What is gas seepage?
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Natural gas seepage is the steady or episodic, slow or rapid, visible or invisible flow of gaseous hydrocarbons from subsurface sources to Earth’s surface.
In petroleum geology literature, use of the term “seepage” is traditionally restricted to the hydrocarbon-rich gas, composed mainly of methane (CH4) and subordinately ethane (C2H6), propane (C3H8), and butane (C4H10), that is formed in petroleum (oil and gas) prone sedimentary basins through microbial or thermal conversions of organic matter in source rocks, generally shales or limestones. Non-hydrocarbon gases, such as CO2, N2, He, and H2S, are generally also present as minor constituents.
Seepage DOES NOT refer to geothermal or volcanic H2O- or CO2-rich gas manifestations (e.g., fumaroles, mofettes, and geysers) where hydrocarbons are a minor component.
from Etiope (2015; Natural Gas Seepage, Springer)
The surface expressions of natural gas seepage can be classified on the basis of spatial dimension, visibility, and fluid typology, as summarised in the following scheme:
Gas seeps (or dry seeps) release only a gaseous phase, such as gas vents from outcropping rocks or through the soil horizon or through river/lake beds. Gas bubbling from groundwater filled wells, or other shallow water bodies, are considered gas seeps since surface water is only being crossed by the gas flow. Dry gas flow through rock and dry soil can produce flames by self-ignition (fire seeps or everlasting fires); more generally, however many vents can be easily ignited artificially.
Mud volcanoes, related to sedimentary volcanism release a three-phase (gas, water and sediment) mixture. Gas is typically released from gryphons, craters or bubbling pools (salses).
Oil seeps release mainly liquid hydrocarbons. The amount of gas in oil seeps decreases during oil exposure to the atmosphere, with subsequent oxidation, biodegradation, and solidification into asphalts and tars (solid seeps).
Water springs (or water-seeps) essentially refer to groundwater issuing from natural springs or in very shallow wells, with a significant concentration of gas in solution; water may have a deep origin and may have interacted with gas during its ascent to the surface.
Miniseepage is the invisible, diffuse exhalation of gas surrounding visible seeps within a macro-seepage zone. It is a sort of halo that surrounds a channelled seep. The concept is very important because it makes a clear distinction between the visible point of gas emission (a crater, a vent, or a flame) and the surrounding soil. A transition area exists where gas flux gradually decreases, dropping to “zero” after tens or hundreds of meters.
Microseepage is the invisible, slow, continual, or episodic exhalation of methane and light alkanes from gas-oil-prone sedimentary basins, independent of the presence of macro-seeps. Microseepage can only be detected using soil-gas analyses, revealing anomalous concentrations of gaseous hydrocarbons in the soil, or using closed-chamber techniques that allow determinations of gas flux to the atmosphere. A series of geophysical indirect methods, such as microbial prospecting, remote sensing and magnetic measurements can also detect microseepage.
Here are some of my papers on seepage:
1. Etiope G., Panieri G., Fattorini D., Regoli F., Vannoli P., Italiano F., Locritani M., Carmisciano C. (2014). A thermogenic hydrocarbon seep in shallow Adriatic Sea (Italy): gas origin, sediment contamination and benthic foraminifera. Marine and Petroleum Geology, 57, 283-293.
2. Etiope G., Drobniak A., Schimmelmann A. (2013). Natural seepage of shale gas and the origin of “eternal flames” in the Northern Appalachian Basin, USA. Mar.Petrol.Geol., 43, 178-186,
3. Hong W.L., Etiope G., Yang T.F., Chang P.Y. (2013). Methane flux of miniseepage in mud volcanoes of SW Taiwan: Comparison with the data from Europe. J. Asian Earth Sci., 65, 3-12.
4. Etiope G., Christodoulou D., Kordella S., Marinaro G. Papatheodorou G. (2013). Offshore and onshore seepage of thermogenic gas at Katakolo Bay (Western Greece). Chem. Geol., 339, 115-126.
5. Etiope G. (2012). Methane uncovered. Nature Geosci., 5, 373-374.
6. Etiope G. , Nakada R., Tanaka K., Yoshida N. (2011). Gas seepage from Tokamachi mud volcanoes, onshore Niigata Basin (Japan): origin, post-genetic alterations and CH4-CO2 fluxes. Applied Geochemistry, 26, 348-359.
7. Etiope G., Zwahlen C., Anselmetti F.S., Kipfer R., Schubert C.J. (2010). Origin and flux of a gas seep in the Northern Alps (Giswil, Switzerland). Geofluids, 10, 476-485.
8. Spulber L., Etiope G., Baciu C., Malos C., Vlad S.N. (2010). Methane emission from natural gas seeps and mud volcanoes in Transylvania (Romania). Geofluids, 10, 463-475.
9. Etiope G., Klusman R.W. (2010). Microseepage in drylands: flux and implications in the global atmospheric source/sink budget of methane. Global Planet. Change, 72, 265-274.
10. Etiope G., Ciccioli P. (2009). Earth’s degassing – A missing ethane and propane source. Science, 323, 5913, 478, doi: 10.1126/science.1165904.
11. Etiope G., Feyzullayev A., Milkov A.V., Waseda A., Mizobe K., Sun C.H. (2009). Evidence of subsurface anaerobic biodegradation of hydrocarbons and potential secondary methanogenesis in terrestrial mud volcanoes. Mar.Petroleum Geol., 26, 1692-1703.
12. Etiope G. (2009). Natural emissions of methane from geological seepage in Europe. Atmosph. Environment, 43, 1430-1443
13. Etiope G., Feyzullayev A., Baciu C.L. (2009). Terrestrial methane seeps and mud volcanoes: a global perspective of gas origin. Mar. Petroleum Geology, 26, 333-344. ,
14. Baciu C., Etiope G., Cuna S., Spulber L. (2008). Methane seepage in an urban development area (Bacau, Romania): origin, extent and hazard. Geofluids, 8, 311-320,
15. Cremonini S., Etiope G., Italiano F., Martinelli G. (2008). Evidence of possible enhanced peat burning by deep originated methane in Po river delta (Italy). J.Geology, 116, 401-413.
16. Etiope G., Lassey K.R., Klusman R.W., Boschi E. (2008). Reappraisal of the fossil methane budget and related emission from geologic sources. Geoph. Res. Lett., 35, L09307, doi:10.1029/2008GL033623.
17. Etiope G., Milkov A.V., Derbyshire E. (2008). Did geologic emissions of methane play any role in Quaternary climate change? Global Planet.Change, 61, 79-88.
18. Baciu C., Caracausi C., Etiope G., Italiano F. (2007). Mud volcanoes and methane seeps in Romania: main features and gas flux. Annals of Geoph., 50, 501-512.
19. Etiope G., Martinelli G., Caracausi A., Italiano F. (2007). Methane seeps and mud volcanoes in Italy: gas origin, fractionation and emission to the atmosphere. Geoph.Res.Lett., 34, L14303, doi: 10.1029/2007GL030341
20. Etiope G., Baciu C.L. (2007). Gas migration in the geosphere: the “geogas” theory. Ambientum, I/1-2, 95-101.
21. Etiope G., Papatheodorou G., Christodoulou D., Ferentinos G., Sokos E., Favali P. (2006). Methane and hydrogen sulfide seepage in the NW Peloponnesus petroliferous basin (Greece): origin and geohazard. AAPG Bulletin., 90, 5, 701-713.
22. Milkov A.V., Etiope G. (2005). Global methane emission through mud volcanoes and its past and present impact on the Earth's climate. Comment. Intern. J. of Earth Sciences, 94, 490-492.
23. Etiope G. (2005). Mud volcanoes and microseepage: the forgotten geophysical components of atmospheric methane budget. Annals of Geophysics, 48, 1-7.
24. Etiope G., Papatheodorou G., Christodoulou D., Favali P., Ferentinos G., (2005). Gas Hazard Induced by Methane and Hydrogen Sulfide Seepage in the NW Peloponnesus Petroliferous Basin (Greece). Terr.Atm.Ocean. Sci., 16, 897-908.
25. Etiope G., Feyzullaiev A., Baciu C.L., Milkov A.V. (2004). Methane emission from mud volcanoes in eastern Azerbaijan. Geology, 32, 6, 465-468.
26. Etiope G., Baciu C., Caracausi A., Italiano F., Cosma C. (2004). Gas flux to the atmosphere from mud volcanoes in eastern Romania. Terra Nova, 16, 179-184.
27. Etiope G. (2004). GEM – Geologic Emissions of Methane, the missing source in the atmospheric methane budget. Atmospheric Environm., 38, 19, 3099-3100.
28. Etiope G., Milkov A.V. (2004). A new estimate of global methane flux from onshore and shallow submarine mud volcanoes to the atmosphere. Environm. Geology, 46, 997-1002.
29. Etiope G. , Klusman R.W. (2002). Geologic emissions of methane to the atmosphere. Chemosphere, 49, 8, 777-789.
Examples of gas seeps and “eternal fires”. a Deleni, Romania; b Yanardag, Azerbaijan; c Giswil, Switzerland; d Baba Gurgur, Iraq; e Chimaera, Turkey; and f Faros-Katakolo, Greece. (Photo credits a, c, e, and f G. Etiope; b L. Innocenzi; d http://www.en.wikipedia.org/wiki/File:P3110004.jpg)
from Etiope (2015; Natural Gas Seepage, Springer)
GEO-CH4 EMISSIONS INTO THE ATMOSPHERE
A global grid map of geo-methane emissions
(project on atmospheric methane budget - NOAA-Colorado University)
Etiope, G., Ciotoli, G., Schwietzke, S., Schoell, M. (2019). Gridded maps of geological methane emissions and their isotopic signature. Earth Syst. Sci. Data, 11, 1-22, https://doi.org/10.5194/essd-11-1-2019.
The discrepancy of global geo-methane emission estimates
Etiope G., Schwietzke S. (2019). Global geological methane emissions: an update of top-down and bottom-up estimates. Elem. Sci. Anth. 7, 47, http://doi.org/10.1525/elementa.383.
Nature (2016) paper on fossil methane emissions
Methane seepage in Italy (PRIN-MIUR 2009 project)
HYSED-IT Hydrocarbon Seeps Database Italy
Example of local measurements
Methane seepage in La Brea (Los Angeles)
Etiope G., Doezema L., Pacheco C. (2017). Emission of methane and heavier alkanes from the La Brea Tar Pits seepage area, Los Angeles. J. Geophys. Res. Atm., 122, 12,008-12,019. doi: 10.1002/2017JD027675
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