The Petroleum System: Source, Reservoir, Seal, Trap and Timing

Every accumulation needs a working system

Oil and gas are found only where a chain of geological events has worked in the right order. Geologists call this chain the petroleum system. If any link is missing, there is no accumulation, however good the other links are. Exploration is the work of testing each link with the data available, and then deciding whether the whole chain is likely enough to justify a well.

Worked example 1: a missing link

A company mapped a large, clear anticline (an arch-shaped fold) on seismic data. The reservoir sandstone was good in nearby wells, and a thick shale seal covered it. The well found the sandstone full of water. A later study showed that the regional source rock was too shallow in that area to have generated oil. Every element was present except the charge. The trap was real, but there was nothing to fill it.

Worked example 2: a timing problem

In another basin, a fault block trap was formed by movement that happened millions of years after the main phase of oil generation. Wells on older traps nearby found oil. The younger trap was dry. The team now checks the age of each trap against the burial history of the source rock before they rank a prospect.