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==Gasification==
 
==Gasification==
Gasification is the conversion of oil to gas resulting from thermal [[cracking]]. It primarily takes place during burial. If oil is spilled from a trap by gas displacement during gasification, the oil may occur in economic [[accumulation]]s updip along the migration pathway.<ref name=ch11r12>Gussow, W. C., 1954, [http://archives.datapages.com/data/bulletns/1953-56/data/pg/0038/0005/0800/0816.htm Differential entrapment of oil and gas: a fundamental principle]: AAPG Bulletin, vol. 38, p. 816–853.</ref>
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Gasification is the conversion of oil to gas resulting from thermal [[cracking]]. It primarily takes place during burial. If oil is spilled from a trap by gas displacement during gasification, the oil may occur in economic [[accumulation]]s updip along the [[migration pathway]].<ref name=ch11r12>Gussow, W. C., 1954, [http://archives.datapages.com/data/bulletns/1953-56/data/pg/0038/0005/0800/0816.htm Differential entrapment of oil and gas: a fundamental principle]: AAPG Bulletin, vol. 38, p. 816–853.</ref>
    
==Predicting and recognizing gasification==
 
==Predicting and recognizing gasification==
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* Geohistory analysis with proper gasification [[kinetics]] can usually predict at what depth [[accumulation]]s have been gasified.
 
* Geohistory analysis with proper gasification [[kinetics]] can usually predict at what depth [[accumulation]]s have been gasified.
* As a rule of thumb, oil should not be expected at subsurface temperatures > [[temperature::150&deg;C]] or a maturation level much above 1.3% R<sub>o</sub>. Dry gas accumulations can occur at shallower depths, but oil is not likely at greater depths.
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* As a rule of thumb, oil should not be expected at subsurface temperatures > [[temperature::150&deg;C]] or a [[maturation]] level much above 1.3% R<sub>o</sub>. Dry gas accumulations can occur at shallower depths, but oil is not likely at greater depths.
 
* Gasification of oil in reservoirs is associated with the formation of pyrobitumen.<ref name=ch11r34>Tissot, B. P., D. H. Welte, 1984, Petroleum Formation and Occurrence, 2 ed.: New York, Springer-Verlag, 699 p. 460–461</ref>
 
* Gasification of oil in reservoirs is associated with the formation of pyrobitumen.<ref name=ch11r34>Tissot, B. P., D. H. Welte, 1984, Petroleum Formation and Occurrence, 2 ed.: New York, Springer-Verlag, 699 p. 460–461</ref>
 
* Displacement of oil from a trap by gas is associated with asphaltene precipitates and/or relatively unaltered oil stain.
 
* Displacement of oil from a trap by gas is associated with asphaltene precipitates and/or relatively unaltered oil stain.
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==Predicting gas destruction==
 
==Predicting gas destruction==
It is not the destruction of methane as much as the lack of economic [[accumulation]]s which occurs at higher maturation levels. Methane occurs in fluid inclusions from lower crustal depths, and shows of methane are not unusual where drilling through low-grade metamorphic rocks—even those at a grade high enough to contain graphite instead of kerogen (R<sub>0</sub> > 8%). For example the Shell Barret #1 well in Hill County, Texas, had a 30-minute methane flare at over [[depth::13,000 ft]] depth in rock described as dolomite and calcite marble with graphitic inclusions.<ref name=ch11r30>Rozendal, R. A., and W. S. Erskine, 1971, [http://archives.datapages.com/data/bulletns/1971-73/data/pg/0055/0011/2000/2008.htm Deep test in Ouachita structural belt of Central Texas]: AAPG Bulletin, vol. 56, p. 2008–2017.</ref>
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It is not the destruction of methane as much as the lack of economic [[accumulation]]s which occurs at higher maturation levels. Methane occurs in fluid inclusions from lower crustal depths, and shows of methane are not unusual where drilling through low-grade metamorphic rocks—even those at a grade high enough to contain graphite instead of [[kerogen]] (R<sub>0</sub> > 8%). For example the Shell Barret #1 well in Hill County, Texas, had a 30-minute methane flare at over [[depth::13,000 ft]] depth in rock described as [[dolomite]] and calcite marble with graphitic inclusions.<ref name=ch11r30>Rozendal, R. A., and W. S. Erskine, 1971, [http://archives.datapages.com/data/bulletns/1971-73/data/pg/0055/0011/2000/2008.htm Deep test in Ouachita structural belt of Central Texas]: AAPG Bulletin, vol. 56, p. 2008–2017.</ref>
    
The following characteristics can help us predict and recognize gas destruction:
 
The following characteristics can help us predict and recognize gas destruction:
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Carbon dioxide, hydrogen sulfide, and nitrogen can constitute a significant percentage of natural gas from some [[accumulation]]s. In some cases, natural gas is uneconomic due to the high nonhydrocarbon gas content.
 
Carbon dioxide, hydrogen sulfide, and nitrogen can constitute a significant percentage of natural gas from some [[accumulation]]s. In some cases, natural gas is uneconomic due to the high nonhydrocarbon gas content.
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Although low concentrations of carbon dioxide can be derived from organic sources or byproducts of silicate reactions at moderate temperatures<ref name=ch11r32>Smith, J. T., and S. N. Ehrenberg, 1989, Correlation of carbon dioxide abundance with temperature in clastic hydrocarbon reservoirs: relationship to inorganic chemical equilibrium: Marine and Petroleum Geology, vol. 6, p. 129–135., 10., 1016/0264-8172(89)90016-0</ref> high concentrations of carbon dioxide are usually associated with igneous intrusion or regional heating of impure limestones.<ref name=ch11r9>Farmer, R. E., 1965, [http://archives.datapages.com/data/specpubs/methodo2/data/a071/a071/0001/0350/0378.htm Genesis of subsurface carbon dioxide], in A. Young, and J. Galley, eds., Fluids in Subsurface Environments: AAPG Memoir No. 4, p. 378–385.</ref>
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Although low concentrations of carbon dioxide can be derived from organic sources or byproducts of silicate reactions at moderate temperatures<ref name=ch11r32>Smith, J. T., and S. N. Ehrenberg, 1989, Correlation of carbon dioxide abundance with temperature in clastic hydrocarbon reservoirs: relationship to inorganic chemical equilibrium: Marine and Petroleum Geology, vol. 6, p. 129–135., 10., 1016/0264-8172(89)90016-0</ref> high concentrations of carbon dioxide are usually associated with [[igneous]] intrusion or regional heating of impure limestones.<ref name=ch11r9>Farmer, R. E., 1965, [http://archives.datapages.com/data/specpubs/methodo2/data/a071/a071/0001/0350/0378.htm Genesis of subsurface carbon dioxide], in A. Young, and J. Galley, eds., Fluids in Subsurface Environments: AAPG Memoir No. 4, p. 378–385.</ref>
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Hydrogen sulfide concentration increases with depth in gas reservoirs with anhydrite, indicating that it, too, is a product of higher maturity.<ref name=ch11r20 /> The methane is reacting with the sulfate to form hydrogen sulfide and carbon dioxide gas. The reaction is probably kinetically controlled.
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Hydrogen sulfide concentration increases with depth in gas reservoirs with [[anhydrite]], indicating that it, too, is a product of higher maturity.<ref name=ch11r20 /> The methane is reacting with the sulfate to form hydrogen sulfide and carbon dioxide gas. The reaction is probably kinetically controlled.
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The origin of nitrogen gas is not well characterized. In nonpetroleum basins, nitrogen may have high concentration because no other gas is present to dilute it. High-nitrogen gas in thermally mature basins is possibly from [[coal]] sources<ref name=ch11r33>Stahl, W., H. Boigk, and G. Wollanke, 1978, Carbon and nitrogen isotope data of upper Carboniferous and Rotliegend natural gases from north Germany and their relationship to the maturity of the organic source material: Advances in Organic Geochemistry 1976, p. 539–559.</ref> or from the mantle or deep crust.<ref name=ch11r17> Jenden, P. D.,and I. R. Kaplan, 1989, [http://archives.datapages.com/data/bulletns/1988-89/data/pg/0073/0004/0400/0431.htm Origin of natural gas in Sacramento basin, California]: AAPG Bulletin, vol. 73, p. 431–453.</ref>
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The origin of nitrogen gas is not well characterized. In nonpetroleum basins, nitrogen may have high concentration because no other gas is present to dilute it. High-nitrogen gas in thermally mature basins is possibly from [[coal]] sources<ref name=ch11r33>Stahl, W., H. Boigk, and G. Wollanke, 1978, Carbon and nitrogen isotope data of upper Carboniferous and Rotliegend natural gases from north Germany and their relationship to the maturity of the organic source material: Advances in Organic Geochemistry 1976, p. 539–559.</ref> or from the [[mantle]] or deep [[crust]].<ref name=ch11r17> Jenden, P. D.,and I. R. Kaplan, 1989, [http://archives.datapages.com/data/bulletns/1988-89/data/pg/0073/0004/0400/0431.htm Origin of natural gas in Sacramento basin, California]: AAPG Bulletin, vol. 73, p. 431–453.</ref>
    
==Predicting burial destruction==
 
==Predicting burial destruction==
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* Analyzing geohistory or mapping maturation indicators can identify reservoir maturation levels where methane [[accumulation]]s may be uneconomic. Most sizable gas accumulations occurring at maturation levels > 2.8% R<sub>o</sub> have thick claystone seals that help preserve the accumulation.
 
* Analyzing geohistory or mapping maturation indicators can identify reservoir maturation levels where methane [[accumulation]]s may be uneconomic. Most sizable gas accumulations occurring at maturation levels > 2.8% R<sub>o</sub> have thick claystone seals that help preserve the accumulation.
 
* Presence of intrusives in the fetch area can indicate a potential for carbon dioxide dilution.<ref name=ch11r29>Parker, C., 1974, Geopressures and secondary porosity in the deep Jurassic of Mississippi: Transactions of the Gulf Coast Association of Geological Societies, vol. 24, p. 69–80.</ref>
 
* Presence of intrusives in the fetch area can indicate a potential for carbon dioxide dilution.<ref name=ch11r29>Parker, C., 1974, Geopressures and secondary porosity in the deep Jurassic of Mississippi: Transactions of the Gulf Coast Association of Geological Societies, vol. 24, p. 69–80.</ref>
* If reservoir rocks are associated with evaporite cements or beds, expect hydrogen sulfide if the reservoir is exposed to temperatures > [[temperature::150&deg;C]] and iron is not present to remove the hydrogen sulfide.
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* If reservoir rocks are associated with [[evaporite]] cements or beds, expect hydrogen sulfide if the reservoir is exposed to temperatures > [[temperature::150&deg;C]] and iron is not present to remove the hydrogen sulfide.
 
* Nitrogen is released during the late stages of coal maturation.<ref name=ch11r18>Jüntgen, V. H., and J. Karweil, 1966, Gasbildung and gasspeicherung in steinkohlenfluzen, I. gasbildung: Erdöl und Kohle-Erdgas-Petrochemie, vol. 19, p. 339–344.</ref> Therefore, if a prospect is [[Calculating charge volume|charged]] by a type III source rock only during its late maturation stage (R<sub>o</sub> > 2.5%), nitrogen dilution is possible. High nitrogen gas content is also characteristic of evaporative settings and hydrocarbon-poor basins.
 
* Nitrogen is released during the late stages of coal maturation.<ref name=ch11r18>Jüntgen, V. H., and J. Karweil, 1966, Gasbildung and gasspeicherung in steinkohlenfluzen, I. gasbildung: Erdöl und Kohle-Erdgas-Petrochemie, vol. 19, p. 339–344.</ref> Therefore, if a prospect is [[Calculating charge volume|charged]] by a type III source rock only during its late maturation stage (R<sub>o</sub> > 2.5%), nitrogen dilution is possible. High nitrogen gas content is also characteristic of evaporative settings and hydrocarbon-poor basins.
 
* Nonhydrocarbon gas concentrations in mature basins can be estimated from evaluating regional gas concentration trends.
 
* Nonhydrocarbon gas concentrations in mature basins can be estimated from evaluating regional gas concentration trends.
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[[Category:Predicting the occurrence of oil and gas traps]]  
 
[[Category:Predicting the occurrence of oil and gas traps]]  
 
[[Category:Predicting preservation and destruction of accumulations]]
 
[[Category:Predicting preservation and destruction of accumulations]]
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[[Category:Treatise Handbook 3]]

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