Step 6 - Seal/Trap Rock
Because of the great amount of pressure thousands of feet beneath the earth's surface, oil tries to move to areas of less pressure. If it is allowed, it will move upwards until it is above ground. This is what happens at oil seeps (once common in Pennsylvania, California, Texas and Louisiana, among other places). While these seeps tell us there is oil below ground, it also tells us that much oil has already escaped, and it may mean that there isn't much left to find. Unlike a reservoir rock, which acts like a sponge, trap rocks act like walls and ceilings, and will not allow fluids to move through. The most common trap rock is shale, which, when compared to many sandstones, has very little room inside for fluids (oil, for example) to move through it.
Though trap rocks block oil from moving through them, they don't always block oil from moving around them. For a trap rock to do its job, we need some kind of geologic trap.
Take 2 pieces (10" x 10") of fine mesh screen, and put a bucket under both. On one screen, smear some mud (as pure as possible). On another, smear sand. While still wet, take a glass of water. Pour half of the water on the screen with sand, and watch it drip through. Do the same with the muddy screen.
Why the difference?
The Paleontological Research Institution