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| isbn = 0-89181-602-X
| isbn = 0-89181-602-X
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Given the strong economic control of petroleum type on development of an accumulation, the conversion of oil to gas or dilution of gas by nonhydrocarbon components in the deep burial environment may make an accumulation uneconomic and, from an exploration point of view, “destroyed.” The following burial processes destroy accumulations by altering the properties of the petroleum:
Given the strong economic control of petroleum type on development of an [[accumulation]], the conversion of oil to gas or dilution of gas by nonhydrocarbon components in the deep burial environment may make an accumulation uneconomic and, from an exploration point of view, “destroyed.” The following burial processes destroy accumulations by altering the properties of the petroleum:
* Gasification
* Gasification
==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 accumulations 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==
The following characteristics can help us predict and recognize gasification.
The following characteristics can help us predict and recognize gasification.
* Geohistory analysis with proper gasification kinetics can usually predict at what depth accumulations 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°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.
* As a rule of thumb, oil should not be expected at subsurface temperatures > [[temperature::150°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., Welte, D., H., 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., Welte, D., H., 1984, Petroleum Formation and Occurrence, 2 ed.: New York, Springer-Verlag, 699 p. 460–461</ref>
==Predicting gas destruction==
==Predicting gas destruction==
It is not the destruction of methane as much as the lack of economic accumulations 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., Erskine, W., S., 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 (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., Erskine, W., S., 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:
* Economic gas accumulations become more unusual with maturation levels > 2.8% R<sub>o</sub>.<ref name=ch11r3>Bartenstein, H., 1980, Coalification in NW Germany: Erdöl und Kohle-Erdgas-Petrochemie: vol. 33, p. 121–125.</ref> This is the traditional base of the gas preservation zone.
* Economic gas [[accumulation]]s become more unusual with maturation levels > 2.8% R<sub>o</sub>.<ref name=ch11r3>Bartenstein, H., 1980, Coalification in NW Germany: Erdöl und Kohle-Erdgas-Petrochemie: vol. 33, p. 121–125.</ref> This is the traditional base of the gas preservation zone.
* The major gas accumulation with the highest well-documented maturity level where charging occurred before or during exposure to the high temperatures occurs at a maturation level 3.5–3.8% R<sub>o</sub> equivalent (Wilburton field, Oklahoma).<ref name=ch11r13>Hendrick, S., J., 1992, Vitrinite reflectance and deep Arbuckle maturation at Wilburton field, Latimer County, OK: Oklahoma Geological Survey Circular 93, p. 176–184.</ref>
* The major gas accumulation with the highest well-documented maturity level where charging occurred before or during exposure to the high temperatures occurs at a maturation level 3.5–3.8% R<sub>o</sub> equivalent (Wilburton field, Oklahoma).<ref name=ch11r13>Hendrick, S., J., 1992, Vitrinite reflectance and deep Arbuckle maturation at Wilburton field, Latimer County, OK: Oklahoma Geological Survey Circular 93, p. 176–184.</ref>
==Gas dilution==
==Gas dilution==
Carbon dioxide, hydrogen sulfide, and nitrogen can constitute a significant percentage of natural gas from some accumulations. 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., Ehrenberg, S., N., 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 Young, A., Galley, J., 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., Ehrenberg, S., N., 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 Young, A., Galley, J., eds., Fluids in Subsurface Environments: AAPG Memoir No. 4, p. 378–385.</ref>
The following characteristics can help us predict and recognize burial destruction.
The following characteristics can help us predict and recognize burial destruction.
* Analyzing geohistory or mapping maturation indicators can identify reservoir maturation levels where methane accumulations 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°C]] and iron is not present to remove the hydrogen sulfide.
* If reservoir rocks are associated with evaporite cements or beds, expect hydrogen sulfide if the reservoir is exposed to temperatures > [[temperature::150°C]] and iron is not present to remove the hydrogen sulfide.