Page 34 - Hydrocarbon

Exploration 21

fore-arc pelagic
basin sediment

continental crust

remnant of
oceanic plate
oceanic plate
Compressional System

Extensional System
Figure 3.3 Sedimentary basins.

algae which live in the upper layers of the oceans, and upon death sink in vast
quantities onto the seabed. Plant-derived source rocks often lead to ‘waxy’ crudes.
An example of a marine source rock is the Kimmeridge clay which has sourced the
large fields in the Northern North Sea. The coals of the carboniferous age have
sourced the gas fields of the Southern North Sea.

3.1.4. Maturation

The conversion of sedimentary organic matter into petroleum is termed maturation.
The resulting products are largely controlled by the composition of the original
matter. Figure 3.4 shows the maturation process, which starts with the conversion of
mainly kerogen into petroleum; but in very small amounts below a temperature of
501C (kerogen: organic rich material which will produce hydrocarbon on heating).
When kerogens are present in high concentrations in shale, and have not been
heated to a sufficient temperature to release their hydrocarbons, they may form
oil shale deposits.
The temperature rises as the sediment package subsides within the basinal
framework. The peak conversion of kerogen occurs at a temperature of about
1001C. If the temperature is raised above 1301C for even a short period of time,
crude oil itself will begin to ‘crack’ and gas will start to be produced. Initially the
composition of the gas will show a high content of C4–C10 components (wet gas
and condensate), but with further increases in temperature the mixture will tend
towards the light hydrocarbons (C1–C3, dry gas). For more detail on the
composition of hydrocarbons, refer to Section 6.2, Chapter 6.