# Source, Trap, and Reservoir

Part of the Geology Merit Badge Digital Resource Guide: [Introduction & Overview](https://merit-badge.university/merit-badges/geology/guide/index.md)

*4. Energy Resources*

> **4b2.** Discuss source rock, trap, and reservoir rock—the three components necessary for the occurrence of oil and gas underground.

Oil and gas do not just sit in giant underground caves waiting to be found. They are part of a geologic system. If one key piece is missing, the resource may never accumulate in a useful amount.

## The Three Essential Parts

### Source Rock

A **source rock** is the rock where the organic material originally built up and, under heat and pressure, was transformed into oil or gas. These are often fine-grained sedimentary rocks such as shale rich in ancient microscopic life or plant material.

### Reservoir Rock

A **reservoir rock** is the rock that can store oil or gas. It needs enough **porosity** for fluids to occupy open spaces and enough **permeability** for fluids to move through connected spaces. Sandstone and some limestones are common examples.

### Trap

A **trap** is the geologic arrangement that stops oil or gas from continuing to migrate upward. Without a trap, the fluids may escape. Traps can be structural, like an anticline or fault trap, or stratigraphic, where rock layers change in a way that blocks movement.

## How the Pieces Work Together

1. Organic-rich sediment becomes a source rock.
2. Heat and pressure generate oil or gas.
3. The fluids migrate through permeable pathways.
4. The fluids collect in a porous reservoir rock.
5. A trap and seal keep them from escaping farther upward.

If you are missing the source, there is no petroleum to begin with. If you are missing a good reservoir, there is nowhere practical for it to collect. If you are missing a trap, it may leak away.

![Geologic cross-section showing source rock, migration, porous reservoir, seal, and an anticline trap with oil and gas accumulation](https://ik.imagekit.io/doulacoop/tr:w-800/mbu-assets/merit-badges/geology/guide/petroleum-system-cross-section?v=05e43b12)

**Three-Part Petroleum System** — What to mention when you discuss this with your counselor

- [ ] **Source rock:** where hydrocarbons form
- [ ] **Reservoir rock:** where hydrocarbons collect and can be stored
- [ ] **Trap:** what keeps them from migrating away
- [ ] **Seal:** often a low-permeability layer such as shale that helps hold the trap closed

> **Tip**
>
> Draw a simple cross-section. Even a basic sketch with a source layer below, a porous reservoir above, and a trap under a sealing layer can make this requirement much easier to explain.

## Official Resources

Video: [Formation of Reservoir Rock (video)](https://youtu.be/_PDOD_FEnNk)

Up next, you will look at the tools geologists use to find these hidden systems underground.

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- Previous: [Power Sources in the United States](https://merit-badge.university/merit-badges/geology/guide/req4b1/index.md)
- Next: [Tools for Finding Oil and Gas](https://merit-badge.university/merit-badges/geology/guide/req4b3/index.md)