Changes

===Molecular composition===

===Molecular composition===

The most common analyses done to determine molecular composition are as follows (see <ref name=pt05r1>Altgelt, K. W., Gouw, T. H., 1979, Chromatography in Petroleum Analysis: New York, Marcel Dekker.</ref>, and <ref name=pt05r154 />, for a description of these analyses and their applications):

The most common analyses done to determine molecular composition are as follows (see <ref name=pt05r1>Altgelt, K. W., Gouw, T. H., 1979, Chromatography in Petroleum Analysis: New York, Marcel Dekker.</ref>, and <ref name=pt05r154 />, for a description of these analyses and their applications):

Molecular compositions can be used to categorize oils (such as waxy or paraffinic and aromatic) and to determine the effects of geological processes (such as biodegraded, water washed, or immature). Bulk properties such as API gravity can sometimes be predicted from molecular composition data<ref name=pt05r94>Kennicutt, M. C., Brooks, J. M., 1988, Surface geochemical exploration studies predict API gravity off California: Oil and Gas Journal: Sept. 12, p. 101–106.</ref>. Chromatographic methods are typically used to determine these characteristics.

Molecular compositions can be used to categorize oils (such as waxy or paraffinic and aromatic) and to determine the effects of geological processes (such as biodegraded, water washed, or immature). Bulk properties such as API gravity can sometimes be predicted from molecular composition data<ref name=pt05r94>Kennicutt, M. C., Brooks, J. M., 1988, Surface geochemical exploration studies predict API gravity off California: Oil and Gas Journal: Sept. 12, p. 101–106.</ref>. Chromatographic methods are typically used to determine these characteristics.

Gas chromatography (GC) data can indicate the geological mechanisms responsible for changes in composition of an oil. For example, during biodegradation, bacteria preferentially remove the ''n''-paraffins that are prominent features in most chromatograms (Figure 2). Therefore the decrease or absence of n-paraffins is a strong indication that an oil is biodegraded. Other geological processes recognizable from chromatograms are thermal immaturity (odd-even predominance of the ''n''-paraffins), water washing (depletion of light aromatics), leaky reservoir seals (loss of light ends), and source characteristics (biomarkers) (Figure 3). Drilling additives and contaminants can also be identified by chromatography (Figure 4).

Gas chromatography (GC) data can indicate the geological mechanisms responsible for changes in composition of an oil. For example, during biodegradation, bacteria preferentially remove the ''n''-paraffins that are prominent features in most chromatograms ([[:file:oil-and-condensate-analysis_fig2.png|Figure 2]]). Therefore the decrease or absence of n-paraffins is a strong indication that an oil is biodegraded. Other geological processes recognizable from chromatograms are thermal immaturity (odd-even predominance of the ''n''-paraffins), water washing (depletion of light aromatics), leaky reservoir seals (loss of light ends), and source characteristics (biomarkers) ([[:file:oil-and-condensate-analysis_fig3.png|Figure 3]]). Drilling additives and contaminants can also be identified by chromatography ([[:file:oil-and-condensate-analysis_fig4.png|Figure 4]]).

If proper standardized procedures are followed, chromatograms can provide reproducible “fingerprints” of oils. These oil fingerprints can then be used to compare and correlate oils. This technique of molecular characterization is more discriminating than bulk property data<ref name=pt05r88>Kaufman, R. L., Ahmed, A. S., Elsinger, R. J., 1990, Gas chromatography as a development and production tool, in Geochemistry of Gulf Coast Oils and Gases: Proceedings of the 9th Annual Research Conference, Gulf Coast Section, Society of Economic Paleontologists and Mineralogists, p. 263–282.</ref>. For ease in interpretation, chromatographic data can also be displayed as polar or star plots of hydrocarbon peak ratios ([[:file:oil-and-condensate-analysis_fig5.png|Figure 5]]).

 

 

 

If proper standardized procedures are followed, chromatograms can provide reproducible “fingerprints” of oils. These oil fingerprints can then be used to compare and correlate oils. This technique of molecular characterization is more discriminating than bulk property data<ref name=pt05r88>Kaufman, R. L., Ahmed, A. S., Elsinger, R. J., 1990, Gas chromatography as a development and production tool, in Geochemistry of Gulf Coast Oils and Gases: Proceedings of the 9th Annual Research Conference, Gulf Coast Section, Society of Economic Paleontologists and Mineralogists, p. 263–282.</ref>. For ease in interpretation, chromatographic data can also be displayed as polar or star plots of hydrocarbon peak ratios (Figure 5).

 

[[file:oil-and-condensate-analysis_fig5.png|thumb|{{figure number|5}}(a) Gas chromatographic data can be displayed as chromatograms at different attenuations, (b) as star diagrams of selected peak height ratios, or (c) as data tables of peak height ratios. The star diagram is a polar plot of the peak height ratio data from the table, which shows data from the chromatographic trace for peaks between ''n''-C₁₇ and ''n''-C₂₅.]]

Gas chromatography should never be used alone to make these interpretations. Supporting analytical data and geological information should be obtained as well. A combination of several processes (that is, multiple sources for oils and/or different thermal maturities) can make interpretation complex.

Gas chromatography should never be used alone to make these interpretations. Supporting analytical data and geological information should be obtained as well. A combination of several processes (that is, multiple sources for oils and/or different thermal maturities) can make interpretation complex.