2. Defining oil reservoir: Oil lake under the surface or a sponge made of rock filled with oil?

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After reading last article you should have a good understanding of the types of companies in the industry. As it was mentioned there, petroleum industry is commonly divided into three sectors: upstream, midstream and downstream.
The upstream sector (better known as E&P sector – Exploration and Production) contains all tasks and operations which deal with bringing oil and gas to the surface. It includes searching for the potential underground crude oil or natural gas fields, drilling exploratory wells and then drilling wells that bring oil.
The midstream sector deals with marketing of the unrefined products: storing in tanks, terminals, salt caverns, creating intermediate products and sending them to refineries through pipelines, long-distance transporting, delivering refined products to distributors.
The downstream sector (also known as refining) deals with all processing hydrocarbons, distribution of commercial products received from oil and gas.

We can now properly describe oil life in 6 points.
1. Oil reservoir.
2. Extraction.
3. Long-distance transport.
4. Refinery.
5. Delivery of refined products.
6. Fuel stations.

Concentrating on the first point, what actually is oil and gas reservoir? Most of you, like me first imagined it as a big lake filled with oil, hidden somewhere under the ground. Wrong? Looking to the dictionary you will find that it is “a subsurface pool of hydrocarbons contained in porous or fractured rock formation”, so the world “pool” can bring the image that petroleum is hidden in “pools” or “lakes” in the caverns under the ground. In fact it more reminds a rock sponge, but since we can’t physically see what is under the ground, drilling companies take core samples of the reservoir to measure rock properties such as porosity and permeability. Core samples are cylindrical sections of rock which are inspected and analyzed by geologists and petrophysicists in specialized labs. As we observe part of the rock carefully, we will see that a reservoir rock has spaces which can be filled with fluids, property which describes number and size of the paces (pores) is named porosity. However, to let petroleum flow, the rock must also have appropriate permeability. Permeability is really an expression about how the pores are connected.


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As temperature and pressure are changed during production, fluids in pores decompress and flow into the well. It is fundamental to understand more advanced topics we are going to go through.


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Unconventional reservoirs has less permeability than conventional ones. Unconventional gas reservoir is an accumulation of the same natural gas as we use in gas stoves for years. Natural gas is a mixture of hydrocarbons (mostly methane – about 90%) and sometimes a small amount of hydrogen sulfide, carbon dioxide and other components.
There are 3 types of unconventional gas reservoirs:
• Shale gas
• Tight gas
• Coalbed methane (Link here you can read more about coalbed methane)

Unconventional oil haven’t been yet strictly defined, but according to definition it is “a type of petroleum that is produced or obtained through techniques other than traditional oil well extraction. Unconventional oil production is commonly seen as more costly than conventional oil production, less efficient, and is likely to cause more environmental damage.” More about the types we are going to discuss later.

In the next article we will try to find out how is petroleum and natural gas formed and name basic drilling string components.

Sources: wikipedia, investopedia
Main Image source: dleng.info

1.Integrated vs service oil companies here you can read 1st part.

About author

Agata Gruszczak

Editor in YoungPetro, fourth year student of Mining and Geology at the Faculty of Drilling, Oil and Gas at AGH University of Science and Technology, member of AGH UST Student Chapter. Interested in enhanced oil recovery and unconventional resources. Keen on Scandinavian culture, reading fantastic literature.

View all posts by Agata Gruszczak