Changes

* Differing thermal maturities of samples

* Differing thermal maturities of samples

* Solution-exsolution at different depths

* Solution-exsolution at different depths

* Biodegradation

* [[Biodegradation]]

* Natural variation among samples

* Natural variation among samples

* Multiple sources for oil in a trap

* Multiple sources for oil in a trap

==Biodegradation==

==Biodegradation==

Although [http://www.oiltracers.com/services/exploration-geochemistry/oil-biodegradation.aspx biodegradation] affects gas chromatograms very strongly, steranes, triterpanes, and isotopes are much less affected. Modern correlation technology can usually overcome differences caused by biodegradation.

Although [[biodegradation]] affects gas chromatograms very strongly, steranes, triterpanes, and isotopes are much less affected. Modern correlation technology can usually overcome differences caused by biodegradation.

==Natural variation among samples==

==Natural variation among samples==

Any group of oils will inevitably exhibit variations among the members. This “natural variation” can stem from true differences, sample handling, analytical error, or differences in technique between laboratories. The amount of acceptable or “normal” difference varies greatly, depending on the range of maturities, biodegradation, expulsion and [[migration]] (the effects of which are especially large when comparing oils with source rock extracts), and variations in source rock facies. Correlations become increasingly difficult and tenuous as distances between samples increase.

Any group of oils will inevitably exhibit variations among the members. This “natural variation” can stem from true differences, sample handling, analytical error, or differences in technique between laboratories. The amount of acceptable or “normal” difference varies greatly, depending on the range of maturities, [[biodegradation]], expulsion and [[migration]] (the effects of which are especially large when comparing oils with source rock extracts), and variations in source rock facies. Correlations become increasingly difficult and tenuous as distances between samples increase.

Vertical and lateral rates of change in source rock facies are highly variable but are generally highest in nearshore and nonmarine environments. Katz (in press), for example, documents the extreme variability in source facies in the lacustrine Pematang Formation of central Sumatra and the consequent variability in oil properties. As he notes, correlations in such settings are very difficult and depend critically upon sample coverage of the source facies.

Vertical and lateral rates of change in source rock facies are highly variable but are generally highest in nearshore and nonmarine environments. Katz (in press), for example, documents the extreme variability in source facies in the lacustrine Pematang Formation of central Sumatra and the consequent variability in oil properties. As he notes, correlations in such settings are very difficult and depend critically upon sample coverage of the source facies.