2 Key ingredients for petroleum accumulation
Petroleum that accumulated as a thin layer at the top of an extensive horizontal reservoir would be uneconomic to extract. That is because many wells, each with only a small rate of production and lifetime, would be needed to extract the petroleum. To be worth working, a sealed petroleum-bearing ‘container’ or trap must be shaped naturally to retain and focus petroleum, rather as the curved upper surface of a balloon traps buoyant hot air. The lower surface of a trap is defined either by a petroleum–water contact or sometimes by another seal.
There are many different styles of trap (see Figure 4) but the most common are structural traps in the form of anticlines produced by tectonic processes, by differential compaction of soft rocks above hard, irregular surfaces and by evaporitic salt masses that rise gravitationally. The low density of salt, combined with its ductility, enables it to rise to form domes and intrusive masses. Because they produce distinctive geological and geophysical features, structural traps are the easiest to find.
Figure 4: Types of traps. Types A to E are explained in the text. A–C are structural traps, D is a stratigraphic trap and E is a combination trap.
About 80% of the world's petroleum reserves are held in structural traps like those shown in Figure 4. They include simple anticlines (A), faulted structures that juxtapose reservoirs against seals (B), and traps created at the flank of a salt dome or in the compaction anticline above it (C). Most fields in the North Sea occur in structural traps.
Stratigraphic traps result from lateral changes in rock type and typically consist of discontinuous sandstone bodies encased in mudstone (D). Sometimes referred to as ‘subtle traps’, they currently contain about 13% of the world's petroleum reserves, but much of the remaining undiscovered petroleum will probably be found in these settings because the more obvious structural traps have long since been exploited.
In practice, traps often form through a sequence of different processes over the course of tens of millions of years. For example, in E the reservoir was first deposited, then folded, uplifted and eroded, before being overlain by a much younger impermeable mudstone. The resulting configuration is appropriately called a combination trap. Provided it was intact before the reservoir received a petroleum charge, it forms a valid trap regardless of how long it took to form.
As petroleum accumulation continues it is possible for traps to fill beyond their natural spill-point, when petroleum can escape sideways to re-migrate to other traps (Figure 4, upper C) or to the Earth's surface where it emerges as oil or gas seeps.
The lowest trap associated with the fault might leak along fractures produced by the faulting, to help charge higher traps.