Petroleum Reservoir – A reservoir is a formation of mostly solid rock with tiny openings, or pores, which can hold fluids.  Reservoir rock usually contains hydrocarbons as well as water (mainly salt water).  Usually, these fluids are layered; gas (the lightest) on top, oil in the middle, and water (the heaviest) at the bottom.  It is preferred that reservoirs contain all three fluids; oil and gas are valuable minerals, and the gas and water often providing the driving force for oil to reach the surface.  This makes pumping unnecessary.

To accumulate a commercially valuable petroleum deposit, a reservoir must have the necessary configuration:

  • Porosity of 10% or more pore space,
  • Permeability of 10% or moreso that pores must be connected so oil, gas, and water can flow through it by moving from one pore space to another, and
  • The right seal and trap mechanisms that keep the petroleum from escaping.