Topic 6 Maturation and Alteration of Reservoired Petroleum
The chemical composition of a crude oil is not fixed but changes in response to changing conditions in much the same way as kerogen composition changes (Topic 4). The evolution of a crude oil involves a process of continuous, irreversible change, usually called “maturation”, which leads from a heavy, NSO-rich “immature” crude oil towards “mature” crudes which are lighter. This trend was recognized as early as 1915 when White found that in areas where coal and oil occur together the higher rank coals were associated with the oils of highest API gravity. Since rank was known to increase with temperature it followed that API gravity also increased with temperature. Barton (1934) documented this trend for Gulf Coast oils where the deepest (and oldest) reservoirs contain lighter and more paraffinic crudes. Similar trends have been observed in many other areas also (although they are not universal) (Figure 6.1).
In documenting the effects of increasing depth on maturation, data is accumulated for many different crude oils taken from reservoirs located over the entire depth range. There are inherent uncertainties in this approach because crude oils generated by different source rocks may have started with very different compositions, although statistical studies involving large numbers of crude oils will tend to smooth out these effects. An alternative approach to the study of the effect of increasing depth of burial on composition is to examine the changes produced in a single crude oil type. Data of this type have been reported by Koons et al.