Why is there oil in the ground?
Oil, or more specifically hydrocarbons, is found in most major basins in the world. How did it get there? And what makes an oil field an oil field?
The shortest answer: An oil field is an “upside-down lake“. But you can’t swim in it…
This statement sounds rather strange at first, but bear with me. The biggest misconception about oil and gas fields is that they are basically cavities in the earth filled with liquid. That is not true. The liquid is stored within the pores of the rock. Take a sponge in your kitchen and soak it with olive oil and you have something more like it. The pores in the rock are much smaller than in your household sponge but the concept is the same.
It might sound strange, but the journey of an oil field starts in the mountains. Humongous mountain ranges like the Alps or the Himalayas.
Mountains rise and crumble over eons. A natural process called erosion. Water, frost and wind tear down solid rocks and transport the end-product (sediment) off to somewhere else. That somewhere else is what we call a sedimentary basin. LINK – What is a sedimentary basin?
Rivers flow over great distances and carry the sediments with them. Initial erosion and the forces of the river break down the sediments into finer and finer sediments. This ranges from coarse sand to finer silt and even finer muds. Finer sediments can be transported further by the water and deposited further away than coarser grained sands for example. What that means is that the single governing factor, what type of sediment settles down at a point in a basin, is the speed of water. In a fast flowing river only larger boulders can be deposited, whereas out in the ocean only the finest sediments are left because the coarser and heavier sediments “didn’t make it that far“.
We often speak of high- and low-energy environments.
This results in a spatial variation in grain size of the sediments. Muds get deposited far out in the basin, whereas sands end up closer to the shore. A perfect example would be a nice sandy beach we all know and love from the old family holidays. Keep those two sediments in mind – a fine grained mudstone and the sandy beach you used to build sand castles out when you were a kid.
Sedimentary basins see very different depositional environments over the course of many millions of years. Sea level rises and falls again and again, tectonic settings change (rifting apart or being pushed together), glaciers grow or shrink, land gets uplifted or “drowned”. This causes a heterogeneous distribution of different sediment types because with each different sea level position, the sediments will end up somewhere else. These different sediment types have properties like porosity and permeability. Some might act as a pretty good container to store a fluid, whereas another might act as a barrier for the same fluid.
You guessed it, the barrier is the fine mud and the container is the sandy beach. Now both sediments will get buried at some point and lie at deeper depths. The loose sediments transform into harder rock called mudstone and sandstone. The process behind this is mainly compaction from the weight of the overlying sediments, but other factors play a role as well, however, I will keep it simple.
If we deposit a fine grained mudstone directly on top of a highly porous sandstone we almost have an oil field. This would be a reservoir with a seal.
Well, right now it would still be empty (or more correct: filled with water). We need to fill it with oil for it to become an oil field.
The finer grained mudstone further out in the basin can sometimes contain high amounts of organic matter. We call it source rock. It got deposited there under anoxic condition (very little or no available oxygen). Anoxic environments typically occur in a restricted basin. A modern day example would be the black sea, high nutrient influx from surrounding landmasses and limited influx of “fresh” sea water through a bottleneck. Most of the oxygen is consumed and the rest of the organic matter gets buried with the fine grained sediments and gets therefore “trapped” in the sediments. The deeper we go down in the earth the warmer it gets. On average by about 30°C per kilometer. That means if the rock is in a depth of (typically) 2km, it gets warm enough for it to generate a simple hydrocarbons-precursor called kerogen, which then later expulses oil and gas. This is often (and not surprisingly) called the kitchen. If it gets deposited deeper, it will generate more gas and less liquids.
Now once the liquid is expulsed from the rock it will rise. Why exactly oil rises I will cover in a different article, but it does. Just like olive oil that you pour into the pot when you cook spaghetti. It wants to come to the surface. It will start moving and it will always choose the path of least resistance. This part of the process is called migration.
In our world on the surface we experience a different behavior. If you pour out a glass of water on a high mountain (and neglect any processes like evaporation and such) it will travel downwards. This glass of water will end up in the ocean. All liquids are driven there by gravity. It might take some time, but it will go there.
This is where I get back to the analogue with the upside-down lakes. If you follow a river down to the ocean and at some point in the middle you give that river a big void that it can fill up (a potential low). This will create a lake. A buffer – which is filled until it flows over and the river continues.
Lake Geneva with the river Rhone flowing through it.
The same thing happens below the earth, just the other way around. If the oil can find a potential low during its way up it will fill up that reservoir until it spills over and the oil can continue to rise like a river. In order to capture the oil in place it is not enough to just have an impermeable layer on top. We also need a trapping style. Something that defines and outlines a geometrical body made up of the reservoir rock. We call this a trap.
Sketch of a petroleum system with mirrored analogue of rivers and lakes on top.
The simplest trap we can think of is basically a hill-like structure called an anticline. In its shape not unlike a lake on the surface. It is the simplest trapping style but there are many different other types of traps that can produce an oilfield.
Now we went through most of the elements that are needed to create an oilfield. A porous reservoir (e.g. a sandstone), a trap that defines a geometric body of said reservoir, an impermeable layer on top of that structure that prevents the oil from rising further to the surface (a seal). Of course we need a source for the oil itself (a source rock) and a way to get it to the reservoir (migration). On top the reservoir, seal and trap needs to be in place before the source rock starts to expulse oil. So timing is crucial as well!
As you can see, special circumstance have to come together for Mother Nature to form an oil field. This is why we don’t find them everywhere. The process of finding these remaining fields is called exploration.
tl;dr: oilfields are highly porous rock formations in the earth that are geometrically isolated by an impermeable layer that keep the oil in place, which migrated into the reservoir over millions of years after it was generated under favorable temperature conditions in a different rock formation called source rock with high organic content.