Changes

==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>

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==

* 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_o. Dry gas accumulations can occur at shallower depths, but oil is not likely at greater depths.

* 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_o. 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.

==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 (R₀ > 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>

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]] (R₀ > 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:

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.

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>

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>

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.

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.

[[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]]