Crude oil and natural gas don't come from dinosaurs. They start as something much smaller: tiny marine organisms like plankton and algae. Millions of years ago, when these organisms died in oceans and ancient seas, they sank to the bottom.

Normally, they would just decompose. But in some places, the water was very still and had little oxygen. Here, the dead organisms mixed with mud and clay, forming an organic-rich sludge. Over time, layers of this sludge built up, one on top of the other, burying the organic matter deeper and deeper.

This buried layer of organic-rich sediment is called a source rock. It’s the kitchen where petroleum is made. Common source rocks are fine-grained shales or limestones.

As more sediment piled on top, the source rock was pushed further into the Earth. This journey downwards subjected it to immense pressure and heat. Think of it like a giant, slow-moving pressure cooker.

The heat and pressure gradually transformed the solid organic matter into a waxy substance called kerogen. As temperatures continued to rise, the kerogen broke down into the smaller, simpler molecules of liquid oil and natural gas. This process is called catagenesis.

Once formed, oil and gas are lighter and less dense than the water that saturates the surrounding rock layers. This buoyancy forces them to slowly seep upwards, migrating out of the source rock and through tiny cracks and pores in the rock above.

For petroleum to form a commercially viable deposit, it can't just leak out onto the surface. It needs to be trapped underground. This requires a specific combination of three rock types, all arranged in the right way.

The reservoir rock must have two key properties: porosity and permeability. Porosity is the amount of empty space within the rock, like the holes in a sponge. Permeability is how connected those spaces are, which allows fluids to flow through the rock. Sandstone and limestone are common reservoir rocks because they can hold large amounts of petroleum and allow it to move.

Above the reservoir rock, there must be a cap rock, or seal. This is a layer of impermeable rock, like shale or a salt dome, that oil and gas cannot easily pass through. As the petroleum migrates upwards into the reservoir rock, it eventually hits this ceiling and can go no further. It accumulates in the pores of the reservoir rock, forming an oil and gas deposit.

Without this complete system—a source, a path to migrate, a reservoir to hold it, and a trap to seal it—a significant oil field won't form. Geologists search for places where these conditions came together millions of years ago.

Now that you understand the basic ingredients for an oil deposit, let's test your knowledge.

Understanding these fundamental geological concepts is the first step in exploring for and producing the energy resources our world relies on.