There has been substantial progress in organic geochemistry, over the past two decades, in the areas of geochemical modeling and analytical methods. However, lesser attention has been paid by most geochemists to the natural variability of geologic systems. An examination of selected stratigraphic units from North America, South America, and southeast Asia suggests that variations in depositional conditions control how source rock intervals are stratigraphically distributed, as well as the character of the organic matter and the nature of the hydrocarbons generated upon thermal maturation.

The distribution of organic-rich intervals within a source sequence impacts the quantity of hydrocarbons generated as well as expulsion efficiency. Variations in the nature of the organic matter within the organic-rich intervals directly impact both the quantity and character of the generated products. Variations in geochemical character of many source rock systems can be substantial enough to raise questions on proposed oil-to-source rock correlations. Available data clearly indicate the need for more geologically controlled sampling programs that take into consideration the ultimate goal of the study.