Both oil and gas are lighter than water.
Explanation
The occurrence and stratification of fluids in a petroleum reservoir are governed by the principle of density. When multiple immiscible fluids are present in a porous rock structure, they separate into distinct layers based on their specific gravities.
Analysis of Density Differences:
- Water: Among the fluids found in a reservoir, saline water (brine) is the densest. Consequently, it occupies the lowest part of the porous rock trap.
- Oil: Crude oil consists of hydrocarbons that are generally lighter (less dense) than water. Therefore, oil floats above the water layer.
- Gas: Natural gas is the least dense of the three components. It accumulates at the highest point of the trap, forming a "gas cap" above the oil.
Since both crude oil and natural gas have lower densities compared to water, they rise through the water-saturated rocks until they are trapped by an impermeable layer (cap rock).
Key Takeaway:
In a petroleum trap, the order of stratification from top to bottom is always Gas → Oil → Water, because both oil and gas are lighter than water.