The term reservoir in this context refers to a geological under ground formation that is porous and permeable. Such reservoirs may be filled with water, gas or oil.
These reservoirs can also be used to store gases as natural gas or hydrogen. The fluids from a reservoir are produced through bore holes.
Reservoir Engineering generally means to calculate the flow of fluids in porous media. It is important for gas storage, production of oil and gas as well as for the recovery of geothermal energy or storage of hydrogen.