Hydrocarbon traps form where permeable reservoir rocks (carbonates, sandstones) are covered by rocks with low permeability (caprocks) that are capable of preventing the hydrocarbons from further upward migration. Typical caprocks are compacted shales, evaporites, and tightly cemented sandstones and carbonate rocks.
The caprock need not be 100% impermeable to water, oil or gas. If the upward loss of hydrocarbons is less than the supply of hydrocarbons from the source rocks to the trap, hydrocarbons may still accumulate…