All the oil created by the source rock won't be useful unless it winds up being stored in an easily accessible container, a rock that has room to "suck it up". A reservoir rock is a place that oil migrates to and is held underground. A sandstone has plenty of room inside itself to trap oil, just like a sponge has room inside of itself to soak up spills in your kitchen. It is for this reason that sandstones are the most common reservoir rocks. Limestones and dolostones, some of which are the skeletal remains of ancient coral reefs, are other examples of reservoir rocks.

Here we see what a reservoir rock would look like through a magnifying lens. The yellow objects represent sand grains that are packed together. Notice, however, the purple areas between the sand grains. These areas (also known as "pore spaces") are where the oil fits into the rock. In a very "porous" rock (one with many pore spaces), oil can move freely through these spaces. Compare this rock to a trap rock.

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