Sedimentary seepage

Gas seepage in sedimentary hydrocarbon-prone (petroliferous) basins includes low-temperature CH4-dominated (generally around 80-99 per cent v/v) gas manifestations and exhalations related to the following four classes:

1 onshore mud volcanoes;

2 onshore seeps (independent of mud volcanism);

3 onshore microseepage;

4 offshore (submarine) macro-seeps (including seafloor mud volcanoes).

Such gas manifestations have historically been an important indicator of subsurface hydrocarbon accumulations, and still drive the geochemical exploration for petroleum and gas today (for example Schumacher and Abrams, 1996; Abrams, 2005). Methane production in these areas can be due to microbial and/or thermogenic processes. Microbial CH4 forms through the bacterial breakdown of organic material in sediments, providing a distinctive isotopic carbon composition, a 813C-CH4 lighter than -60 parts per mil. At greater depths, thermogenic CH4 produced through the thermal breakdown of organic matter or heavier hydrocarbons has a 813C-CH4 composition ranging from -50 to -25 parts per mil. The migration and accumulation of fossil CH4 in stratigraphic and structural traps has been extensively described in the literature of petroleum geology (Hunt, 1996). Methane gas in these areas is released naturally into the atmosphere mainly through active and permeable faults and fractured rocks, after long-distance migration driven by pressure or density gradients (Etiope and Martinelli, 2002).

Onshore mud volcanoes

Today a large amount of scientific information can be found regarding mud volcanoes, including information describing their formation mechanisms and distributions (for example Milkov, 2000; Dimitrov, 2002; Kopf, 2002). Mud volcanoes are cone-shaped structures formed by the emission of gas, water and sediments, sometimes with oil and/or rocky breccia, in areas where thick sequences of sedimentary rocks are compressed tectonically and often subjected to buoyancy-driven movements. Mud volcanoes, more than 900 structures on land and more than 300 on the ocean's shelves, are distributed along faults, over oil and gas reservoirs of the Alpine-Himalayan, the Pacific Ocean and the Caribbean geological belts (Figure 4.1). The gas of onshore mud volcanoes is mainly thermogenic CH4 (Etiope et al, 2009) and can be released through continuous (steady-state) exhalations from craters (Figure 4.2), vents (gryphons, bubbling pools or salses) and surrounding soil, or intermittent blow-outs and eruptions (for example Etiope and Milkov, 2004, and references therein).

Other macroseeps

All gas manifestations that are independent of mud volcanism can be referred to as 'other seeps', and include 'water seeps' and 'dry seeps' (Etiope et al, 2009). Water seeps release an abundant gas phase accompanied by a water discharge (bubbling springs, groundwater or hydrocarbon wells), where the water may have a deep origin and may have interacted with gas during its ascent to the surface. Dry seeps release only a gaseous phase, such as the gas venting from outcropping rocks or through the soil horizon or through river/lake beds. Gas bubbling from groundwater filled wells, or from other shallow water bodies, should be considered as dry seeps, since surface water is only crossed by gas flow. Dry gas flow through rocks and dry soils can produce fascinating flames (Figure 4.3). Many seeps naturally burn in the dry and

Figure 4.1 Global distribution of geological sources of hydrocarbons

Note: Dots are the main petroleum seepage areas; crosses are the main geothermal and volcanic areas. Source: Modified from Etiope and Ciccioli (2009)

Figure 4.2 Examples of mud volcano gas exhalations

Note: a) Dashgil, Azerbaijan; b) Regnano, Italy; c) Bakhar satellite, Azerbaijan; d) Paclele Beciu, Romania. Sources: a) C. Baciu, Babes-Bolyai University; b) G. Etiope, INGV; c) L. Innocenzi, INGV; d) G. Etiope, INGV

Figure 4.3 Examples of everlasting fire seeps

Note: a) Yanardag, Azerbaijan; b) Chimaera, Turkey; c) Monte Busca, Italy; d) Andreiasu, Romania Sources: a) L. Innocenzi, INGV; b) H. Hosgormez, University of Istanbul; c) G. Etiope, INGV; d) G. Etiope, INGV

Figure 4.3 Examples of everlasting fire seeps

Note: a) Yanardag, Azerbaijan; b) Chimaera, Turkey; c) Monte Busca, Italy; d) Andreiasu, Romania Sources: a) L. Innocenzi, INGV; b) H. Hosgormez, University of Istanbul; c) G. Etiope, INGV; d) G. Etiope, INGV

summer seasons, or throughout the year. Many vents can be easily ignited artificially.

Some fires are called 'everlasting' or 'eternal', since the presence of a flame has been continuously reported in historical records. Several continuous seeps are related to ancient religious traditions (such as those related to Zoroastrism in Azerbaijan; see Etiope et al, 2004a) and are still active today in archaeological sites (for example Chimaera seep in Turkey; see Hosgormez et al, 2008). Active seeps occur in almost all of the 112 countries hosting total petroleum systems (TPSs). More than 10,000 seeps are assumed to exist on land (Clarke and Cleverly, 1991) and can be found in all petroliferous areas (Figure 4.1) in correspondence with active tectonic faults.

Microseepage

Microseepage is the slow, invisible, but continuous loss of CH4 and light alkanes from sedimentary basins. It is the pervasive, diffuse exhalation of CH4 from the soil and may be responsible for positive fluxes or for a decrease in negative CH4 flux in dry lands, indicating that methanotrophic consumption in the soil could be lower than the input from underground sources (Etiope and Klusman, 2002; 2010). Positive fluxes are typically a few or tens of mg m-2 d-1, and may reach hundreds of mg m-2 d-1 over wide tectonized and faulted areas. All petroleum basins contain microseepage, as shown by innumerable surveys performed for petroleum exploration (for example Hunt, 1996; Saunders et al, 1999; Wagner et al, 2002; Abrams, 2005, Khan and Jacobson 2008). More than 75 per cent of the world's petroliferous basins contain surface seeps (Clarke and Cleverly, 1991). Klusman et al (1998, 2000) assumed that microseeping areas potentially include all of the sedimentary basins in dry climates, with petroleum and gas generation processes at depth, an area that has been estimated to be ~43.4 X 106km-2. Flux data available today suggest that microseepage corresponds closely with the spatial distribution of hydrocarbon reservoirs, coal measures and portions of sedimentary basins that are, or that have been, at temperatures >70°C (thermogenesis). Accordingly, Etiope and Klusman (2010) assumed that microseepage may occur within a TPS, a term used in petroleum geology (Magoon and Schmoker, 2000) to describe the whole hydrocarbon-fluid system in the lithosphere including the essential elements and processes needed for oil and gas accumulations, migration and seeps. 42 countries produce 98 per cent of the world's petroleum, 70 countries produce 2 per cent, and 70 countries produce 0 per cent. So a TPS, and consequently the potential for microseepage, occurs in 112 (42 + 70) countries, suggesting that microseepage is potentially a very common phenomenon and widespread on all continents.

The global area of potential microseepage was assessed using an analysis for the distribution of oil/gas fields within all of the 937 petroleum provinces or basins, reported using a GIS data set from the US Geological Survey World Petroleum Assessment 2000 and related maps (Etiope and Klusman, 2010). For each province, a polygon was drawn that enclosed gas/oil field points in interactive maps, and the area was estimated using graphic software. Using this method, it was determined that significant gas/oil field zones occur in at least 120 provinces. The total area of gas/oil field zones was estimated to be between 3.5 and 4.2 million km2 (Etiope and Klusman, 2010), approximately 7 per cent of global dryland area.

Submarine emissions

Methane is released from the seafloor in sedimentary basins mainly through cold seeps, mud volcanoes and pockmarks (see Judd and Hovland, 2007, for a comprehensive review). Unlike the flow in the subaerial environment, CH4 seeping into the marine environment meets significant hindrances before entering the atmosphere. Methane passing through seafloor sediments is normally oxidized at the sulphate-methane transition zone (Borowski et al, 1999); if the supply of CH 4 overcomes anaerobic consumption, CH4 bubbles are able to escape into the water column where they can be partially or completely dissolved and oxidized. The degree of dissolution in seawater depends mainly on the depth of the water, the temperature and the size of the bubbles rising towards the surface. In general, models and field data indicate that only submarine seeps occurring at depths less than 100-300m have a significant impact on the atmosphere (for example Leifer and Patro, 2002; Schmale et al, 2005).

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