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2) Geoscientific illustration

3 phases of reservoir formation explained simply

Formation of crude oil and natural gas

This graphic on the subject of reservoir formation was created for the science magazine “Spektrum der Wissenschaft” (German-language edition of “Scientific American”).

Not a given

The formation of crude oil and natural gas is by no means a geological matter of course. A number of factors have to come together and take place in a certain time and sequence in order for these valuable raw materials to be formed at all.

First of all, a suitable deposition or sedimentation environment is required. Organic material (the dead remains of living organisms, predominantly microorganisms such as plankton) must sink over a fairly large area and over a geologically long period of time into deeper water where there is no oxygen, into a so-called “anaerobic” environment. This can only happen if it is an isolated tank without large bottom currents and the associated water mixing. An example from today is the Black Sea – large quantities of “sludge”, technically known as “sapropel”, are currently forming in its depths, from which crude oil and natural gas can later be produced. This is shown in the first step of the drawing:

reservoir formation step 1 Reservoir formation step 1

Pressure, heat and time

In order for the organic matter in these sediments to be converted into the corresponding hydrocarbons, exactly the right amount of time, temperature and pressure is required. The pressure is created when the deposits with a lot of organic matter are covered by a seal and heavy, i.e. thick sediments and the whole structure is displaced by tectonic movements into deeper layers of the earth’s crust. This is where the oil and gas can then “mature”. This is illustrated in the second step:

reservoir formation step 2 Reservoir formation step 2

Geological traps

After all, the oil and gas must be compressed and stored in such a way that it can be easily extracted by humans. This can be ensured by so-called “geological traps”. For example, a thick layer that is impermeable to gas and oil can lie on top of the oil-bearing layer and be bent and folded by tectonic processes. Fractures and displacements in the surface layers can also lead to upward and lateral impermeable structures. Mobile raw materials can then migrate into these areas and concentrate there. They can then be mined here. See step three:

reservoir formation step 3 Reservoir formation step 3

Fracking, oil sands and more

If the final step in geological development is missing, it is also possible to access these raw materials, but at considerably greater expense and with even greater environmental impact. Fracking, for example, extracts oil and gas from oil sands and oil shales in which these hydrocarbons have not yet collected and concentrated in geological “traps”.

50% of a deposit has been mined – is there still 50% left?

There is one issue which is often forgotten or considered somewhat naively in debates about how many raw materials we still have available – quite apart from the question of whether it is wise to burn naturally occurring hydrocarbons – if only because of the impact on the climate and their use in numerous and important value chains. And that issue is the question of how extraction changes over time. This is because for most deposits, the following is true: in the beginning, you can draw from the full, the raw material is abundant and can be extracted comparatively cheaply.

But the less there is in the source, the more difficult it becomes to get hold of the rest. Some of it gets trapped in small cavities that are hardly worth extracting, the pressure decreases, which is why the oil no longer comes out of the ground “voluntarily”, and so on. In other words, it is becoming more and more costly and uneconomical to get at the resources, and some of them can no longer be accessed at all.

What else?

What’s special about this drawing is that it is – besides my “block image of the skin” – my “most stolen” graphic on the Internet, which I have already come across in various places (for example > here: zoom on Greenland). Which also speaks for it ;-)

And by the way: This series of illustrations was created for a publication on the subject of “Nord Stream 2”.

The “Spektrum der Wissenschaft” editorial team

I have been working for the very nice and friendly Spektrum editorial team since 2004, and it is always a great pleasure and very exciting to illustrate the various topics. For a long time, I mainly illustrated land and thematic maps, mostly on historical topics. One of the magazines I worked for was called “epoc“, and when I have to illustrate a geological topic, as in this case, I’m really in my element.