Natural reservoir is a natural storage of oil, gas and water, the shape of which is determined by the interrelation of reservoir rocks with low permeability rocks. This notion is largely similar to that of a field. A natural reservoir is specified by the type of reservoir rock, its capacity, hydrodynamics and energy level.

The main types of natural reservoirs are as follows:

• formation reservoirs – limited by impermeable rocks across a significant space of the top and the bottom;
• massive reservoirs – their sizes are consistent in three dimensions, they are surrounded by impermeable rocks and represented by, for instance, reef formations, structural curves of thick permeable rocks;
• lenticular reservoirs – sandstone lenses within clay, fractured areas in various impermeable rocks.