Question: How does oil form underground?

Cees Van der Land answered on 25 Jun 2012:

Thanks for your question!

In the sea, dead sea animals slowly fall to the bottom and over time (thousands of years) form layers on top of each other. This way one layer of sediment slowly gets buried deeper and deeper. The overburden (overlying sediments) leads to high pressures and temperatures in this buried sediment layer. If this layer contained a lot of “organics” (for example plant material or algae and plankton from the sea) they will slowly become liquid due to these high pressures and temperatures and form oil. This is called the “source rock”.

In the underground the oil slowly rises to the surface (if the rock is porous enough) and sometimes gets trapped in highly porous rocks capped by non-porous rock (which stops the oil from flowing further upward, the “caprock”). These porous rocks which contain the oil (after formation and flowing) are called “reservoir rocks” and that’s where we drill wells in to get the oil out.