Oil is found in underground pools of oil called reservoirs. This oil location is not what one might typically expect when considering the term “pool”. It is impossible to go swimming in these pools! Industry experts use the term “pool” to define accumulations of hydrocarbon in zones of subsurface rock. (see oil accumulation). What? How can oil reside in rock?
Well, if you were to zoom in on a chunk of rock, let’s say a sandstone (down to about 0.000001 meters!), you would see thousands of little flecks of stone stuck together with spaces in between them. These spaces are called pores and the term porosity refers to the percentage of pore space to little stones over a given area.
It is within these pore spaces that oil, gas and water reside.
The rock containing the hydrocarbon is called the reservoir rock, and can be a variety of rock types, but is typically a sandstone or limestone. This is due to the relatively high porosities these rock types possess. High quality sandstone reservoirs can have porosities in excess of 25%.
High porosities usually mean higher reserves potential.
Another term used to describe the quality of a reservoir is permeability. Permeability is the measure of the connectivity of the pore spaces to each other. If the pore spaces were not connected to each other, oil would not be able to flow, regardless of whether or not there was good porosity.
Industry experts experience great difficulty (i.e., expense) producing poor permeability reservoirs.