View source for Burial destruction

{{publication 
 | image   = exploring-for-oil-and-gas-traps.png
 | width   = 120px
 | series  = Treatise in Petroleum Geology
 | title   = Exploring for Oil and Gas Traps
 | part    = Predicting the occurrence of oil and gas traps
 | chapter = Predicting preservation and destruction of accumulations
 | frompg  = 11-1
 | topg    = 11-30
 | author  = Alton A. Brown
 | link    = http://archives.datapages.com/data/specpubs/beaumont/ch11/ch11.htm
 | pdf     = 
 | store   = http://store.aapg.org/detail.aspx?id=545
 | isbn    = 0-89181-602-X
}}
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
* Gas destruction
* Gas dilution

==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>

==Predicting and recognizing gasification==
The following characteristics can help us predict and recognize gasification.

* 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.
* 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>
* Displacement of oil from a trap by gas is associated with asphaltene precipitates and/or relatively unaltered oil stain.
* Absence of an oil leg in the trap prior to charging by gas is indicated by the absence of oil stain with heavy molecular components.
* In accumulations that have been gasified, the presence of pyrobitumen can significantly reduce reservoir [[permeability]] due to gas or condensate.

==Gas destruction==
Methane is the most thermodynamically stable hydrocarbon in sedimentary basins.<ref name=ch11r16>Hunt, J., M., 1979, Petroleum Geochemistry and Geology: San Francisco, W., H. Freeman, 617 p.</ref> Methane apparently can be destroyed only by oxidation. The most common form of oxidation in the burial environment is thermogenic sulfate reduction.<ref name=ch11r20>Krouse, H., R., 1979, Stable isotope geochemistry of non-hydrocarbon constituents of natural gas: Proceedings of the Tenth World Petroleum Congress, vol. 4, p. 85–91.</ref> The presence of oxidized iron can also remove methane at high temperatures.

Studies by Barker and Takach.<ref name=ch11r2>Barker, C., Takach, N., E., 1992, [http://archives.datapages.com/data/bulletns/1992-93/data/pg/0076/0012/0000/1859.htm Prediction of natural gas composition in ultradeep sandstone reservoirs]: AAPG Bulletin, vol. 76, p. 1859–1873.</ref> indicate water can oxidize methane to carbon dioxide and hydrogen gas at temperatures as low as [[temperature::200&deg;C]], assuming systems are at thermo-dynamic equilibrium. Where oxygen fugacity is buffered at modestly reducing conditions, methane is calculated to remain stable to temperatures > [[temperature::400&deg;C]]<ref name=ch11r10>Green, D., H., Falloon, T., J., Taylor, W., R., 1987, Mantle-derived magmas—roles of variable source peridotite and variable C-H-O fluid compositions, in Mysen, B. O., ed., Magmatic Processes: Physiochemical Principles: The Geochemical Society Special Publication No. 1, p. 139–153.</ref>

==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., 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:

* 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>

==Gas dilution==
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>

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.

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., Boigk, H., Wollanke, G., 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., Kaplan, I., R., 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==
The following characteristics can help us predict and recognize burial destruction.

* 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>
* 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., Karweil, J., 1966, Gasbildung and gasspeicherung in steinkohlenfluzen, I. gasbildung: Erdöl und Kohle-Erdgas-Petrochemie, vol. 19, p. 339–344.</ref> Therefore, if a prospect is 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.

The table below summarizes techniques that help us predict hydrocarbon destruction during burial.

{| class = "wikitable"
|-
! Process || colspan=2 |  Prediction techniques
|-
| Gasification
| colspan=2 |  
*  Geohistory analysis 
*  Mapping maturation indicators (no oil, where reservoir R<sub>o</sub> > 1.3%)
|-
| Gas destruction
| colspan=2 |  
*  Geohistory analysis 
*  Mapping maturation indicators (gas unlikely where reservoir Ro> 2.8%)
|-
| rowspan=2 | Gas dilution || ''Identified by'' || ''Indicates potential for''
|-
| 
*  Intrusives in fetch area 
*  Evaporite cements or beds at depths where temperature > [[temperature::150&deg;C]] 
*  Gas sourced from coal, high thermal maturity 
*  Low methane [[Calculating charge volume|charge]]
|
*  Carbon dioxide 
*  Hydrogen sulfide 
*  Nitrogen 
*  Carbon dioxide, nitrogen
|}

==See also==
* [[Petroleum destruction]]
* [[Near-surface destruction]]

==References==
{{reflist}}

==External links==
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[[Category:Predicting the occurrence of oil and gas traps]] 
[[Category:Predicting preservation and destruction of accumulations]]