Oil fields don’t just happen. Each accumulation of hydrocarbons has a unique history associated with it. For example: At some point in the Earth’s past a river deposited a thick sedimentary layer of sand. Sea levels began to rise and burried the land in a deep sea. Fine particles settled and formed a shaley layer that will later create a seal.
Many eons pass and both layers get burried under more and more sediments. The area is slowly ripped apart by the forces of Earth’s plate tectonics. The process creates different structures. Highs and lows. The lows get burried by more and more sediments whereas the highs remain high as the rifting activity fades over time.
Many eons later an earlier deposited black shale, rich in organic content is burried deep enough to generate oil. It slowly releases the oil which then begins to rise and flow upwards towards the high.
Once it gets there it is stopped by the seal and it forms an accumulation.
This concept, which includes all the major play elements
- Reservoir
- Seal
- Source
- Trap
- HGMT (Hydrocarbon generation, Migration & Timing)
is the general idea of how oil fields are created. The unique combination of all 5 is called a play, or play concept.
Now, the reality of oilfields is way more complicated than that. In frontier basins simple concepts need to be proven. In very prolific basins like the North Sea where a lot of hydrocarbons were deposited the charge-history of a field can be very complicated. Multiple charge phases from different charge-routes etc.
Different play concepts can be broken down into just a few words like. Jurassic sandstones charged by Jurassic source rock in a rotated fault-block. A large part of the North Sea fields work that way.
When people talk about geological plays on a very high level the terms for the individual plays often get broken down to just a simple “Jurassic Play”. It often implies the most obvious component of the play concept, the reservoir, in this case a Jurassic reservoir.
The NPD, the Norwegian Petroleum Directorate has issued maps for the most common plays on the Norwegian Continental Shelf (NCS)
As depositional environment, thermal history, and structural elements vary strongly from basin to basin these concepts can’t be transfered 1-to-1 but often similar elements exist.