10. Oil and Sedimentary Rocks

Part of

Today we are very lucky to have a guest post from Rachel Meacock who is an environmental consultant working within the energy industry.

Oil is one of the most useful resources in the world. It is used to provide energy, power industry forward, heat our home and provide fuel for vehicles, which in turn carry people and goods all over the world. It’s vital in the production and manufacturing of many everyday items, such as plastic, laundry detergent, paint, printer ink, building materials, and even medicines! Oil is a very useful product, but where does it come from?

Where do we find oil?

Oil is found within reservoirs, deep underground. These reservoirs are made up of sedimentary rocks. There are two types of sedimentary rocks – Clastic and Biochemical.

Clastic rocks are formed following movement, building up and settling of solid particles, like sand and pebbles created from the weathering of larger rocks. These particles are carried to a point of deposit, for example via the movement of a river, and the layer of particles settles. This process repeats, with more and more layers being settled on top of previous ones. Higher layers exert a force known as overburden on the lower layers, which results in physical and chemical changes to the lower layers. This results in the formation of clastic rocks. Examples of clastic rocks include sandstone and shale.

Biochemical rocks are formed in much the same way as clastic rocks, but from marine life remains rather than particles. Objects such as shell fragments, corals and animal skeletons make up biochemical rocks. Once again, layers of these objects build up, and overburden causes changes to the physical and chemical properties. Examples of biochemical rocks include limestone, chalk and dolomite.

When we talk about reservoirs, it’s easy to imagine a big hole full of fluid. But oil reservoirs are not formed like this at all. Oil is held in spaces between sedimentary rock particles, and these spaces which hold the oil are known as pore spaces. The number of pore spaces, or the porosity of the rock, is an important feature of your reservoir. A higher porosity means more oil.

Another feature of the sedimentary rock which is important when extracting oil is the permeability of the rock, or how well connected the pore spaces are. A higher permeability level means the oil is easier to extract, as it flow more easily through the rock.

The actual formation of the oil itself is not known, but the most widely supported theory is the organic theory. This theory summarises that organic remains, such as those of small plants or animals, where covered by rock deposits and sealed from the air. Over time, the compression of these remains led to changes in the physical and chemical structure of the organic remains, leading to the formation of oil. It is very similar to the formation the sedimentary rocks described above. However, oil doesn’t stay in one place. The place it forms in is known as the source rock. The oil then migrates to the reservoir rock, which is where we extract it from. On top of the reservoir is a rock layer called impervious rock – the oil cannot travel past this layer. The petroleum reservoir accumulates under this rock layer.

Here is a simple diagram of a reservoir as described. Of course in reality things are a bit more complex, but you get the idea!

Diagram of an underground oil reservoir
Figure 1: Diagram of an underground oil reservoir (Source: Energy Education, University of Calgary https://energyeducation.ca/).

Activity: Understanding Sedimentary Rocks

You will need:

  • 2-4 containers (empty yoghurt pots or plastic cups work well)
  • Some dry sand
  • Water
  • Oil (cooking oil works well, or light machine oil)

Activity 1

Take your sand and place it into one of your containers. Fill another container with water. Now, take the container filled with water and slowly pour some into the container of sand. What happens?

Answer: The water disappears into the sand. Sand has spaces between its grains too small to be seen by the human eye, and the water is being held within these. This demonstrates how oil is held in the pore spaces in a sedimentary rock.

Activity 2

Note - coarse sand or grit, free from mud and clay, work best here.

Fill one of your containers with water, and the other with a small layer of oil (no more than ¼”). Place your sand or grit into the container with oil. Pour water over your grit until there is a visible water layer on the surface, and leave overnight. What happens?

Answer: There should be a film of oil on the surface of the water. This demonstrates the permeability of sedimentary rocks, and how the oil migrates through the rock to the surface.

How do we find oil?

The first step in extracting oil from it’s reservoirs is to find it. There are several methods for doing this. For land based reservoirs, it can be easier to spot. Faults, domes and other structural contours can be seen at the earth’s surface. An aerial survey may be used to identify these contours on a larger scale. Once a potential reservoir is identified, a field survey will be carried out by geologists on the ground to identify if oil is present. This field survey will look at the shape of the contour or the rock type in the area to see if it fits the profile of an oil reservoir.

For underwater reservoirs, it’s usually more difficult to see contours on the seabed. So visual surveys and field surveys may be undertaken by scuba divers, or a sonar survey (reflected sound waves) can be used to map the seabed.

Prior to drilling into a potential reservoir, a geological survey is carried out to provide some confirmation that the contour is indicative of a reservoir. These surveys can also provide hints as the how deep down beneath the Earth’s surface the reservoir is. A seismic survey is commonly used, and this involves sending a shock wave from the surface of the earth and down through the rock layers below. At the boundary of each layer of rock, part of the wave is reflected back to the surface. This returning wave is picked up by either a geophone (on land) or hydrophone (on water). The returned waves are analysed by computers, which can build up a good picture of the rock layers below. Using this information, the best drilling site can be identified.

Once a drilling site is identified, a well can be drilled using a rig. An exploration well is drilled to see if oil is present as expected. And if we find oil, the real production begins! Once a well is drilled into the reservoir, the oil begins to move up the wellbore to the surface. This oil is collected, stored, and transported to a refinery for processing in the first step for creating all the useful products it is used for.

Rachel Meacock
Rachel Meacock

Environmental consultant working within the energy industry.

Rachel Meacock is an environmental consultant working within the energy industry. She obtained an undergraduate bachelors degree from the University of Aberdeen in Marine Biology, and a masters degree in Marine Planning for Sustainable Development from Heriot Watt University’s Orkney campus. Her professional interests involve sustainable offshore development, protection and conservation of the marine environment, and marine planning policy.

Outside of work she is a regular beauty pageant competitor, currently holding the Ms Aberdeen Saltire 2020/21 and Ms Crown and Glory Aberdeen 2021 titles.

Rachel uses the platform her pageant titles give her to inspire more young girls and women to get into scientific fields.

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