In underground reservoirs, the layer containing petroleum oil and gas is invariably found above the water layer because:

Correct Option

The correct option is

Petroleum and gas are less dense than water and immiscible.

Explanation

The arrangement of fluids in underground petroleum reservoirs is governed by the physical principles of density (specific gravity) and immiscibility. When fluids are trapped within porous rock formations, gravity causes them to separate into distinct layers based on their relative densities.

Option Analysis:

• Petroleum and gas are heavier than water. is Incorrect: Petroleum (crude oil) and natural gas are composed of hydrocarbons, which have a lower density than water. If they were heavier, gravity would pull them to the bottom, below the water layer.
• Petroleum and gas are less dense than water and immiscible. is Correct:
  • Density Difference: Natural gas is the lightest fluid, followed by crude oil, while saline water (brine) is the heaviest. Consequently, gas accumulates at the very top (forming a gas cap), oil floats in the middle, and water settles at the bottom.
  • Immiscibility: Hydrocarbons are non-polar, whereas water is polar. This chemical difference makes them immiscible (they do not mix). This property ensures that the fluids remain in separate, distinct layers rather than forming a solution.
• The impervious rock layer is located below the water. is Incorrect: An impervious rock layer (cap rock) is required to trap the hydrocarbons, but it is typically located above the reservoir to prevent upward migration. The internal layering of gas, oil, and water is determined by fluid properties, not the position of the rock layer relative to water.
• Water is formed after petroleum and settles down. is Incorrect: The sequence of formation or migration does not dictate the final arrangement of fluids. Over geological time, gravity segregation ensures that fluids arrange themselves according to density, regardless of which fluid entered the reservoir first.

Key Takeaway:

In underground reservoirs, fluids stratify due to density differences: Gas (lightest) > Oil > Water (heaviest). This separation is maintained because hydrocarbons are immiscible with water.