The first stage of oil and gas development is known as exploration. During this phase, geologists are trying to figure out where to drill for new reserves of oil and gas, or how to expand an existing oil or gas field.
What petroleum geologists are really looking for are complete petroleum systems.
A petroleum system is a self-contained collection of geologic formations that has produced oil, allowed it to migrate, and trapped it in place until the present. Since each of these steps requires a very specific set of geologic conditions, complete petroleum systems are relatively uncommon.