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Introduction 

Natural hydrocarbon seepage has for many years served petroleum exploration as a direct indicator of gas and/or oil subsurface accumulations (Link, 1952; Jones and Drozd, 1983; Rhakmanov, 1987). Surface macro-seeps (visible gas vents or oil leaks from the soil or rock outcrops) are generally an indication of a fault in an active Petroleum Seepage System (Abrams, 1995) belonging to a Total Petroleum System (Magoon and Schmoker, 2000; Etiope et al., 2009a). The assessment of the origin and flux of the seeping gas is therefore a key task for understanding, without drilling, the subsurface hydrocarbon potential, genesis and quality; e.g., the presence of shallow microbial gas, deeper thermogenic accumulations, oil and nonhydrocarbon undesirable gases (CO₂, N₂, H₂S). The global number of terrestrial seeps seem to exceeds 10,000 (Clarke and Cleverly, 1991), but only a small number has been directly investigated. A global analysis of more than 200 onshore seeps worldwide (including 143 mud volcanoes) revealed that methane is thermogenic in about 80% of the cases, microbial gas is in only 4% of seeps, mixed gas in the remaining cases (Etiope et al, 2009a). Other studies have demonstrated how gas seeps can indicate subsurface petroleum biodegradation (Etiope et al, 2009b), which has an important impact on hydrocarbon quality, and it may influence exploration and production strategies.

More generally, gas and oil seeps are important for four main reasons:

1. Seeps can be indicators of petroleum or natural gas reservoirs.

2. Seeps indicate the occurrence of a fault.

3. Seeps can represent a geo-hazard for societal community and industry.

4. Seeps are natural sources of greenhouse gas.

Indicators of Petroleum or Natural Gas Reservoirs 

1. Seeps have driven petroleum exploration in many countries. They can assist hydrocarbon exploitation in the assessment of geochemical and pressure variations during fluid extraction, and they are fundamental for the definition of the Petroleum Seepage System (Abrams, 2005). The geochemical analysis of seeping gas, in particular, can be effective to understand the potential and nature of subsurface accumulations.

Occurrence of a Fault 

2. Seeps are effective indicators of tectonic discontinuities and rock formations with enhanced secondary permeability, providing information on the location and depth of gas-bearing faults. Mud volcanism, in particular, has been extensively studied for its sensitivity to seismic activity (Mellors et al., 2007).

Geo-Hazard for Societal Community and Industry 

3. Seeps may represent hazards for humans and buildings. Explosions and sudden flames may occur in gas-rich environments (boreholes, soil), if methane concentrations reach explosive levels of 5-10% in the presence of air. When methane is accompanied by hydrogen sulfide (e.g., in salt diapirism zones), seeps can be toxic or even lethal under some circumstances (Etiope et al., 2006). Hazardous conditions can be also induced by variable density mud, especially in the mud volcanoes, which can lead to formation of “quicksand”, with risks for people and animals. Seeps and mud volcano plumbing can damage building and infrastructures by gas-pressure build-up in the subsoil or by general degradation of geotechnical properties of soil foundations.

Natural Sources of Greenhouse Gas 

4. Offshore and onshore seepage, including microseepage, is an important source of greenhouse gas (methane; Etiope et al, 2008) and photochemical pollutants (ethane and propane; Etiope and Ciccioli, 2009). Onshore and offshore seeps (together with diffuse microseepage) are estimated to be the second most important natural source of atmospheric methane, after wetlands, both on global scale and European scale (Etiope, 2009 [and references therein]). The evaluation of the gas flux is, then, an important task for understanding the potential of underground gas accumulations, the possible hazard, and the impact into the atmosphere. In this respect, the gas flux from a given category of source (in this case “natural gas seep”) is a fundamental parameter for the definition of the “emission factor”, which is the basic element for upscaling procedures and greenhouse gas emission estimates on large scales (Etiope et al, 2007a).

Seep Databases 

Although petroleum seeps are very important for both scientific and energy exploration purposes, presently only a few and limited global onshore seep databases are available which report complete geographic and geochemical data, with special reference to gas seeps. Available seep databases are commercial products for oil industry (e.g., GIRTM by Fugro Robertson Ltd) and are derived mainly from older datasets (Link, 1952; BP SEEPS in Clarke and Cleverly, 1991; Simon Petroleum Technology, 1992). These databases, however, have incomplete or ambiguous descriptions of seeps, with repetitions and inactive impregnations being included. Most seeps refer to oil seeps and impregnations, and very few gas seeps are reported, rarely including gas geochemical (compositional and isotopic) data. A number of gas seeps studied and described in recent literature are not included.  

Following a long research on gas seepage, a new global dataset of onshore gas and oil seep has been recently developed. It includes gas seeps or mud volcanoes previously unreported, and many gas geochemical data which are fundamental for the evaluation of subsurface accumulation linked to the seeps. The dataset, named GLOGOS (Global Onshore Gas-Oil Seep), is described in the present note.  

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Description of the Dataset 

GLOGOS dataset is the result of ten years of studies and investigations on natural hydrocarbon seepage phenomena. It is a list of more than 1150 terrestrial (onshore) seeps from 84 countries (version August 2009), including a series of geographical and geochemical data, based on original research and extensive literature and internet web surveys. Seeps are classified in mud volcanoes, gas seeps, and oil seeps. 

Mud volcanoes release a three-phase (gas, water and sediment) mixture. Gas is typically released from gryphons, craters or bubbling pools (salses). There is a wide literature on genesis, typology, distribution and significance of mud volcanoes (e.g., Kopf, 2002; Milkov, 2005; Etiope et al., 2007b). 

Gas seeps refer to gas manifestations which are independent of mud volcanism. They may include:

• Water-seeps, which release an abundant gaseous phase accompanied by water discharge (bubbling springs, groundwater or hydrocarbon wells); water may have a deep origin and may have interacted with gas during its ascent to the surface.

• Dry-seeps, which 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, should be considered dry-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.

GLOGOS also reports some non-natural seeps;, i.e., anthropogenically induced seeps such as those produced by coal mining.  

GLOGOS Structure 

The dataset is integrated in a single Excel file and subdivided into 6 continental regions:

EUROPE (including Azerbaijan and Russia), ASIA, AFRICA, NORTH-AMERICA, CENTRAL-SOUTH- AMERICA, OCEANIA. 

The following types of information are reported:

1. Country

2. Estimated total number of seeps occurring in the country (according to published literature, web resources).

3. Longitude/Latitude geographic coordinates (variable format).

4. Name of the petroliferous basin or geographic area or region.

5. Type of seep: Gas, Oil, MV (mud volcano). For MV, the name can refer to a group or cluster of seeps. They are reported in different colours: Black: Gas seeps, Blue: Mud volcanoes, Green: Oil seeps, Brown: non-natural seeps (likely generated by anthropogenic activity).

6. Seep name or sample name.

7. Reference: bibliographic or web source (the complete reference list is in the last page of the Excel file). 

The following data are also provided for gas seeps and mud volcanoes:

• δ¹³C₁: isotopic ratio of carbon of CH₄ (‰, PDB) ₄

• δD₁: isotopic ratio of hydrogen of CH₄ (‰, SMOW)

• CH₄: methane concentration (%)

• C2: ethane concentration (%)

• C3: propane concentration (%)

• C1/(C2+C3): "Bernard" ratio 

Other gases (such as CO₂, N₂, Ar, He, H₂S, C4+ alkanes) and isotopic ratios (δ¹³CO₂, ³He/⁴He, δ¹⁵N) can be available for some seeps and included upon request. For a few seeps also gas flux data (flux from the ground to the atmosphere) can be available.

An example of a data table extracted from GLOGOS is shown in Figure 1.

The GLOGOS data are checked and selected in order to avoid seep repetitions (other databases may report duplicate or more data for the same seep, leading to a "false" -overestimated- total number of seeps), to distinguish non-natural seeps, and omit gas manifestations clearly related to geothermal processes (e.g., CO₂- rich gas unrelated to petroleum occurrences). 

Table 1 summarizes the number of seeps for each continent and typology for the version GLOGOS-AUG09. The percentage of attributes in version GLOGOS-AUG09 is shown in Table 2.

Bibliography and Auxiliary Material 

All seeps are referenced, by published scientific articles, reports or by www sources. The bibliographic material is available as electronic files. A Google-Earth kmz file for the visualization of many seeps (especially mud volcanoes, e.g., Figure 2), a series of photos (such as those in Figure 3), and specific maps are also available. 

Conclusion 

GLOGOS is a new global dataset of onshore gas and oil seeps which includes more than 1150 seeps from 84 countries (version August 2009). The dataset includes geographical and gas-geochemical data (molecular and isotopic composition of the main gases). Thanks to specific studies, GLOGOS includes many exclusive seeps, recently discovered or never reported in other databases. Seeps are catalogued by country and classified in three types: gas seeps, oil seeps and mud volcanoes. All seeps have a bibliographic or www reference.   

Seeps are valuable indicators of underground hydrocarbon resources; in this respect, GLOGOS is a cost-effective tool for a re-evaluation of petroleum potential studies in already explored basins and/or for an assessment of new prospects in frontier and unexplored areas. GLOGOS and related information can be requested by contacting the author of this paper.  

References 

Abrams, M.A., 2005, Significance of hydrocarbon seepage relative to petroleum generation and entrapment: Marine Petroleum Geology, v. 22, p. 457-477.   

Clarke, R.H., and R.W. Cleverly, 1991, Leakage and post-accumulation migration, in W.A. England, and A.J. Fleet, eds., Petroleum migration: Geological Society Special Publication no. 59, p. 265–271.   

Etiope G., 2009, Natural emissions of methane from geological seepage in Europe: Atmospheric Environment, v. 43, p. 1430-1443, doi:10.1016/j.atmosenv.2008.03.014.   

Etiope G., and P. Ciccioli, 2009, Earth’s degassing – A missing ethane and propane source: Science, v. 323, no. 5913, p. 478, doi: 10.1126/science.1165904.   

Etiope G., A. Feyzullayev, and C.L. Baciu, 2009a, Terrestrial methane seeps and mud volcanoes: a global perspective of gas origin: Marine Petroleum Geology, v. 26, p. 333-344, doi:10.1016/j.marpetgeo.2008.03.001.   

Etiope G., A. Feyzullayev, A.V. Milkov, A. Waseda., K. Mizobe, and C.H. Sun, 2009b, Evidence of subsurface anaerobic biodegradation of hydrocarbons and potential secondary methanogenesis in terrestrial mud volcanoes: Marine and Petroleum Geology, v. 26, doi:10.1016/j.marpetgeo.2008.12.002.  

Etiope G., T. Fridriksson, F. Italiano, W. Winiwarter, and J. Theloke J, 2007a, Natural emissions of methane from geothermal and volcanic sources in Europe: Journal of Volcanology and Geothermal Research, v. 165, p. 76-86.   

Etiope G., K.R. Lassey, R.W. Klusman, and E. Boschi E., 2008, Reappraisal of the fossil methane budget and related emission from geologic sources. Geophysical Research Letters, v. 35, L09307, doi:10.1029/2008GL033623.   

Etiope G., G. Martinelli, A. Caracausi, and F. Italiano, 2007b, Methane seeps and mud volcanoes in Italy: gas origin, fractionation and emission to the atmosphere. Geophysical Research Letters, v. L14303, doi: 10.1029/2007GL030341.   

Etiope G., G. Papatheodorou, D. Christodoulou, G. Ferentinos, E. Sokos, and P. Favali, 2006, Methane and hydrogen sulfide seepage in the NW Peloponnesus petroliferous basin (Greece): origin and geohazard: AAPG Bulletin, v. 90, p. 701-713.   

Jones, V.T., and R.J. Drozd, 1983, Predictions of oil and gas potential by near-surface geochemistry: AAPG Bulletin, v. 67, p. 932-952.   

Kopf, A.J., 2002, Significance of mud volcanism: Reviews of Geophysics, v. 40, no. 2, 1005, doi: 10.1029/2000RG000093.  

Link, W.K., 1952, Significance of oil and gas seeps in world oil exploration: AAPG Bulletin, v. 36, p. 1505-1540.

Magoon, L.B., and J.W. Schmoker, 2000, The Total Petroleum System - the natural fluid network that constraints the assessment units: U.S. Geological Survey World Petroleum Assessment 2000— Description and results: USGS Digital Data Series 60, World Energy Assessment Team, 31 p. 

Milkov, A.V., 2005, Global distribution of mud volcanoes and their significance in petroleum exploration, as a source of methane in the atmosphere and hydrosphere, and as geohazard, in G. Martinelli and B. Panahi, eds., Mud volcanoes , Geodynamics and Seismicity. IV: Earth and Environmental Sciences, v. 51, p. 77-87. NATO Science Series, Springer, p. 29-34. 

Rhakmanov, R.R., 1987, Mud volcanoes and their importance in forecasting of subsurface petroleum potential (in Russian), Nedra, Moscow.   

Simon Petroleum Technology Ltd., 1992, Hydrocarbon Seeps: A Global Digital Database. Non-exclusive data product.

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Copyright © AAPG. Serial rights given by author. For all other rights contact author directly.

Copyright © AAPG. Serial rights given by author. For all other rights contact author directly.