were found in the mud volcano area of the
Central Black Sea abyssal plain [21] and more recently in 1996 in Feodosiya mud volcano region in Sorokin
trough [21, 22]. All samples were taken from the crests of mud volcano structures and occur within mud breccias
at depths of 0.6 to 2.85 m below the seafloor.
Analyzing well log data from hole 379 of DSDP Ginsburg and Soloviev [9] believe that the sediments lying
380 to 450 m below seafloor are gas hydrates. Within these strata high values (up to 2,400 m/s) of sound velocity
were measured, which sharply changed to 1,400 m/s at the bottom of the interval as well as decreasing of the
sediment density and lowering of the drilling speed two and half times. All these suggest the presence of gas
hydrates there.
Besides the direct observations of gas hydrates some seismic evidence has been reported. Bottom Simulating
Reflectors (BSR) are observed in water depth deeper than 1,000 m along the northern Caucasian coast (Tuapse
trough and Shatsky ridge) and eastern part of the Kuban’ fan [11, 24]. Meisner and coworkers [25] reported that
more than 200 km2 of continuous BSR are mapped in these regions. It is also found in the Feodosiya mud volcano
region (Sorokin trough); on the eastern part of the Danube [26]; on the southern Bulgarian continental slope and
in front of the Bosphorus Strait (Fig. 1). Usually BSR occur at 0.2–0.4 s TWTT below seabed, “exactly coinciding
with the lower hydrate stability zone” [11]. The results from velocity analysis [27] show that the sound velocity
above the BSR reaches up to 2,800 m/s (the background ones are 1,800–1,900 m/s average). It dramatically
decreases to 1,200 m/s below suggesting a possible high gas saturation of the sediments. It should be noted that
all documented BSR are situated on the continental slopes and are not observed in the abyssal plain. An explanation
could be that BSR is relatively easy to be recognized on the slopes where gradients are great, while the sediment
strata lie almost horizontally in the abyssal plain and BSR, if occurs, will be hardly masked by reflections from
stratigraphic boundaries. Another extraordinary reflection connected also with the possible presence of gas hydrates
in the sediments is observed in the abyssal plain. The so-called VAMP’s (Velocity AMPlitude features) have been
documented at the crests of gentle anticlines in the western Black Sea Basin and southern Bulgarian continental
slope [11].
Several large (up to 3 km long and more than 400 m high) isometric acoustic anomalies related to gas
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Fig. 1. Black Sea gas hydrate and shallow gas occurrences. 1 — Gas hydrate sampling (see Table 1 for numbering); 2 — areas with seismic
indications of gas hydrates, BSR, VAMP’s; 3 — areas of high gas hydrate prospect; 4 — mud volcanoes; 5 — areas of intensive fluid
discharging; 6 — gas seepage and seabed pockmarks; 7 — mine submarine fans.
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emissions from the sea bottom at depths of 900–1,800 m were recorded in 1989 [28]. They were found about 25
nautical miles southwest of Sevastopol [17] and were studied in more detail in 1992. Moreover, about
200 gas-venting sites have been inventoried on the upper part of the slope in this area [29, 30] and extensive
seafloor landslides have been localized there [26]. All these phenomena may have a common origin connected
with destabilization of gas hydrate layers along fault zones.
In addition, there are numerous indications of gas presence in the sediments such as bright spots, enhanced
reflectors and acoustic turbidity, gas seeps, pockmarks, foaming sediments, slides in the upper part of continental
slope possibly caused by hydrate melting, etc. This information, together with above mentioned data, help to outline
promising areas for gas hydrate findings.
These are the continental slopes: Southwestern area (South Bulgarian and Bosphorus slope and apron); Danube
fan area; South Crimea area (Sorokin trough); Kuban’ fan area; Caucasian area (Tuapse trough and Shatsky ridge);
Southeastern area (Georgia slope and apron and Gurian trough); Central Turkish Area (Giresun and Sinopian
troughs and Arkhangelsky Ridge); and Central abyssal area (Central Mud Volcano Province and Andrusov Ridge).
The rest areas of the Black Sea abyssal plain are of low possibility for gas hydrate discovery.
METHODOLOGY
The location and dimensions of the methane hydrate stability zone in the Black Sea are estimated in a grid
of 6′×6′ (in longitude and latitude), based on water depths, sea floor temperatures, the calculated temperature
gradients and equation governing the methane hydrate stability, gas composition, porosity and hydrate content in
the sediments.
Bathymetry. The creation of the input database was started with data of the bottom relief of the Black Sea.
These were data of the Institute of Oceanology for the Bulgarian part of the Black Sea, and data on the rest of
the Black Sea were taken from the Internet site “Measured and estimated seafloor topography” [31]. The initial
data were arranged in grids of 2′×2′. Since the other input data (sea bottom temperatures and geothermal gradient
or heat flow) were far from this density, the initial data grid was transformed to 6′×6′ which corresponds to a map
scale of one to five million (scale 1:5,000,000).
Gas composition. The main gas component of the Black Sea hydrates is methane, 93.3–99.7%. The gas
sampled in the Feodosiya region contains negligible amounts of higher methane homologs (0.02–0.045%), while
they are up to 6.7% in the gas from central mud volcano area. Small amount of CO2 (0.86–0.9%) is found in the
gases of the Feodosiya polygon while in the gases from the eastern part of Danube fan the portion of CO2 is as
Table 1
Black Sea Gas Hydrate Samples (see Fig. 1 for Location)
Nos. Locality Water
depth, m Reference Notice
NE
1 43°35′31°08′1950 [15, 16] one sample, Danube fan
2 44°17′40′′ 34°58′40′′ 2050 [9, 18] 4 (+3) samples in a circle of
200 m2, Sorokin mud volcano area
3 N.d.. N.d. N.d. [19, 20] 4 samples on two mud volcanoes in
the central abyssal plain
4 43°00′29′′ 36°00′68′′ 2171 [19, 20] Site DSDP-379
5 44°30′–
44°45′
34°41′–
35°09′
1850–2050 [21, 22] 5 samples in a belt 3 km wide
70 km long in Sorokin mud volcano
area
6 44°33′75′′ 31°43′90′′ N.d. [17], M. Kruglyakova,
2000 (personal
communication)
one sample, gas discharging area of
Danube fan
7 N.d. N.d. N.d. [12] N.d.
8 N.d. N.d. N.d. E. Sakvarelidze, 2000
(personal communication)
N.d.
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high as 24% [17], and no CO2 is recorded in samples from other regions. Although it is not detected by analysis,
all samples have the specific smell of hydrogen sulfide.
The carbon isotope ratio of the gas from the Feodosiya region is about –61.8‰ PDB, and from –43‰ to
–61.8‰ in the central abyssal plain. These results, together with data of gas composition, suggest that gas is a
mixture of biochemical and thermocatalytic gases in the first case while in the rest the prevalence of the thermogenic
gas is obvious. A supposition was made [9] that hydrate from the very first finding in the Danube fan includes
biochemical methane formed in situ. This is supported by Nitrogen enriched sample in the eastern part of the fan
[17], and the general gas composition (CH4 — 68.1%; CO2 — 24.1%; N2 — 7.7%) of this sample suggests recent
microbial gas generation.
For the purpose of calculation it was accepted that the gas hydrate is formed by pure methane. This permitted
us to use the gas hydrate stability curve for methane–sea water system described by Kuustraa and Hammershaimb
[32].
Bs = exp(38.9803 – 8533.8/Ts)
for temperatures in the range of 273 < Ts < 298 K and where Bs is the stability pressure in KPa and Ts is the
stability temperature in K.
Sea-floor temperatures and geothermal field. The Black Sea geothermal database is created by available
published data and contains measurements in 487 geothermal stations. These measurements have been made during
several expeditions by different scientific teams and research vessels [13, 14, 33–42]. Because of irregularly
scattered geothermal data (Fig. 3) they are interpolated and extrapolated numerically to obtain the bottom sediments
thermal conditions for the Black Sea in a regularly spaced two-dimensional grid with a step of 6×6 minutes (in
longitude and latitude). The transformation and mapping were performed using Kriging algorithm from software
programs GS Scripter and Surfer (Golden Software, Inc.). Based on these data, the bottom temperature field and
the heat flow of the deep Black Sea basin were mapped.
Sea-floor water temperatures were measured at an approximately regular set of 2320 points. These data are
collected from international monitoring stations for last 100 years. There are at least few measurements at every
single point, which were averaged before creating the final grid. As it is clearly shown (Fig. 2), the bottom
temperatures are almost constant all over the deep Black Sea basin, slightly oscillating around 8.9 °C with an
amplitude of about 0.3 °C.
Since the variation of the temperatures in depth of sediments is needed for estimating the thickness of gas
hydrate stability zone (GHSZ), they were estimated on the basis of mapped heat flow data. Two methods are used
to determine temperature cross-section at each grid node.
1. The curves of temperature-depth measurements in deep marine wells are nearly straight lines [43]. In a
first approximation the temperature conditions in sediments could be determined from measured bottom temperature
and temperature gradients for water depths exceeded the zone of insulation (about 500 m in the Black Sea). It is
the fastest and easiest approach for estimation and could be performed when conductivity and heat flow data are
not available. Thus, geothermal gradients vary from 10 to 80 °C/km (Fig. 4), in accordance with the mapped heat
flow being between 20 and 70 mW/m2 (Fig. 3).
2. More accurate is the assumption that the heat flow varies with depth [8]. This approach gives the
temperature T(z) at subbottom depth z:
T (z) = T0 + Q ⋅ ∫
z0
z
[1/k (z)] ⋅ dz ,
where T0 is the sea floor temperature in K, Q is heat flow in mW/m2 and z0 is the water depth in m. Thermal
conductivity k (z) is:
k (z) = kw
P ⋅ ks
(1 − P ),
where kw = 0.57 W/(m⋅K) is water conductivity and ks = 2.4 W/(m⋅K) is the conductivity of dry sediments.
Hyndman and Davis [43] provide a formula for porosity P:
P = P0 ⋅ exp [−(z − z0)/L]
where P0 = 0.69, L = 1500 m and the results agree with the data by Von Huene and School [44] and by Erikson
and Pindell [45].
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Fig. 2. Sea bottom temperatures (°C) in the Black Sea (both in the near-bottom seawater and sediments measurements are considered). Input
data collected from the archive of Institute of Oceanology, department “Physics of the sea”, Dr. D. Trukhchev and citations in the references
[13, 14, 33–35, 37–42]. Black circles show station’ locations and in many cases — group of stations.
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Fig. 3. The Black Sea heat flow (mW/m2) at the seafloor. Input data collected from citations in the references [13, 14, 33–35, 37–42]. Stations
are shown with small black circles.
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Methane hydrate stability zone. Methane hydrates start to form in the Black Sea at water depths from 600 m.
Three isolated zones with initial depths of about 550–570 m were localized in the central parts of the Danube and
Kuban’ fans and at the beginning of the Arkhangel’sky Ridge. They are caused by extremely low heat flow there.
The planar area of the Black Sea suitable for gas hydrate formation was calculated to be 288,080 square
kilometers, i.e., about 68.5% of the total Black Sea area (420,800 km2) or almost 91% of the deep waters (Fig. 5).
The surface area of the sea floor possibly embraced by gas hydrates was estimated to be 295,080 to 295,276 square
kilometers.
The thickness of the Methane Hydrate Stability Zone (MHSZ) was calculated for every single point of data
grid using two methods for predicting variation of the temperatures in depth of the sediment cover: the Gornitz
and Fung [8] relation and in situ measured thermal gradients. From the initial small thickness it gradually reaches
average value of about 160 m at 1,000 m water depth running from 80 to more than 300 m. At depth of 1,500 m
it averages about 260 m (from 110 to 650 m) and at 2,000 m is about 350 m. The MHSZ thicknesses vary about
450–500 m in the deepest part of the Black Sea basin at water depth of about 2,240 m. A very good coincidence
is observed in hole 379 of DSDP between supposed bottom of the HSZ at 450 m below seafloor and the calculated
thickness by in situ measured thermal gradients of 454 m while those obtained by Gornitz and Fung [8] relation
is few tens of meters higher.
Two areas of maximum thickness of the HSZ (Fig. 5) are clearly identified: the large one occurs in the
middle of Danube fan where it reaches up to 865 m and the second is in front of Caucasus coast where the
thickness exceeds 1,000 m at several points. The average MHSZ thickness for the whole Black Sea is 356 and
303 m according to corresponding method of calculation.
The sediment volume of the Black Sea hydrate stability zone was estimated to be in the range of 85,310 to
100,280 cubic kilometers.
Hydrate content within sediments. The content of gas hydrates within sediments is one of the most important
parameters in evaluating their quantity and at the same time it is the most uncertain and highly variable (from
around zero up to 100%) as laterally as in depth and is difficult for determination.
It could be concluded from descriptions of the Black Sea hydrate samples that they occur in massive to
nodular form (according to Sloan classification [46]) in areas of gas migration paths and from disseminated to
nodular in areas of in situ gas formation. They show a heterogeneous distribution pattern in both cases and comprise
on average 2–4% of the sediment by volume with peak up to 10% [18]. It was roughly accepted that gas hydrates
uniformly fill pore space of the Hydrate Stability Zone and represent 3.5% of the total sediment volume all over
the Black Sea. This value is in correspondence with recently obtained data in the Gulf of Mexico — 4.6% and
Nigeria delta — 2.5% [47], on the Blake Ridge and Cascadia Margin — 3.3–5.8% [48].
Generally, the methane content in the marine sediments increases linearly with depth in the areas with
biochemical origin of the gas and roughly follows a distribution of bulged exponential function in areas of migrated
thermogenic gas from deep sources [49]. This suggests that hydrate contents in the pore space behave similarly
in the areas with similar gas sources and the supposed distributions can be applied to calculation at every single
point of the data grid.
In cases of biogenic origin of the gas, e.g., in submarine fans, the hydrate content was assumed to be 5%
of the sediment porosity near sea bottom, gradually increasing up to 45% at the base of the HSZ. In the areas of
thermogenic gas it exponentially increased with depth from about 1% to 7–10% and then more smoothly up to
60%. Similar distribution in particular, is reported on the Cascadia continental slope [50] in the Nankai Trough [51].
It should be noted that all the Black Sea hydrate samples have been found below the sea floor, usually more
than 0.4 m depthward [9]. This could have different explanations, in particular, that the Holocene deposits are not
favorable for hydrate formation (very small dimensions of the pore space; low gas content; seasonal temperature
variations; etc.) and do not contain hydrates at all. For this reason the first 50 centimeters of sediment cover
representing 145 km2 of the HSZ were eliminated from calculations as empty ones.
RESULTS
Observations in the Black Sea and worldwide show that about 10% of the gas hydrate stability zone may
actually contain hydrates [9].
These values can be used for approximate estimation of the hydrate content on the assumption that about
3.5% of the obtained volume are pure hydrate, which gives about (0.3−0.35)⋅1012 cubic meters of hydrate.
Multiplying this value by the commonest gas-expanding factor of 140 yields (42−49)⋅1012 cubic meters of methane
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Fig. 4. The Black Sea geothermal gradients (mK/m). Input data are taken from [13, 14, 33–35, 37–42]. Stations are shown with small black circles.
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Fig. 5. The Black Sea hydrate prone area and calculated thickness (m) of methane hydrate stability zone based on in situ measured
temperature gradients.
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existing in the Black Sea sediments in the form of hydrate. This volume is close to the estimation made by
Korsakov et al. [11] ((40−50)⋅1012 cubic meters).
A more precise approach is to quantify hydrates in the most promising areas based on their distribution
according to gas origin. The promising areas (about 100,800 km2) represent nearly 35% of the whole Black Sea
prone area or about 30% of MHSZ (25,600–30,000 km3). But even in these areas hydrates occur occasionally, in
discrete accumulations. Not dwelling on details of the methodology of estimation of the reserves of hydrates and
gas, we mention only that our estimates appeared to be four times lower: (77−90)⋅109 m
3 hydrate and
(10.1−12.6)⋅1012 m
3 methane, respectively.
CONCLUSIONS
Using a database of 6-minute grid we have defined parameters of the Black Sea Methane Hydrate Stability
Zone such as: prone area, initial water depths of hydrate formation, and thickness of methane hydrate stability
zone. Judging by the values obtained at the DSDP site 379 checkpoint the approach of in situ measured temperature
gradients gives more precise results than the calculation using the Gornitz and Fung equation [8] although the
latter is theoretically more valid.
The hydrate quantity and methane content within them were evaluated generally as percentage of the volume
of hydrate stability zone in the range of (300−350)⋅109 m3 ((42−49)⋅1012 of gas methane) and more accurately by
defining prospect areas and specific distribution of hydrate in pore space to (77−90)⋅109 m
3 of pure hydrate
((10.1−12.6)⋅1012 m
3 of gas respectively) with preferences tending to the lower limit.
Acknowledgements. The authors are grateful to Dr. Gabriel Ginsburg, who gave impetus to this work and
to whom it is dedicated. He will stay in our memory as an excellent scientist with enormous erudition, a splendid
man and creative optimist.
We thank Dr. D. Trukhchev for kindly given data about water temperatures at the sea floor of the Black Sea
and useful observations made.
The GASHYDRAT, an European Commission MAST III funded project with coordinator Prof. Jean Klerkx
is acknowledged for financial support to present the topics of this work at the 6th Conference “Gas in Marine
Sediments” in St. Petersburg on September, 2000.
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Received 2 March 2001
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