An oil and gas reservoir is a formation of rock in which oil and natural gas has accumulated within. The oil and gas collect in the small, connected pore spaces of rock and are sealed below ground surface by an impermeable layer of rock. These reservoirs are not "puddles" or "lakes" of oil beneath the surface, as there are no vast open cavities that contain oil.
Reservoirs are made of rock known as reservoir rock - shown in Figure 1, part II - these are types of rocks that are accessible and contain considerable amounts of oil and gas.  In order to be a reservoir rock, it must be both porous and permeable. If both porous and permeable, there are small pockets within the rock where oil or gas can settle and small channels connecting these pockets to allow the oil or gas to flow out of this rock easily when drilled for. These spaces between grains can develop as the formation of rock occurs or afterwards, usually as a result of groundwater passing through the rock and dissolving some of the sediment.
For a reservoir to exist, oil and gas from the source rock (the rock containing kerogen) - shown in Figure 1, part III - must migrate into the reservoir rock, which takes millions of years. This migration occurs because oil and gas are less dense than water. This difference in density causes the oil and gas to rise towards the surface so that they are above groundwater with the gas settling above the oil because of their different densities. Migration pathways - a set of well connected fractures - must exist for this rising to occur.
Finally, there must be some rock preventing the oil and gas from escaping the reservoir rock that they can travel so easily through. For a reservoir to exist the oil and gas must be trapped underground so that they do not produce an oil seep. Traps create the top layer of the reservoir, preventing the fossil fuels from exiting the reservoir rock. There are two main components to a trap: the seal rock and a proper arrangement. The seal rock - shown in Figure 1, part I - is the impermeable rock that lies above the reservoir rock, trapping the oil and gas in. Lastly, for a trap to exist there must be the proper arrangement of rocks in the reservoir to create a small, restricted area for the oil and gas to accumulate.