Geology · Exploration
Seal (Cap Rock)
A layer of rock with very low permeability above the reservoir that keeps the oil and gas from escaping upward.
What it does
Without a seal the buoyant hydrocarbons keep migrating until they reach surface. Shales, anhydrite and salt are the common seals.
Basics
| Also called | Caprock |
| Common rocks | Shale, anhydrite, salt |
| Role | Essential element of the petroleum system |
Common failures
- Seal too thin or fractured, so the trap leaks and holds only a small column
Troubleshooting
- Symptom
- Short oil column below a good structure
- Likely cause
- Leaky or breached seal
- What to check
- Review seal thickness and faults crossing it