Changes

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.

A major part of the art of correlation is deciding whether an observed difference is significant or is better considered to be the result of natural variation. The differences between the various members of a family of oils are often more subtle than the discussions in this chapter suggest. In fact, sometimes the variations within a single family (especially those that cover wide geographic areas or those from nonmarine sources) are as large as the variations between families.

A major part of the art of correlation is deciding whether an observed difference is significant or is better considered to be the result of natural variation. The differences between the various members of a family of oils are often more subtle than the discussions in this chapter suggest. In fact, sometimes the variations within a single family (especially those that cover wide geographic areas or those from nonmarine sources) are as large as the variations between families.