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Geological carbon flows

Posted on: August 27, 2019


Seeps Give a Peek Into Plumbing



“Oil and gas production lowers the pressure that forces out the natural hydrocarbon seeps. Without oil and gas production, the seepage rate will increase and undermine the apparent advantage to the climate of not producing oil and gas.”








  1. The theory of Anthropogenic global warming and climate change (AGW) rests entirely on the ability of the combustion of fossil fuels by the industrial economy to change atmospheric composition.
  2. Statistical analysis of emissions and atmospheric CO2 data are presented in related posts [LINK]  [LINK] . These tests of the hypothesis that changes in atmospheric CO2 must be understood in terms of fossil fuel emissions of the industrial economy fail to show the assumed relationship. The data do not show that changes in atmospheric composition are driven by fossil fuel emissions.
  3. A further argument in climate science for human cause is that the source of the carbon causing atmospheric CO2 to rise has to be fossil fuels because this carbon id devoid  of the 14C and 13C isotopes. It is noted that this argument does not exclude geological carbon because that carbon also devoid of 13C and 14C isotopes. It is not possible to distinguish fossil fuel carbon from geological carbon. 
  4. It is proposed therefore that natural flows of CO2 to the atmosphere that may have been missed or underestimated in carbon cycle dynamics must be reconsidered specifically the known carbon flows having to do with out-gassing from oceans, geological flows to the oceans, and geological flows directly to the atmosphere. Although eruptions of land volcanoes do introduce significant quantities of carbon both as hydrocarbons and as carbon dioxide into the atmosphere, these eruptions are rare and therefore their effect on atmospheric composition cannot be interpreted in terms of the short time scale required by the theory of AGW.
  5. Here we present more continuous sources of natural geological flows in terms of submarine volcanic activity, submarine mud volcanoes, and both submarine and terrestrial natural seeps of hydrocarbons both oil and natural gas. A bibliography of sources used in this assessment is listed below. Of particular note is the Barber 1986 paper in paragraph#8 as it relates to detailed geological processes that create these hydrocarbon flows. An extensive presentation on mud volcanoes is published online by Science Direct [LINK] . These papers are presented in a separate section below labeled as “Mud Volcano Research”.
  6. These sources indicate significant flows of hydrocarbons and carbon dioxide, both from fossil fuels and from inorganic geological carbon. The following quantitative estimates are taken from the bibliography presented below.
  7. Of hydrocarbon flows to the surface, approximately half are spills and leakages from oil and gas production with the other half being natural flows and seepages.
  8. “seepage rates of 2.2–30 m3 1000 km−2 d−1 and 1.4–18 m3 1000 km−2 d−1, respectively. Generalizing to an annual rate suggests that total natural seepage in this region is of the order of at least 20,000 m3 yr−1 (120,000 barrels yr−1)” (MacDonald 1993)
  9. The estimated total number of known and inferred deep-water mud volcanoes is 10,000 to 100,000  (Milkov 2000).
  10. Size of mud volcanoes. “They occur in an upper slope environment seen as 1–2 km circular features at the seabed.  Graue (2000).
  11. A second approach involved broad assumptions based on considerations of the availability of methane for seepage from known global geological sources.about 50 Tg/yr of methane seeps from the seabed and about 30 Tg/yr of methane reaches the atmosphere. A second approach involved broad assumptions based on considerations of the availability of methane for seepage from known global geological sources through geologic time.The methods used in this approach were extrapolated for natural gas from two earlier research studies dealing with crude oil [Kvenvolden and Harbaugh, 1983; Miller, 1992]. Preliminary assumptions were made that need further refinement. The total geologic reservoir of methane was estimated to be between 104 and 108Tg, while the half-life of the total geologic methane reservoir was set at 108 yr.With these and other assumptions concerning the length of time for reservoir depletion (108-1012 yr),the flux of methane was estimated to be about 30 Tg/yr from the seabed and about 10 Tg/yr into the atmosphere.Thus, the results from this theoretical approach and the observational approach were similar within a factor of three. If the atmospheric input of geologically sourced methane is about 20 Tg/yr  (Kvenvolden 2001)
  12. The manifestation of continuous gas migration through mud volcanoes is clarified, and the attempt made to quantify the gas release shows that mud volcanoes are one of the significant natural sources of atmospheric methane emitting a total amount of about 10.3 to 12.6 Tg annually.  (Dimitrov 2002)
  13. Recent global estimates of crude-oil seepage rates suggest that about 47% of crude oil currently entering the marine environment is from natural seeps, whereas 53% results from leaks and spills during the extraction, transportation, refining, storage, and utilization of petroleum. The amount of natural crude-oil seepage is currently estimated to be 600,000 metric tons per year, with a range of uncertainty of 200,000 to 2,000,000 metric tons per year.  (Kvenvolden 2003)
  14. With a carbon store of over 1,200 Pg, the Arctic geologic methane reservoir is large when compared with the global atmospheric methane pool of around 5 Pg. As such, the Earth’s climate is sensitive to the escape of even a small fraction of this methane. Here, we document the release of 14C-depleted methane to the atmosphere from abundant gas seeps concentrated along boundaries of permafrost thaw and receding glaciers in Alaska and Greenland, using aerial and ground surface survey data and in situ measurements of methane isotopes and flux. We mapped over 150,000 seeps. (Anthony 2012)
  15. CONCLUSION: We conclude from the information presented in the bibliography and the analysis above, that there are significant natural flows of carbon from geological sources such as hydrocarbon seeps, methane hydrates, submarine volcanism, and submarine mud volcanoes and that these flows make it difficult to interpret changes in atmospheric CO2 exclusively in terms of the use of fossil fuels in the industrial economy.  In that context, it should also be noted that the bibliography below shows that oil and gas production lowers the pressure that forces out the natural hydrocarbon seeps. Without oil and gas production, the seepage rate will increase and undermine the apparent advantage to the climate of not producing oil and gas. It is noted that humans began using fossil fuels from seeps and natural outflows. It was only after its utility became obvious that seeps were no longer sufficient. It was then that humans began to look for the sources of those seeps. 


  1. Hedberg, Hollis D. “Relation of methane generation to undercompacted shales, shale diapirs, and mud volcanoes.” AAPG Bulletin 58.4 (1974): 661-673. Organic matter commonly makes up a substantial part of freshly deposited muds. During postdepositional history, much of this organic matter is decomposed by biochemical and thermochemical processes, with accompanying generation of methane within the sediments. Likewise during postdepositional history, weight of overburden brings about compaction of mud sediments through expulsion under pressure of interstitial pore water. However, at the same time as pressure on the fluid is tending to increase because of compaction and other factors, the permeability of the sediment tends to be reduced by closer packing of clay particles. Thus it may happen not infrequently that the escape of fluids from a mud or shale body may fail temporarily to keep up with the causes tending to increase fluid pressure, and in such cases these bodies will become overpressured and undercompacted with respect to surrounding sediments. The generation of methane gas within these mud and shale bodies is an important added factor which tends further to accentuate, or even create, their overpressured, undercompacted state, both by building up additional internal pressure, and also by further impeding fluid expulsion because of the development of a second phase (gas) in the pore fluid. Because of the unstable, semifluid nature of methane-charged undercompacted muds or shales, they frequently find expression in mud or shale diapirs or in mud volcanoes. The importance to petroleum exploration of a causal relation between methane generation and undercompacted shales, shale diapirs, and mud volcanoes is principally that these features thus become indicators of areas or intervals where hydrocarbons have been, or are, actively originating. Although the nearby area of these overpressured features may be unfavorable for petroleum accumulation because of a lack of extensive reservoirs, the surrounding facies, above, below, and laterally adjacent, may be particularly favorable in their relation to both reservoir and source environments.
  2. Bernard, Bernie B., James M. Brooks, and William Malcolm Sackett. “Natural gas seepage in the Gulf of Mexico.” Earth and Planetary Science Letters 31.1 (1976): 48-54.  Hydrocarbon compositions and δ13C values for methane of fourteen natural seep gases and four underwater vents in the northwestern Gulf of Mexico are reported. The C1/(C2 + C3) ratios of the seep gas samples ranged from 68 to greater than 1000, whereas δPDB13C values varied from −39.9 to −65.5‰. Compositions suggest that eleven of the natural gas seeps are produced by microbial degradation whereas the remaining three have a significant thermocatalytically produced component. Contradictions in the inferences drawn from molecular and isotopic compositions make strict interpretation of the origins of a few of the samples impossible.
  3. Bernard, Bernie, James M. Brooks, and William M. Sackett. “A geochemical model for characterization of hydrocarbon gas sources in marine sediments.” Offshore Technology Conference. Offshore Technology Conference, 1977.  A geochemical model has been developed to distinguish biogenic from petroleum-related gas in marine sediments. Analyses of seep and sediment gases from the Gulf of Mexico indicate that the characteristic composition of pathogenic gas can be altered during migration through sediments and by mixing with biogenic gas. Measurements of carbon isotopic ratios of methane and hydrocarbon ratios generally provide a definitive interpretation of hydrocarbon gas sources. It is suggested that prospecting for reservoir hydrocarbons include coring on or near observed geologic features to first establish the source of existing hydrocarbon gas in a region.
  4. Levy, E. M., and M. Ehrhardt. “Natural seepage of petroleum at Buchan Gulf, Baffin Island.” Marine Chemistry 10.4 (1981): 355-364.  Analyses by fluorescence spectrophotometry and computerized gas chromatography/mass spectrometry of the hydrocarbons present in surficial bottom sediments from Buchan Gulf, Baffin Island established that natural seepage of petroleum is occurring from the seabed. In addition, compounds of recent biosynthetic origin and from atmospheric fall-out of combustion products were identified.
  5. Philp, R. P., and P. T. Crisp. “Surface geochemical methods used for oil and gas prospecting—a review.” Journal of Geochemical Exploration 17.1 (1982): 1-34.The majority of the world’s oil and gas deposits have been discovered by drilling in the vicinity of natural petroleum seeps, and to date the most successful geochemical prospecting methods still rely upon the surface detection of hydrocarbons. Gas chromatographic techniques are now commonly used in the analysis of hydrocarbon gases for prospecting both onshore (analysis of soils and rocks) and offshore (analysis of near-bottom waters and sediments). Detection of helium fluxes has been partially successful as a geochemical prospecting technique. Many indirect techniques such as the determination of isotope and metal-leaching anomalies in surface rocks and the measurement of radon fluxes have not been widely used. Onshore geochemical prospecting appears to have more problems associated with it than offshore prospecting due to the more complex migration mechanism of near-surface waters containing dissolved gases. No onshore prospecting studies have been published which thoroughly consider this factor and the success of onshore prospecting remains equivocal. In offshore prospecting “sniffers” have been used to detect hydrocarbon anomalies in near-bottom waters, and coring equipment has been used for the detection of hydrocarbons in near-surface sediments. Success is claimed using these techniques. Geochemical prospecting methods are complementary to the widely used geophysical methods. Geochemical methods can provide direct evidence for the presence of petroleum accumulations and are relatively cheap and rapid. Failures in prospecting to date are attributable to the simplistic manner in which data have been interpreted; insufficient attention has been paid to the hydrological and geological factors which modify the upward migration of indicator species to the surface. As oil and gas deposits become more difficult to locate, greater attention should be paid to geochemical prospecting techniques, especially as a regional exploration tool.
  6. Westbrook, G. K., and M. J. Smith. “Long decollements and mud volcanoes: Evidence from the Barbados Ridge Complex for the role of high pore-fluid pressure in the development of an accretionary complex.” Geology 11.5 (1983): 279-283.  Mud volcanoes in front of the Barbados Ridge accretionary complex, discovered in multichannel seismic reflection profiles, provide evidence of very high pore-fluid pressures in the sedimentary cover on the oceanic lithosphere that is being subducted beneath the Lesser Antilles island arc. These high pore-fluid pressures can be attributed to the load imposed on the sedimentary cover by the weight of the advancing accretionary wedge. They offer a mechanical explanation for the very wide decollement that separates the Barbados Ridge Complex accretionary wedge from undeformed sediments on the oceanic lithosphere beneath it. Changes in level of this decollement may be induced by local variations in pore-fluid pressure.
  7. Brooks, James M., et al. “Hydrates, oil seepage, and chemosynthetic ecosystems on the Gulf of Mexico slope.” Eos, Transactions American Geophysical Union 66.10 (1985): 106-106.  The northern Gulf of Mexico continental shelf has been the site of a number of recent discoveries that may dramatically alter our understanding of biological and chemical processes on the continental slope. The Geochemical and Environmental Research Group at Texas A&M University reported in Science last year (vol. 225, p. 409, 1984) the first occurrence of thermogenic gas hydrates in deep ocean sediments. The hydrate discovery sites were located at water depths of 530‐560 m on the Louisiana slope. These hydrates, obtained by piston coring, ranged in size from minute crystals to objects several centimeters in diameter and were composed of methane through butane hydrocarbon gases. The hydrates were often dispersed in carbonate rubble within the cores and were distributed from the top of the core to a sediment depth of at least several meters.
  8. Barber, A. J., S. Tjokrosapoetro, and T. R. Charlton. “Mud volcanoes, shale diapirs, wrench faults, and melanges in accretionary complexes, eastern Indonesia.” AAPG Bulletin70.11 (1986): 1729-1741. In Timor, eastern Indonesia, where the northern margin of the Australian continent is colliding with the Banda Arc, Australian continental margin sediments are being incorporated into an imbricate wedge, which passes northward into a foreland fold and thrust belt. Field mapping in Timor has shown that scaly clays, containing irregularly shaped or phacoidal blocks (up to several meters long) and composed of a wide range of lithologies derived from local stratigraphic units, occur in three environments: along wrench faults, as crosscutting shale diapirs, and associated with mud volcanoes. A model is proposed linking these phenomena. Shales become overpressured as a result of overthrusting; this overpressure is released along vertical wrench faults, which cut through the ove thrust units; overpressured shales containing blocks of consolidated units rise along the fault zones as shale diapirs; and escaping water, oil, and gas construct mud volcanoes at the surface. An extensive melange deposit in Timor, the Bobonaro Scaly Clay, has been interpreted by Audley-Charles and subsequent workers as an olistostrome. Our study interprets the Bobonaro Scaly Clay as the product of shale diapirism. Shale diapirs are likely to be generated wherever water-saturated sediments are incorporated into accretionary complexes. Diapirs have not, however, been recorded as occurring in ancient accretionary complexes, although melanges, usually interpreted as olistostromes, are commonly reported. Criteria are proposed by which the products of shale diapirism might be recognized, including: (1) shape of the diapiric body; (2) its relationship to the surrounding rocks; (3) nature of the matrix and of the enclosed blocks; (4) shapes of the blocks and their relationship to he matrix; and (5) effects of deformation. The importance of shale diapirism in the formation of melanges has not been fully appreciated, and the products of shale diapirism constitute a major component of ancient and present-day accretionary complexes. (blog author’s note: shale diapirism generates methane flows.)
  9. Brooks, James M., et al. “Association of gas hydrates and oil seepage in the Gulf of Mexico.” Organic Geochemistry 10.1-3 (1986): 221-234.  Gas hydrates were recovered from eight sites on the Louisiana slope of the Gulf of Mexico. The gas hydrate discoveries ranged in water depths from 530 to 2400 m occurring as small to medium sized (0.5–50 mm) nodules, interspersed layers (1–10 mm thick) or as solid masses (> 150 mm thick). The hydrates have gas:fluid ratios as high as 170:1 at STP, C1/(C2 + C3) ratios ranging from 1.9 to > 1000 and δ13C ratios from −43 to −71‰. Thermogenic gas hydrates are associated with oil-stained cores containing up to 7% extractable oil exhibiting moderate to severe biodegradation. Biogenic gas hydrates are also associated with elevated bitumen levels (10–700 ppm). All gas hydrate associated cores contain high percentages (up to 65%) of authigenic, isotopically light carbonate. The hydrate-containing cores are associated with seismic “wipeout” zones indicative of gassy sediments. Collapsed structures, diapiric crests, or deep faults on the flanks of diapirs appear to be the sites of the shallow hydrates.
  10. Brooks, James M., et al. “Hydrates, oil seepage, and chemosynthetic ecosystems on the Gulf of Mexico slope: An update.” Eos, Transactions American Geophysical Union68.18 (1987): 498-499.  In 1985, the Geochemical and Environmental Research Group (GERG) at Texas A&M University (College Station, Tex.) reported in Eos [Brooks et al, 1985] the discovery of chemosynthetic organisms (bivalves and tube worms) at two sites on the Gulf of Mexico continental slope. The presence of gas hydrates at five sites, some associated with oil‐stained sediments, was also detailed. In the subsequent year, follow‐up cruises and submersible dives (using the Johnson Sea‐Link, owned and operated by the Harbor Branch Foundation, Fort Pierce, Fla., and the Navy NR‐1) investigated the physiology, biochemistry, and distribution of these chemosynthetic organisms.
  11. Kennicutt, M. E., J. M. Brooks, and R. A. Burke Jr. “Hydrocarbon seepage, gas hydrates, and authigenic carbonate in the northwestern Gulf of Mexico.” Offshore Technology Conference. Offshore Technology Conference, 1989.  Rapid deposition of organic rich sediments onto thick Jurassic salt deposits created conditions conducive to the formation and entrapment of large volumes of oil and gas as well as active salt diapirism. Resulting hydrocarbon seepage, brine seepage, hydrate formation and decomposition, methane oxidation, oil degradation, and authigenic carbonate precipitation have significantly effected the geology, geochemistry, and morphology of the continental slope of the northern Gulf of Mexico. Regional topography is largely controlled by salt diapirism and slumping, and is characterized by pock marks, mud volcanoes, disrupted sediments, and large accumulations of carbonate. Sediment chemistry is driven by the influx of oil, gas and brine seepage and secondarily altered by the enhanced microbial and benthic biology.
  12. MacDonald, IanR, et al. “Natural oil slicks in the Gulf of Mexico visible from space.” Journal of Geophysical Research: Oceans98.C9 (1993): 16351-16364.  Natural oil seepage in the Gulf of Mexico causes persistent surface slicks that are visible from space in predictable locations. A photograph of the sun glint pattern offshore from Louisiana taken from the space shuttle Atlantis on May 5, 1989, shows at least 124 slicks in an area of about 15,000 km2; a thematic mapper (TM) image collected by the Landsat orbiter on July 31, 1991, shows at least 66 slicks in a cloud‐free area of 8200 km2 that overlaps the area of the photograph. Samples and descriptions made from a surface ship, from aircraft, and from a submarine confirmed the presence of crude oil in floating slicks. The imagery data show surface slicks near eight locations where chemosynthetic communities dependent upon seeping hydrocarbons are known to occur on the seafloor. Additionally, a large surface slick above the location of an active mud volcano was evident in the TM image. In one location the combined set of observations confirmed the presence of a flourishing chemosynthetic community, active seafloor oil and gas seepage, crude oil on the sea surface, and slick features that were visible in both images. We derived an analytical expression for the formation of floating slicks based on a parameterization of seafloor flow rate, downstream movement on the surface, half‐life of floating oil, and threshold thickness for detection. Applying this equation to the lengths of observed slicks suggested that the slicks in the Atlantis photograph and in the TM image represent seepage rates of 2.2–30 m3 1000 km−2 d−1 and 1.4–18 m3 1000 km−2 d−1, respectively. Generalizing to an annual rate suggests that total natural seepage in this region is of the order of at least 20,000 m3 yr−1 (120,000 barrels yr−1).
  13. MacDonald, I. R., et al. “Gas hydrate that breaches the sea floor on the continental slope of the Gulf of Mexico.” Geology22.8 (1994): 699-702.  We report observations that concern formation and dissociation of gas hydrate near the sea floor at depths of ∼540 m in the northern Gulf of Mexico. In August 1992, three lobes of gas hydrate were partly exposed beneath a thin layer of sediment. By May 1993, the most prominent lobe had evidently broken free and floated away, leaving a patch of disturbed sediment and exposed hydrate. The underside of the gas hydrate was about 0.2 °C warmer than ambient sea water and had trapped a large volume of oil and free gas. An insitu monitoring device, deployed on a nearby bed of mussels, recorded sustained releases of gas during a 44 day monitoring period. Gas venting coincided with a temporary rise in water temperature of 1 °C, which is consistent with thermally induced dissociation of hydrate composed mainly of methane and water. We conclude that the effects of accumulating buoyant force and fluctuating water temperature cause shallow gas hydrate alternately to check and release gas venting.
  14. Sassen, Roger, et al. “Bacterial methane oxidation in sea-floor gas hydrate: significance to life in extreme environments.” Geology 26.9 (1998): 851-854.  Samples of thermogenic hydrocarbon gases, from vents and gas hydrate mounds within a sea-floor chemosynthetic community on the Gulf of Mexico continental slope at about 540 m depth, were collected by research submersible. Our study area is characterized by low water temperature (mean =7 °C), high pressure (about 5400 kPa), and abundant structure II gas hydrate. Bacterial oxidation of hydrate-bound methane (CH4) is indicated by three isotopic properties of gas hydrate samples. Relative to the vent gas from which the gas hydrate formed, (1) methane-bound methane is enriched in 13C by as much as 3.8‰ PDB (Peedee belemnite), (2) hydrate-bound methane is enriched in deuterium (D) by as much as 37‰ SMOW (standard mean ocean water), and (3) hydrate-bound carbon dioxide (CO2) is depleted in 13C by as much as 22.4‰ PDB. Hydrate-associated authigenic carbonate rock is also depleted in 13C. Bacterial oxidation of methane is a driving force in chemosynthetic communities, and in the concomitant precipitation of authigenic carbonate rock that modifies sea-floor geology. Bacterial oxidation of hydrate-bound methane expands the potential boundaries of life in extreme environments.
  15. Heggland, Roar. “Gas seepage as an indicator of deeper prospective reservoirs. A study based on exploration 3D seismic data.” Marine and Petroleum Geology 15.1 (1998): 1-9.  Three periods of sustained gas seepage in geological time have been revealed in Danish block 5604/26 in the North Sea by the use of exploration 3D seismic data. The most recent period is indicated by a cluster of seismic chimneys which ties in to buried craters near the seabed, and possible present gas escape through the seabed, along with amplitude anomalies indicating a shallow gas sand charged by gas migrating from a deeper level. The cluster of seismic chimneys indicative of vertical gas migration is visible down to 1.5 s TWT (1500 meters), and therefore the gas is interpreted as migrating from a deeper stratigraphic horizon. Below this level it becomes difficult to see the chimneys due to complex faulting. The faults may work as gas migration pathways. The geometry of the cluster of seismic chimneys indicates that the gas has been migrating from one point. The nearest possible source of the gas is an underlying prospect where an oil and gas discovery has been made. Two earlier periods of gas seepage are indicated by mounds, possibly carbonate buildups over gas seepages in Pliocene time, and,similarly, buried craters formed by gas seepages in an earlier period in Pliocene time. The results of this study are that gas seepage is a periodical process in geological time and that its presence and associated features can be used as an indicator of deeper prospective reservoirs.
  16. Quigley, Derek C., et al. “Decrease in natural marine hydrocarbon seepage near Coal Oil Point, California, associated with offshore oil production.” Geology 27.11 (1999): 1047-1050.  Prolific natural hydrocarbon seepage occurs offshore of Coal Oil Point in the Santa Barbara Channel, California. Within the water column above submarine vents, plumes of hydrocarbon gas bubbles act as acoustic scattering targets. Using 3.5 kHz sonar data, seep distribution offshore of Coal Oil Point was mapped for August 1996, July 1995, and July 1973. Comparison of the seep distributions over time reveals more than 50% decrease in the areal extent of seepage, accompanied by declines in seep emission volume, in a 13 km2area above a producing oil reservoir. Declines in reservoir pressure and depletion of seep hydrocarbon sources associated with oil production are the mechanisms inferred to explain the declines in seep area and emission volume.
  17. Aloisi, Giovanni, et al. “Methane-related authigenic carbonates of eastern Mediterranean Sea mud volcanoes and their possible relation to gas hydrate destabilisation.” Earth and Planetary Science Letters 184.1 (2000): 321-338.  Nautile submersible investigations of mud volcanoes and brine seep areas of the eastern Mediterranean Sea during the MEDINAUT cruise in November 1998 discovered extensive areas of authigenic carbonate crusts associated with methane emissions. Carbonate crusts form pavements, round slabs and circular mounds on the central, most active parts of mud volcanoes and in a fault-related valley where brines have accumulated to form a submarine brine lake. Authigenic carbonate nodules have been recovered from the same areas during the MEDINETH cruise in July 1999. Large 13C depletions of authigenic calcite, aragonite and dolomite indicate methane as a major carbon source for the carbonate. Crust pavements are formed when methane from a freshly emplaced, methane-charged mud flow is oxidised at the seafloor. In this environment, where bottom waters provide the sulphate and magnesium, aragonite is favoured versus calcite and accounts for the majority of the methane-related authigenic carbonates. Calcite, when present, contains significant amounts of Mg2+ (high-Mg calcite), and possibly other divalent ions in its crystal lattice. In areas of brine seep and accumulation, dolomitic nodules are present at shallow depth in the sediment. The 18O enrichment of the authigenic carbonates (up to 4‰ greater than calculated values for carbonates precipitating from modern eastern Mediterranean bottom waters) is interpreted as due to precipitation from 18O-rich fluids rather than as a temperature effect. The source of the 18O-rich fluids may be multiple and possibly includes the destabilisation of gas hydrates present at shallow subbottom depth, and the seepage of relic Messinian brines.
  18. Milkov, A. V. “Worldwide distribution of submarine mud volcanoes and associated gas hydrates.” Marine Geology167.1-2 (2000): 29-42.  The list of known and inferred submarine mud volcanoes is presented in this paper. They occur worldwide on shelves, continental and insular slopes and in the abyssal parts of inland seas. Submarine mud volcanoes are distributed on the Earth more extensively than their subaerial analogs. The estimated total number of known and inferred deep-water mud volcanoes is 103–105. There are two key reasons for the formation of submarine mud volcanoes—high sedimentation rate and lateral tectonic compression. Submarine mud volcanoes form by two basic mechanisms: (1) formation on the top of a seafloor-piercing shale diapir; (2) formation due to the rise of fluidized sediments along faults. Fluid migration is critical to the formation of a mud volcano. Gas hydrates are often associated with deep-water mud volcanoes and have many common features from one accumulation to another. Gas hydrates form by conventional low-temperature hydrothermal process around the central part of a mud volcano and by metasomatic processes at its periphery. A preliminary global estimate of methane accumulated in gas hydrates associated with mud volcanoes is about 1010–1012 m3 at standard temperature and pressure.
  19. Graue, K. “Mud volcanoes in deepwater Nigeria.” Marine and Petroleum Geology 17.8 (2000): 959-974. Detailed study of 3D seismic data from deepwater Nigeria has revealed the presence of features interpreted to be mud volcanoes. They occur in an upper slope environment seen as 1–2 km circular features at the seabed. Seabed cores from the mud volcanoes contain oil, gas and sand/shale–clast content richer than the seabed background. Pliocene fossils have been identified in the cores, demonstrating transport of material from depth. The features show a high seabed seismic amplitude above a chimney of chaotic seismic reflections and data wipe-out. The mud volcanoes in the study area shows two distinct clusters located over deeper structural culminations. Four active mud volcanoes (Area 1) are located above a rollover anticline in the central part of the area. Cuspate listric faults reach the seabed on the up-slope side of these mud volcanoes. Four abandoned mud volcanoes have also been identified, with progressively older ages towards the crest of the underlying structure. These abandoned features are associated with an extremely chaotic seismic signature. A combination of over-pressured carrier beds and low integrity top seals are believed to be responsible for the formation of these mud volcanoes. It is further believed that gas expansion, subsequent to seal failure, was the main driving force for what must have been violent eruptions. The long lived mud volcano activity over the deep structural closure suggests a plumbing system that focuses on escaping compaction water and hydrocarbons through time. In the south west of the study area, another cluster of five mud volcanoes is located above a shale diapir. The seabed expression of the members of this cluster is more varied. Some show a positive relief at the seabed while others show circular depressions. The largest feature represents an assemblage of many smaller mud volcanoes with a common root system. Their seismic expression is also different from that of Area 1, with a well-defined sediment wedge at surface and a shallower root system. These features are believed to represent a less violent type of mud volcano characterised by ductile flow. Tectonic stress, due to growth of the underlying diapir, is thought to have played an important role during eruption, in addition to focused methane escape and the low mechanical strength of the overburden.
  20. Schumacher, Dietmar “Deet. “Surface geochemical exploration for oil and gas: new life for an old technology.” The Leading Edge 19.3 (2000): 258-261.  Surface geochemical exploration for petroleum is the search for chemically identifiable surface or near-surface occurrences of hydrocarbons, or hydrocarbon-induced changes, as clues to the location of oil and gas accumulations. It extends through a range of observations from clearly visible oil and gas seepage (macroseepage) at one extreme to identification of minute traces of hydrocarbons (microseepage) or hydrocarbon-induced changes at the other.
  21. Sassen, Roger, et al. “Thermogenic vent gas and gas hydrate in the Gulf of Mexico slope: Is gas hydrate decomposition significant?.” Geology 29.2 (2001): 107-110.  Samples of vent gas and gas hydrate on the Gulf of Mexico slope were collected by research submersible (∼540 m water depth) and by piston coring (∼1060–1070 m water depth). Although gas hydrate that crops out is transiently unstable, the larger volume of structure II gas hydrate in the gulf is stable or increasing in volume because gas from the subsurface petroleum system is venting prolifically within the gas hydrate stability zone. Vent gas from gas hydrate shows no meaningful molecular evidence of gas hydrate decomposition. Gas hydrate fabrics, mainly vein fillings, are typical of ongoing crystallization. Once crystallized, most hydrocarbons are protected from bacteria within the crystal lattice of gas hydrate. A leaky petroleum system is proposed to be the main source of thermogenic greenhouse gases in the central gulf. Stable gas hydrate sequesters large volumes of greenhouse gases, suggesting that gas hydrate may not be a significant factor in models of climate change at present.
  22. Kvenvolden, Keith A., Thomas D. Lorenson, and William S. Reeburgh. “Attention turns to naturally occurring methane seepage.” Eos, Transactions American Geophysical Union82.40 (2001): 457-457.  Methane is the most abundant organic compound in the Earth’s atmosphere. As a powerful greenhouse gas, it has implications for global climate change. Sources of methane to the atmosphere are varied. Depending on the source, methane can contain either modern or ancient carbon. Methane exiting from swamps and wetlands contains modern carbon, whereas methane leaking from petroleum reservoirs contains ancient carbon. The total annual source of methane to the atmosphere has been constrained to about 540 teragrams (Tg) per yearCicerone and Oremland, 1988”. Notably absent from any identified sources is the contribution of geologically sourced methane from naturally occurring seepage. Two approaches to making global estimates of geological methane seepage rates were used.The first was to compile existing information on the local, regional, and global seepage rates that have already been estimated and reported. This kind of information is currently scattered in the scientific literature, but consensus estimates were obtained by synthesis of data in hand. Based on this approach, it was estimated that about 50 Tg/yr of methane seeps from the seabed and about 30 Tg/yr of methane reaches the atmosphere. A second approach involved broad assumptions based on considerations of the availability of methane for seepage from known global geological sources through geologic time.The methods used in this approach were extrapolated for natural gas from two earlier research studies dealing with crude oil [Kvenvolden and Harbaugh, 1983; Miller, 1992]. Preliminary assumptions were made that need further refinement. The total geologic reservoir of methane was estimated to be between 104 and 108Tg, while the half-life of the total geologic methane reservoir was set at 108 yr.With these and other assumptions concerning the length of time for reservoir depletion (108-1012 yr),the flux of methane was estimated to be about 30 Tg/yr from the seabed and about 10 Tg/yr into the atmosphere.Thus, the results from this theoretical approach and the observational approach were similar within a factor of three. If the atmospheric input of geologically sourced methane is about 20 Tg/yr—the average of the observational and theoretical approaches—then the magnitude of the estimate is significant and should be included in the inventory of annual methane release rates from identified sources [Cicerone and Oremland, 1988].The participants recognize that these global estimates are first approximations to be refined as more information is obtained.
  23. Dimitrov, Lyobomir I. “Mud volcanoes—the most important pathway for degassing deeply buried sediments.” Earth-Science Reviews 59.1-4 (2002): 49-76.  This paper discusses the nature of the phenomenon of “mud volcanism” with respect to degassing of deeply buried sediments. Mud volcanoes are defined as geological structures and their main elements are described. Based on the nature of activity, mud volcanoes are grouped in to three main types and the relationship between each type and corresponding morphological expression is discussed. The presented up-to-date data of the world geographical distribution of mud volcanoes show that they approximate to 1800 individuals. A detail overview of geological environments in which they occur helps to deduce some necessary conditions for mud volcano formation. The basic mechanisms of mud volcano formation are discussed, specifically the fluid-pressure hypothesis, and some triggering events are identified. The most common investigation approach and some criteria for recognizing of a submarine mud volcano on side-scan sonar records and seismic sections are given. Mud volcanism in the Mediterranean Ridge (an accretionary complex) and Black Sea Abyssal Plane (a back ark basin with tectonic regime of extension) are described as case studies to show variety in morphology and common factors in development of mud volcanoes. The manifestation of continuous gas migration through mud volcanoes is clarified, and the attempt made to quantify the gas release shows that mud volcanoes are one of the significant natural sources of atmospheric methane emitting a total amount of about 10.3 to 12.6 Tg annually.
  24. Kholodov, V. N. “Mud volcanoes, their distribution regularities and genesis: communication 1. Mud volcanic provinces and morphology of mud volcanoes.” Lithology and Mineral Resources 37.3 (2002): 197-209.  The article discusses regularities in the distribution of mud volcanoes and characterizes most important mud volcanic provinces of the world. A new morphogenetic classification of mud volcanoes substantiated by results of their study in the Crimean–Caucasian and West Turkmenian regions is proposed.
  25. Judd, A. G., et al. “The geological methane budget at continental margins and its influence on climate change.” Geofluids 2.2 (2002): 109-126.  Geological methane, generated by microbial decay and the thermogenic breakdown of organic matter, migrates towards the surface (seabed) to be trapped in reservoirs, sequestered by gas hydrates or escape through natural gas seeps or mud volcanoes (via ebullition). The total annual geological contribution to the atmosphere is estimated as 16–40 Terragrammes (Tg) methane; much of this natural flux is ‘fossil’ in origin. Emissions are affected by surface conditions (particularly the extent of ice sheets and permafrost), eustatic sea‐level and ocean bottom‐water temperatures. However, the different reservoirs and pathways are affected in different ways. Consequently, geological sources provide both positive and negative feedback to global warming and global cooling. Gas hydrates are not the only geological contributors to feedback. It is suggested that, together, these geological sources and reservoirs influence the direction and speed of global climate change, and constrain the extremes of climate.
  26. Planke, S., et al. “Mud and fluid migration in active mud volcanoes in Azerbaijan.” Geo-Marine Letters 23.3-4 (2003): 258-268.  Mud volcanic eruptions in Azerbaijan normally last for less than a few hours, and are characterized by vigorous extrusion of mud breccias, hydrocarbon gases, and waters. Recent fieldwork and mapping on four active mud volcanoes show that dormant period activity ranges from quiet to vigorous flow of mud and fluids. Geochemical analyses of expelled waters show a wide range in solute concentrations, suggesting the existence of a complex plumbing system. The mud and fluids have a deep origin, but are sometimes stored in intermediate-depth mud chambers. A mixing model between deep-seated saline waters and shallow meteoric water is proposed.
  27. Kvenvolden, K. A., and C. K. Cooper. “Natural seepage of crude oil into the marine environment.” Geo-Marine Letters23.3-4 (2003): 140-146.  Recent global estimates of crude-oil seepage rates suggest that about 47% of crude oil currently entering the marine environment is from natural seeps, whereas 53% results from leaks and spills during the extraction, transportation, refining, storage, and utilization of petroleum. The amount of natural crude-oil seepage is currently estimated to be 600,000 metric tons per year, with a range of uncertainty of 200,000 to 2,000,000 metric tons per year. Thus, natural oil seeps may be the single most important source of oil that enters the ocean, exceeding each of the various sources of crude oil that enters the ocean through its exploitation by humankind.
  28. Olu-Le Roy, Karine, et al. “Cold seep communities in the deep eastern Mediterranean Sea: composition, symbiosis and spatial distribution on mud volcanoes.” Deep Sea Research Part I: Oceanographic Research Papers 51.12 (2004): 1915-1936.  Two mud volcano fields were explored during the French–Dutch MEDINAUT cruise (1998) with the submersible NAUTILE, one south of Crete along the Mediteranean Ridge at about 2000 m depth (Olimpi mud field) and the other south of Turkey between 1700 and 2000 m depth (Anaximander mud field) where high methane concentrations were measured. Chemosynthetic communities were observed and sampled on six mud volcanoes and along a fault scarp. The communities were dominated by bivalves of particularly small size, belonging to families commonly found at seeps (Mytilidae, Vesicomyidae, Thyasiridae) and to Lucinidae mostly encountered in littoral sulfide-rich sediments and at the shallowest seeps. Siboglinid polychaetes including a large vestimentiferan Lamellibrachia sp. were also associated. At least four bivalve species and one siboglinid are associated with symbiotic chemoautotrophic bacteria, as evidenced by Transmission Electronic Microscopy and isotopic ratio measurements. Among the bivalves, a mytilid harbors both methanotrophic and sulfide-oxidizing bacteria. Video spatial analysis of the community distribution on three volcanoes shows that dense bivalve shell accumulations (mainly lucinids) spread over large areas, from 10% to 38% of the explored areas (2500–15000 m2) on the different volcanoes. Lamellibrachia sp. had different spatial distribution and variable density in the two mud volcano fields, apparently related with higher methane fluxes in the Anaximander volcanoes and maybe with the instability due to brines in the Olimpi area. The abundance and richness of the observed chemosynthetic fauna and the size of some of the species contrast with the poverty of the deep eastern Mediterranean. The presence of a specialized fauna, with some mollusk genera and species shared with other reduced environments of the Mediterranean, but not dominated by the large bivalves usually found at seeps, is discussed.
  29. Pohlman, John W., et al. “The origin of thermogenic gas hydrates on the northern Cascadia Margin as inferred from isotopic (13C/12C and D/H) and molecular composition of hydrate and vent gas.” Organic Geochemistry 36.5 (2005): 703-716.  The isotopic (δ13C and δD) and hydrocarbon compositions of hydrate-bound and vent gas collected from the seafloor of Barkley Canyon (northern Cascadia Margin, offshore Vancouver Island, Canada) were evaluated to characterize the gas and infer the type and maturity of the source rock kerogen. The hydrate gas contained methane having δ13C values from −43.4‰ to −42.6‰δD values from −143‰ to −138‰ and a large percentage (14.9% to 31.9%) of C2 to C5+ hydrocarbons. These data are consistent with a thermogenic gas source. The data from Barkley Canyon are interpreted within the context of similar data from the Gulf of Mexico and Caspian Sea thermogenic hydrates, which occur in regions where the petroleum systems supporting gas and oil generationare better understood. A stable carbon isotope-based natural gas plot model and D/H data from the hydrate gas indicate that the source rock for the Barkley Canyon hydrate and vent gas had primarily Type III kerogen mixed with a small fraction of Type II kerogen. Oil from a seep having a pristane/phytane ratio of 3.2 was identified as a gas condensate, which supports the conclusions drawn from the gas data. A mechanism for explaining how fluids are conducted from the deep petroleum reservoir to the seafloor of Barkley Canyon is proposed.
  30. León, R., et al. “Sea-floor features related to hydrocarbon seeps in deepwater carbonate-mud mounds of the Gulf of Cádiz: from mud flows to carbonate precipitates.” Geo-Marine Letters 27.2-4 (2007): 237-247.  Underwater images taken from deepwater carbonate-mud mounds located along the continental margin of the Gulf of Cádiz (eastern Central Atlantic) have identified a great variety of hydrocarbon seep-related geomorphic features that exist on the sea floor. An extensive photographic survey was made along the Guadalquivir Diapiric Ridge, after detailed examination of the main mounds identified on previous swath bathymetry coverage, high-resolution seismic imagery, dredge and gravity core data. Recognised fluid-induced geomorphic features include seep precipitates, named here generically as hydrocarbon-derived authigenic carbonates (HDACs), mud-breccia flows and piping/rills, at scales ranging from metres to centimetres. Based on the viscosity, texture, morphology, and the nature of observed features, we have categorized the geomorphic seeps into the following types: mud-breccia flows and liquid seepages, which can be grouped as highly viscous and viscous mud-breccia flows, gassy mud-breccia flows, and small-scale piping/rills; HDACs types, including massive crusts, “honeycombed” carbonate crusts, nodular aggregated crusts, steeply dipping to vertical slabs, and pipe-like formations (chimneys). These widespread geomorphic features observed along the carbonate-mud mounds reveal alternate periods of (1) active mud-flow extrusion (mud-volcano formation), (2) reduced seepage activity, with the formation of extensive carbonate features by chemosynthetic organisms, and (3) formation of hardgrounds and colonisation by non-chemosynthetic organisms such as deepwater corals (e.g. Lophelia pertusaMadrepora oculata). The formation of large amounts of HDACs is related to the microbially mediated oxidation of hydrocarbon fluids (biogenic and thermogenic) during periods of slower fluid venting. This has led to the hypothesis that these carbonate-mud mounds could be built up by alternating episodes of varying fluid-venting rates, with peaks that may have been triggered by tectonic events (e.g. high-seismicity periods) and slower rates controlled by climate/oceanographic factors (e.g. glacial to interglacial climatic transitions, increasing shallow subsurface hydrate formation, and sealing of sea-floor fluid venting).
  31. Zhu, Wei-yao, et al. “Low-velocity non-Darcy gas seepage model and productivity equations of low-permeability water-bearing gas reservoirs.” Natural Gas Geoscience 5 (2008).  The flow behaviour of fluids in water-bearing gas reservoirs was analyzed based on the experimental study of low-velocity non-Darcy percolation. Three kinds of mathematical models were established under the influences of movable water, inmovable water and combined water. Deliverability equations of gas well were obtained with the gas slippage effect and the threshold pressure gradient existed respectively and simultaneously. The case study proved that the study was applicable and the corresponding equations could provide a necessary theoretical basis for defining reasonable gas-well deliverability under different water conditions.
  32. Manga, Michael, Maria Brumm, and Maxwell L. Rudolph. “Earthquake triggering of mud volcanoes.” Marine and Petroleum Geology 26.9 (2009): 1785-1798.Mud volcanoes sometimes erupt within days after nearby earthquakes. The number of such nearly coincident events is larger than would be expected by chance and the eruptions are thus assumed to be triggered by earthquakes. Here we compile observations of the response of mud volcanoes and other geologic systems (earthquakes, volcanoes, liquefactionground water, and geysers) to earthquakes. The compilation shows a clear magnitude–distance threshold for triggering, suggesting that these seemingly disparate phenomena may share similar underlying triggering mechanisms. The compilation also shows that pre-existing geysers and already-erupting volcanoes and mud volcanoes are much more sensitive to earthquakes than quiescent systems. Several changes produced by earthquakes have been proposed as triggering mechanisms, including liquefaction and loss of strength, increased hydraulic permeability or removing hydraulic barriers, and bubble nucleation and growth. We present new measurements of the response of erupted mud samples to oscillatory shear at seismic frequencies and amplitudes, and find that loss of strength occurs at strain amplitudes greater than 10−3. This is much larger than the peak dynamic strains associated with earthquakes that may have triggered eruptions or influenced already-erupting mud volcanoes. Therefore, we do not favor loss of strength as a general triggering mechanism. Mechanisms involving bubbles require significant supersaturation or incompressible mud, and neither condition is likely to be relevant. We analyze the response of the Niikappu group of mud volcanoes in Japan to several earthquakes. We find that this system is insensitive to earthquakes if an eruption has occurred within the previous couple of years, and that static strain magnitudes are very small and not correlated with triggering suggesting that triggering likely results from dynamic strain. Moreover, triggering may be frequency-dependent with longer period seismic waves being more effective at triggering. Available data are insufficient, however, to determine whether triggering characteristics at Niikappu are representative of triggered eruptions in general. Nor can we determine the exact mechanism by which dynamic (long-period) strains induce eruption, but given the apparent failure of all mechanisms except increasing permeability and breaching barriers we favor these. More observations and longer records are needed. In particular, gas measurements and broadband seismic data can be collected remotely and continuously, and provide key information about processes that occur during and immediately after the arrival of seismic waves.
  33. Wang, Cuiping, et al. “PAHs distribution in sediments associated with gas hydrate and oil seepage from the Gulf of Mexico.” Marine pollution bulletin 62.12 (2011): 2714-2723.  Six sediment samples collected from the Gulf of Mexico were analyzed. Total concentrations of the PAHs ranged from 52 to 403 ng g−1 dry weight. The lowest PAH concentration without 5–6 rings PAHs appeared in S-1 sample associated with gas hydrate or gas venting. Moreover, S-1 sample had the lowest organic carbon content with 0.85% and highest reduced sulfur level with 1.21% relative to other samples. And, analysis of the sources of PAHs in S-1 sample indicated that both pyrogenic and petrogenic sources, converserly, while S-8, S-10 and S-11 sample suggested petrogenic origin. The distribution of dibenzothiophene, fluorine and dibenzofuran and the maturity parameters of triaromatic steranes suggested that organic matters in S-1 sample were different from that in S-8, S-10 and S-11 sample. This study suggested that organic geochemical data could help in distinguish the characteristic of sediment associated with gas hydrate or with oil seepage.
  34. Anthony, Katey M. Walter, et al. “Geologic methane seeps along boundaries of Arctic permafrost thaw and melting glaciers.” Nature Geoscience 5.6 (2012): 419.  Methane, a potent greenhouse gas, accumulates in subsurface hydrocarbon reservoirs, such as coal beds and natural gas deposits. In the Arctic, permafrost and glaciers form a ‘cryosphere cap’ that traps gas leaking from these reservoirs, restricting flow to the atmosphere. With a carbon store of over 1,200 Pg, the Arctic geologic methane reservoir is large when compared with the global atmospheric methane pool of around 5 Pg. As such, the Earth’s climate is sensitive to the escape of even a small fraction of this methane. Here, we document the release of 14C-depleted methane to the atmosphere from abundant gas seeps concentrated along boundaries of permafrost thaw and receding glaciers in Alaska and Greenland, using aerial and ground surface survey data and in situ measurements of methane isotopes and flux. We mapped over 150,000 seeps, which we identified as bubble-induced open holes in lake ice. These seeps were characterized by anomalously high methane fluxes, and in Alaska by ancient radiocarbon ages and stable isotope values that matched those of coal bed and thermogenic methane accumulations. Younger seeps in Greenland were associated with zones of ice-sheet retreat since the Little Ice Age. Our findings imply that in a warming climate, disintegration of permafrost, glaciers and parts of the polar ice sheets could facilitate the transient expulsion of 14C-depleted methane trapped by the cryosphere cap.


  1. Mud volcano refers to the seabed protrusion terrain that can spraymud, gas, and fluid. According to the studies of R.S. Newton et al. (1980), mud volcano is often triggered by high-pressure gas, fault activity, diapir effect, sedimentation, and other factors. Mud volcanowill increase the seabed slope gradient, reduce the safety coefficient of slope, and thus trigger subsea slope instability. This kind of instability is usually closely related to the rapid accumulation, compressive fault, diaper, and high-pressure gas of sediments.
  2. Mud volcanoespockmarks, and other fluid escape structures are common along the Iberian margin of the Gulf of Cadiz. They are interpreted as indicators of gas-rich, overpressured sediments occurring at different depths (300–1,200 m) [5]. Mud volcanoes are conduits for fluid venting and consequent carbonate precipitation within the sediments or at the seafloor. Some of the widespread shallow fluid venting on the seafloor is attributed to the local destabilization of gas-hydrate-rich sediments due to MOW warming [4]. Three major sites of seepage-related cluster structures can be identified along the Iberian upper continental slope [3,7,8] (Figure 61.1) that are representative of the fluid venting phenomena in three different geological environments: contouritic drifts, megasliding deposits (on the continental margin), and diapiric ridges.
  3. El Laberinto: This region is located on the upper slope (Figure 61.1) and comprises a cluster of separate, circular volcanoes outcropping over an extensive contouritic drift, covering 201 km2 (Figure 61.2). The volcanoes are surrounded by rings of subcircular pockmarks, formed as a product of degassing diapirs. Notable characteristic features include three mud volcanoes (Anastasya, Tarsis, and Pipoca) together with an adjacent prominent diapiric ridge and contouritic channels. Anastasya mud volcano has a regular cone geometry (diameter <1.5 km) covering 7.5 km2 and located at a depth of 452 m, with a height of 80 m above the level of surrounding seafloor. Sediments are plastic, mousse-like muds that are methanesaturated and draped by a thin layer of recent medium-grained sand. Tarsis mud volcano has a regular geometry (diameter <1 km) covering 4.5 km2, occurring at 762 m depth, with a height of 60 m. It has a 50-m deep pockmark depression circling the cone. The volcano is entirely surrounded by a sheeted contourite drift that is densely burrowed by decapods. Pipoca mud volcano (diameter <1.5 km) covers 6.6 km2 and displays a mud flow lobe flowing downslope and placed between two MOW moats.
  4. Hesperides Complex: This complex is representative of megasliding deposits and consists of a cluster of five single-cone mud volcanoes, covering ca. 7 km2 (680–730 m depth) [3] (Figure 61.3). The complex is affected by a MOW flow filament that descends along a channel, warming the deposits and thereby triggering the destabilization of hydrates and generating several pockmarks. Three cone morphotypes include: (a) twin circular and regular cones, (b) twin slightly flat cones with smooth surfaces and irregular contour lines, and (c) elongated single cone with strongly irregular and asymmetric hillsides. Scattered over the complex are carbonate chimneys and crusts providing hard substrata for benthic fauna. A large pockmark of 2.1 km2, with a maximum depth of 150 m, next to the complex contains bottom mud deposits without evidence of gas-enriched sediments or hard structures.
  5. Hormigas Ridge: The ridge is formed by a set of single mud volcanoes on top of several small hills from the deepest sector of one of the diapiric ridges, covering 45 km2 (890–935 m depth) and containing numerous carbonate chimneys, crusts, and slabs. The largest mound is the Cornide mud volcano (diameter <1.2 km) (Figure 61.4), covering 0.5 km2 (935 m depth) and located next to the Cadiz Channel. It is conformed by a twin flat mound with a height of 35 m below the diapir summit and with a cliff of ca. 235 m above the channel. The crest of the cone yielded large amounts of brown carbonate crusts and chimneys, and a polygenic matrix breccia covered by sandy sediments with a strong H2S smell.
  6. Geomorphic features related to fluid flow and availability of hard bottoms (e.g., chimneys, slabs) is influenced by the erosion patterns that affect each volcano. Therefore, different habitat types occur in association with different volcanoes. For example, exhumed chimneys and slabs can be seen in those volcanoes where strong deep currents have eroded the cone that originally enclosed these structures. Soft bottoms, from coarse sands to mud, generally occur adjacent to the volcanoes or in the pockmarks. Three major types of substrate for benthic and demersal fauna are as follows. Stable hard bottoms: These include crusts, pavements, and slabs as well as carbonate chimneys that are generally not covered by mud or sand. These types of bottoms are normally located in eroded volcanoes (Hespérides and Cornide), close to the summit or at the side of the volcano. This is a rare type of substrate compared to soft bottoms (Figures 61.5–61.7) and represents an ideal habitat for colonial organisms (e.g., corals, sponges).

  7. Mixed bottoms: These include scattered chimneys and crusts that occur in a matrix of sandy mud to muddy sediments. This mixture of substrates generally occurs within mud volcanoes, scattered between the stable hard substrates or at the base of the volcano, where chimneys and crusts are deposited (Hespérides and Córnide) (Figures 61.6 and 61.7). This substrate is less abundant than soft bottoms but exhibits similar coverage to rocky bottoms, especially in the El Laberinto and Cornide mud volcanoes (Figures 61.5 and 61.6). A mixed fauna occupies this habitat. Soft bottoms: These are mainly contouritic sands and muds occurring between the volcanoes and in the pockmark deposits. Soft-bottom types have the greatest spatial coverage and are composed of different sediment types (coarse sands, fine sands, and mud). Fisheries of commercial species in the vicinity of some mud volcanoes (El Laberinto) occur in these types of bottoms.

  8. Distribution and occurrence of cold-seepage and mud-volcano provinces
    Cold seeps and mud volcanoes are most common in areas of recent tectonic activity, that is, within the mobile tectonic belts of the Alpine-Himalayas, Pacific and Central-Asia and in areas with thick (5–20 km) sedimentary successions. Numerous field studies in the last decades have revealed that spontaneous discharge of subsurface fluids is a worldwide phenomenon, and such discharges are now known to be a common feature on active and passive continental margins. The areas of known seeps on the European and north African margins are shown in Fig. 6.13.
  9. The Håkon Mosby mud volcano (HMMV), one of the most thoroughly studied SMVs associated with gas hydrates, is located on the Norwegian shelf in 1250 m water depth (Sauter et al., 2006). Its diameter is 1500 m with rim rising less than 10 m above the seafloor. Thermal profiles and mass emissions of HMMV were reported by de Beer et al. (2011). As measured 750 m from the center outwardly, the HMMV has three concentric regions: (1) high-temperature center of 50 m diameter exhibiting periodic mass expulsions, (2) annulus around a high-temperature center in which white Beggiatoamats cover the surface, and (3) elevated outer ring populated by tube worms and gas hydrates.

  10. The total area of mud volcanism in Eastern Azerbaijan is 16,000 km2, including more than 200 mud volcanoes (Figure 2-1). Scientists believe that there are 150 underwater mud volcanoes in the Southern Caspian Sea and 9 mud-volcanic islands. It is ascertained that mud volcanoes are confined to the most deformed portions of late geosynclinal trend (i.e., to molasse troughs), to the periphery of folded systems (i.e., to foredeeps), periclinal troughs of active geosynclinal folded regions, where thickness of sedimentary fill exceeds 10 km. The following factors are prerequisites for generation of mud volcanoes: anticlinal structure, dislocations with breaks of continuity, plastic clays, buried formation water, accumulation of hydrocarbon gases and abnormally-high formation pressure.
  11. Mud volcanoes can form on shallow seafloor (eg, Caspian Sea), on top of seamounts (South Chamorro Seamount), and on islands (eg, Andaman Islands). Occasionally, mud volcanoes erupt with spectacular flames, sending tons of mud flowing down the sides of the craterODP Leg 195 drilled Site 1200 on the South Chamorro Seamount(Shipboard Scientific Party, Site 1200, 2002), one of 14 large (up to 50 km in diameter and rising up to 2 km above the surrounding seafloor) active serpentinite and blueschist mud volcanoes on the Mariana forearc (see Fryer and Fryer, 1987; Fryer, 1996; Fryer et al., 2000, 2006). Note that blueschist, also called glaucophane schist, is a metavolcanic rock that forms by the metamorphism of basalt and rocks with similar composition at high pressures and low temperatures, approximately corresponding to a depth of 15–30 km and 200°C to ~ 500°C. The blue color of the rock comes from the presence of the predominant minerals glaucophane and lawsonite. Drilling on the mud volcano on the South Chamorro Seamount allowed the composition of this deep-slab-derived water to be determined. Mottl et al. (2003a,b) found that the water ascending through the mud volcanoes has a pH of 12.5 and, relative to seawater, is enriched in sulfatealkalinity, NaCl, K, Rb, B, light hydrocarbons, ammonia, 18O, and deuterium. Surprisingly, it was found that within the upper 20 m below seafloor at South Chamorro Seamount, a microbial community operating at pH 12.5, made up overwhelmingly of extremophilic microbes, mainly Archaea, is oxidizing methane from the ascending fluid to carbonate ion and organic carbon, while reducing sulfate to bisulfide and probably dissolved nitrogen to ammonia. This microbial community is found to be not only extremophile but subsists on a source of chemical energy delivered from as deep as 27 km below the seafloor. Note that an extremophile (from Latin extremus meaning extreme and Greek philia meaning love) is an organism that thrives in physically or geochemically extreme conditions (eg, extraordinarily hot or acidic) that are detrimental to most life on Earth (see Rothschild and Mancinelli, 2001; Rampelotto, 2010). According to Mottl et al. (2003a,b), the sulfur enrichment near the seafloor results from Fe sulfide minerals that precipitate as a result of microbial (archaeal) sulfate reduction. The zone of sulfur enrichment extends to 25 m below seafloor (mbsf) at the cold spring to at least 15 mbsf 20 m away, and to only 5 mbsf 80 m from the spring. It is essentially absent on the summit of Conical Seamount, which also lacks a macrofauna. The enrichment of organic carbon at 0–3 mbsf farthest from the cold spring correlates with a high concentration of foraminifera. Mottl et al. (2003a,b) reckons that microbial sulfate reduction accompanied by production of carbonate, bisulfide, and ammonia at 1–3 and 13 mbsf is remarkable for three reasons: it is accomplished at pH 12.5, by an assemblage overwhelmingly dominated by Archaea rather than bacteria, in a setting that has essentially no sedimentary organic carbon. Bacterial biomass ranged from ≈ 104 to ≈ 106 cells/g dry wt. It was highest at 0–0.1 mbsf but was undetectable in the deepest sample at 53.8–53.9 mbsf. The surface sample had a diverse community structure, including ≈ 10% fatty acids that are common in anaerobic lithotrophic bacteria. Whereas bacteria were concentrated near the seafloor, archaeal biomass was exceptionally high in subsurface samples. In a further study, Wheat et al. (2008) made borehole observations of fluid flow from South Chamorro Seamount, an active serpentinite mud volcano in the Mariana forearc (see Fig. 1.43). Analysis of sediment samples recovered by surface coring from this area was carried out to ascertain mineral composition and sediment physical properties. A majority of these samples, collected in an area where pore water upwelling speeds are ~ 0.5 cm/year, were blue muds. The flow of mud was found to support a megafaunal assemblage on the summit of the seamount. The assemblage includes mussels, gastropods, tube worms, and galatheid crabs (see Fryer and Mottl, 1997a,b). Subsurface fauna supported by this fluid include Archaea (see Mottl et al., 2003a,b) and bacteria (see Takai et al., 2005). Typically, microbial sulfate reduction in Mariana mud volcanoes is limited to the upper meter of sediment (see Mottl et al., 2003a,b; Takai et al., 2005). In addition to this near-surface population, South Chamorro Seamount has a secondary microbial population. In ODP Hole 1200E, this population is at a depth of ~ 20 m. It has been found that the shallow “primary” microbial population is distinctly different from the deeper “secondary” population. Mottl et al. (2003a,b) showed that the formation fluid that upwells in South Chamorro Seamount is reduced by Archaea via anaerobic oxidationof methane. Interestingly, Wheat et al. (2008) found that at South Chamorro Seamount, the deeper secondary microbial zone correlates with the pore water upwelling speed; faster pore water upwelling speeds are associated with a deeper secondary microbial zone. Wheat et al. (2008) reckon that the deep microbial community observed at South Chamorro Seamount is related to the local geologic setting and hydrologic flow pattern. According to their studies, serpentinite mud is slowly ascending from its depth towards the summit of South Chamorro Seamount. In some areas, pore water ascends faster than the mud, at a few centimeters to tens of centimeters per year. There appears to be a single source for the formation fluid based on systematic variations in the chemical composition of shallow pore waters and from borehole fluids that ascended from a 200-m depth in ODP Hole 1200C. Generally, an active subsurface biosphere is evident only near the seafloor. The exception is South Chamorro Seamount, which has a secondary deep biosphere comprised mainly of sulfate-reducing Archaea. This deep biosphere appears to be associated with the high permeability zone and does not occur ubiquitously throughout the seamount, as indicated by the presence of sulfate in shallow pore waters elsewhere in the seamount. Wheat et al. (2008) argue that if this biosphere were ubiquitous, then there would be no measurable dissolved sulfate in any of the pore water samples, except for samples near the seafloor where diffusive exchange with seawater sulfate occurs. This apparent hydrologic restriction for the biosphere could be related to specific chemical and thermal conditions that are appropriate for metabolism. For example, highly permeable zones in the formation may attract and concentrate ions and/or dissolved gases (eg, methane, hydrogen) required for metabolic activity. A fundamental component for metabolic activity is likely methane, which is present in pore waters with a concentration of ~ 17 mmol/kg at the depth of sulfate reduction zone in ODP Hole 1200E (see Mottl et al., 2003a,b).
  12. Dashgil mud volcano is a 2-km-wide dome located in Azerbaijan near the western coast of the Caspian Sea. The summit area of the Dashgil volcano (Fig. 20.5) presents the typical structure of a mud volcano summit caldera. It is one of the most active mud domes of the Earth. Beyond its paroxysmal activity, it also represents a good example of a mud volcano with high seep activity in the dormant period. Gas, mud, and water seepage occurs at a number of different structures. In the southwestern sector of the summit area, a north–south-oriented field of active gryphons and mud cones exist. A deep bubbling sound and a faint oil smell are recorded inside at least some of the cones. Toward the eastern edge of the caldera, two lakes of low-viscosity mud (salsas) exist. In both, gas bubbles rise from the bottom of the lakes, which reach maximum depths of about 10 m (salsa A) and 8 m (salsa B), respectively (Kopf et al., 2009). Monitoring the activity of the Dashgil mud volcano in its “dormant” phase was carried out by using a portable short-period seismometerTromino (Albarello et al., 2012). Three sessions of seismic measurements were carried out in the crater area in June 2006 and July 2007 (sites I–III in Fig. 20.5A). Each session lasted 20 min and a sampling rate of 128 Hz was adopted. The two time series I (near crater) and II (near the lake of low-viscosity mud, salsa A) appear to be quite different to visual inspection (Fig. 20.6). Ground motion amplitude at the mud cone (site I) is about one order of magnitude larger than at the site II near salsa A. This is the effect of energy bursts (see their shape in the inset A1 in Fig. 20.6) that are present in the first series and absent in the second series. The 2007 measurement session was performed near mud cones (site III, Fig. 20.5). The energy bursts were clearly observed again (see the inset C1 in Fig. 20.6) confirming the possible presence of phenomena reflecting persistent activity in the summit area of mud volcano. The shape of the energy burst signals recorded at Dashgil dormant mud volcano may be comparable with the shape of the high-amplitude, high-frequency signals recorded at the newborn Lusi mud volcano (see Fig. 20.4). At the same time, they are quite different in spectral content, having the dominant frequencies within the interval between 3 and 6.5 Hz for Dashgil volcano and within the interval between 15 and 40 Hz for Lusi volcano.
  13. In 2010, the Besmaya mud-volcano field in the Yli depression (Figure KZ 1) was investigated by a consortium of geologists from Russia, Kazakhstan, and Israel. These researchers concluded from their work that the Besmaya mud-volcano field has gryphons and salsas like the mud-volcano fields of the Kerch and Taman Peninsulas, Russia (Shnyukov et al., 2009), and Azerbaijan (Vapnik, 2012: this volume). As in Taman and Azerbaijan, mud volcanism and the scoria-like paralavas in the Yli depression are linked to hydrocarbon-gas combustion.  Mud volcanoes in the Besmaya field occur along joints and strike-slip faults in the central part of the Yli depression, at the boundary between Oligocene-Quaternary sedimentary strata in the Almatinskiy Basin in the west and Triassic-Quaternary sedimentary strata in the Panfilov Basin in the east. Combustion foci and gryphons occur within an 18-km2 area. Besmaya mud volcanoes are represented by about 60 gryphons and dried up salsa lakes that contained silt, similar to the salsas of active mud volcanoes. The gryphons are flattened conical edifices, commonly about 80 cm tall; rarely exceeding 1.5–2 m in height. They occur within silty, dried up salsas. The silty regions are much more extensive than any gryphon and can be hundreds of square meters. Barchan dunes, 2–10 m high, commonly wrap around mud-volcano calderas (Figure KZ 2).

  14. Palchygh Pilpilasi (meaning “Mud Volcano” in Azery language) Oil Field is located in the Caspian Sea east of Baku and southeast of Pirallaghi Adasi (see Figure 6-3). The main base for exploration and development is at Neft Dashlary Field, 4 km southeast. Seismic surveys conducted during 1953–1957 outlined the structure between Chalov Adasi and Neft Dashlary and discovered another anticline Palchygh Pilpilasi (Samedov et al., 1960). Later, in 1963—1965, one more uplift, named Azi Aslanov, was discovered between Chalov Adasi and Palchygh Pilpilasi structures. By 1953, commercial oil saturation was established in nine units on both flanks of the Neft Dashlary anticline. The initial flow rates were high, over 50 tpd (365 bpd). Because Palchygh Pilpilasi area lies on the northwestern extension of the Neft Dashlary Oil Field and includes the same Middle Pliocene formations, it was suggested that the same units may be productive there. During the first few years of exploration (1952–1955), the objective was to penetrate the entire Productive Series and to study its lithology, stratigraphy, structure and petroleum potential. The first well (22) was spudded on August 10, 1952, over the most elevated area of the southwestern flank. This well (TD = 1,003 m), along with platform, was destroyed by a severe storm on December 11, 1952. By 1956, eight wells were completed. The crestal Well 20 tested oil with gas from Kala Suite and 0.5 to 1 tpd (3.6 to 7.3 bpd) of heavy oil (density = 0.945 g/cm3) from Podkirmaku Suite. Commercial flows from the exploratory wells spaced over 4 km were considered as indications of the high potential of the structure. An appraisal drilling program for the Palchygh Pilpilasi area was prepared by F. I. Samedov and A. M. Polaudin and approved on June 30, 1956. The program envisioned the drilling of 20 wells with proposed depth ranging from 1,000 to 1,900 m along seven profiles extended across the strike of the structure and encompassing both of its flanks. The spacing of profiles was 800 to 1,000 m. The appraisal drilling program was supposed to take two years (including the construction of offshore platforms). Some wells were proposed to be deviated and drilled from the existing platforms or platforms under construction at that time. The drilling program for 1956 included 12,000 meters with the remainder to be drilled in 1957. To study the KaS accumulation discovered in the northwestern area of the Neft Dashlary Oil Field, the drilling of four wells was proposed. Actually, five wells were drilled in 1956, with two wells yielding commercial oil production (18 and 45 tpd or 131 and 328 bpd). Four wells out of those drilled in 1957 flowed oil (15 to 22 tpd or 109 to 160 bpd), whereas one well flowed gas from Unit KaS4. Four of these wells, along with the other two which were not completed, have been destroyed by the hurricane on November 21, 1957. The hurricane severely affected the exploration and development program: no wells were drilled in 1958, and only one, in 1959. Nevertheless, the initial drilling program with some adjustments, was completed by 1958. As a result, oil accumulations have been discovered in KaS and PK suites and substantial amount of knowledge was gained about the geology and petroleum potential of the region. The most intensive drilling was conducted in 1960–1961. During that period, 12 wells were drilled that delineated discovered oil accumulations. Five of the wells tested oil (10 to 50 tpd or 73 to 365 bpd), four were water wet, one was plugged and abandoned, and two wells remained uncompleted. No wells were drilled in 1963. One well was drilled each year in 1964 and 1965, three in 1966, two in 1967 and five in 1968. A total of 46 exploratory and appraisal wells were drilled by 1969. Thirty-five of these wells were cored. The average profile spacing was 1,000 m, whereas the well spacing was 300 m. The profile spacing corresponded to the drilling program, whereas the well spacing was smaller due to drilling of some infill wells required by the complexity of the structure and lithology.

6 Responses to "Geological carbon flows"

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