How Shale Acts as a Seal for Sandstone Reservoirs

Why do natural gas, oil, and water not move upward through shale compared to sandstone?

A. Shale has higher permeability and porosity than sandstone.

B. Sandstone has lower permeability and porosity than shale.

C. Shale has very low permeability and porosity compared to sandstone.

Answer:

The correct answer is C. Shale has very low permeability and porosity compared to sandstone.

Shale acts as a seal for sandstone reservoirs because of its low permeability and porosity. Permeability refers to the ability of rocks to allow fluids to pass through them, while porosity is the space or volume in a rock that can hold fluids.

The porous nature of sandstone allows it to hold natural gas, oil, and water in its spaces. On the other hand, shale is made up of tiny clay and mineral particles that are closely packed together, leaving little space for fluids to move. This makes shale less permeable and less porous than sandstone.

Therefore, the shale forms a barrier or seal for the sandstone reservoirs, preventing natural gas, oil, and water from escaping and moving upward to the surface. This trapped hydrocarbons can be extracted through techniques like hydraulic fracturing or fracking.