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Line 14: − ==What is Py-GC?==+ − Pyrolysis gas chromatography (Py-GC) is anhydrous thermal decomposition of a material that leads to the conversion of kerogen to hydrocarbon compounds. Py-GC can be conducted on whole rock or isolated kerogen samples to obtain a visual signature or “fingerprint” of the organic material present.+ + + + + + − − + Line 40:
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Pyrolysis gas chromatography (view source)
Revision as of 17:50, 29 January 2014
, 17:50, 29 January 2014merging article from chapter 8
| isbn = 0-89181-602-X
| isbn = 0-89181-602-X
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'''Pyrolysis gas chromatography''' (Py-GC) is anhydrous thermal decomposition of a material that leads to the conversion of kerogen to hydrocarbon compounds. Py-GC can be conducted on whole rock or isolated kerogen samples to obtain a visual signature or “fingerprint” of the organic material present.
When kerogen is pyrolyzed and the products are analyzed by [[gas chromatography]], those products can be similar to those present in oils. One can compare the pyrolysis products either with the products of natural generation (oil) or with the products of laboratory pyrolysis of the asphaltenes from oils. Both comparisons are useful for oil–[[source rock]] correlations. Oil–oil correlations can also be carried out by comparing products of asphaltene pyrolysis from the various oils.
==Dry vs hydrous pyrolysis==
Two different pyrolysis techniques can be employed. Dry [[Rock-Eval]] pyrolysis is technically simpler but yields products rather different from those in natural systems. It is therefore more difficult and risky to use as a detailed correlation tool.
In contrast, hydrous pyrolysis techniques, where source rock candidates are artificially matured at relatively high temperatures in the presence of liquid water, can be very helpful in correlating oils to [[source rocks]] since it yields products that are fairly similar to those obtained by natural [[maturation]]. Hydrous pyrolysis experiments are also particularly helpful when only immature source rock candidates are available. These source rocks can be “matured” experimentally, and the resulting expelled “oil” can be compared to other oils.
==How to read gas chromatograms==
==How to read gas chromatograms==
[[file:evaluating-source-rocks_fig6-5.png|thumb|{{figure number|1}}See text for explanation.]]
[[file:evaluating-source-rocks_fig6-5.png|thumb|{{figure number|1}}See text for explanation.]]
Gas chromatography generally is a qualitative tool. It is not typically used as a quantitative measurement of hydrocarbon molecules. However, the patterns generated in the chromatograms can help us determine if a source rock will be oil or gas prone.
[[Gas chromatography]] generally is a qualitative tool. It is not typically used as a quantitative measurement of hydrocarbon molecules. However, the patterns generated in the chromatograms can help us determine if a source rock will be oil or gas prone.
The X-axis of a gas chromatogram is retention time, and the Y-axis is the relative quantity of each compound. Each spike in the chromatogram represents a particular hydrocarbon compound, beginning with lowest number of carbon atoms in the compound on the left and going to higher chains of carbons to the right. The height of the spike represents the relative abundance of the compound pyrolized from the sample's kerogen. Typical gas chromatogram examples for types I, II, and III kerogen are shown in [[:file:evaluating-source-rocks_fig6-5.png|Figure 1]].
The X-axis of a gas chromatogram is retention time, and the Y-axis is the relative quantity of each compound. Each spike in the chromatogram represents a particular hydrocarbon compound, beginning with lowest number of carbon atoms in the compound on the left and going to higher chains of carbons to the right. The height of the spike represents the relative abundance of the compound pyrolized from the sample's kerogen. Typical gas chromatogram examples for types I, II, and III kerogen are shown in [[:file:evaluating-source-rocks_fig6-5.png|Figure 1]].
| Contain very few carbon compounds above C<sub>10</sub>
| Contain very few carbon compounds above C<sub>10</sub>
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==Example==
The figure below shows sterane m/z 217 mass fragmentograms of an immature rock extract (bottom left), a hydrous pyrolyzate (top right), and the correlatable oil (bottom right). The carbon-number distribution changes during pyrolysis, indicating a fundamental difference in composition between the immature extract and the kerogen. The pyrolyzate, however, is molecularly very similar to the oil.
[[file:oiloil-and-oilsource-rock-correlations_fig8-44.png|thumb|{{figure number|8-44}}Modified.]]
==See also==
==See also==
* [[Gas chromatography]]
* [[Data obtained by gas chromatography]]
* [[How is GC/MS done?]]
* [[Examples of correlations using GC/MS]]
* [[Limitations of GC/MS]]
* [[Data obtained by high-performance liquid chromatography]]
* [[Evaluating source rock quality]]
* [[Evaluating source rock quality]]
* [[Kerogen types]]
* [[Kerogen types]]
* [[Evaluating quality using rock-eval HI/OI]]
* [[Evaluating quality using rock-eval HI/OI]]
==External links==
==External links==
* [http://store.aapg.org/detail.aspx?id=545 Find the book in the AAPG Store]
* [http://store.aapg.org/detail.aspx?id=545 Find the book in the AAPG Store]
[[Category:Critical elements of the petroleum system]]
[[Category:Critical elements of the petroleum system]] [[Category:Evaluating source rocks]] [[Category:Oil–oil and oil–source rock correlations]] [[Category:Geochemistry]]
[[Category:Evaluating source rocks]]