Click here to skip to the content

Petroleum Systems


Nicki Atkinson, Exploration Manager – New Brunswick, Southwestern Energy, Houston Texas

I wanted to chat about what a petroleum system is. What does a petroleum system mean. Petroleum actually means ‘rock oil’, so where does rock oil come from and how is it formed. A petroleum system is actually, as the name suggests, a whole system. It actually has a number of components to it. It has a source rock, a seal rock, a type of trapping mechanism, and a place in which the petroleum actually can be stored, called a reservoir. So how do all of these components actually come together. The source rock itself is typically generated by organic matter, that once it dies it falls to the ground, it decays, it gets compacted and over millions and millions of years, in typically anoxic environments it transforms itself from organic into petroleum products, and typically what you see today is that in the form of oil and natural gas.

So once that has formed with the subsurface, again it takes millions of years in order to do this, it then migrates its way through into different types of rocks and ultimately it ends up in what we call a reservoir, and this is a type of rock with enough porosity, so enough holes, like a swiss cheese if you will, that actually collects the oil and natural gas. So, in order for it to be collected it needs a cap rock, it needs a very tight piece of rock sitting on top of it, in order to collect the accumulation. So the reservoir itself contains the petroleum product, then has a tight rock sitting on top of it, and that is called a seal.

Now the mechanism in which this trap is formed can be structural, it can be stratigraphic, so we can either have what you see in the form of an anticline, or a fault trap, or in terms of a stratigraphic trap we can have changes in the type of rock, both laterally and vertically. So we have changes in types of porosity and permeability that actually seal the petroleum product within it.

So a hydrocarbon system, a petroleum system, is a very dynamic system. It has four major components to it; source rock, seal, reservoir and trap, and it takes all of those features to come together in order to have a system, and an oil field or a gas field that we see today, and that we’re currently exploring for.

Mother Nature is very creative and she’s very clever and it takes a set of very experienced geologists, geophysicists, scientists of all disciplines in the exploration process. And that’s what we do as oil and gas finders. We are explorers at heart and every day it’s unique, it’s fun and we love what we do.