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What is Shale?

Mary McMahon
Updated Jun 04, 2024
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Shale is a very common form of sedimentary rock found in deposits all over the world. It is distinguished by being soft and highly fissile. Shale has a number of ornamental and practical uses, in addition to being a rich source of fossil depositions which can provide information about different eras in Earth's geological history.

This sedimentary rock starts as a deposition of sand, mud, and clay, with very fine particles of other minerals such as quartz. Over time, the deposition slowly compresses, eventually turning into shale. One of the defining characteristics of this type of rock is the layers of material which can be seen when the rock is cut crosswise, showing the subsequent layers of deposition which occurred, with the rock tending to sheer or fracture along these deposits. The very fine grain of the particles is another distinguishing characteristic.

Shales come in an array of colors. Some are so dark that they are almost black, while others may be greenish, bluish, brown, or cream, depending on the precise composition of the rock. In the case of shale with fossil deposits, the deposits form as a result of biological material which became trapped in the shale while it was being deposited and subjected to pressure. In some cases, parts of the organism may be represented in mineralized form, while in others, the rock holds only the impression of a organism which decayed after the rock started to harden.

In manufacturing, shale is used as a filler material for concrete and brick. People sometimes confuse this rock with slate, a much harder rock which can be used for a wide variety of purposes including roofing, tiling, blackboards, and so forth. In the case of slate, the rock is subjected to metamorphic processes which change the structure and properties of the rock, making it harder and more durable. Although slate also bears distinctive layers and tendency to split along these layers when stressed, it is much harder than shale.

A type of rock known as “oil shale” is of interest to some people who work in the energy field. Oil shale is not necessarily a shale, although it sometimes is, and it shares many of the traits associated with true shales. It bears a mixture of chemical compounds which can be extracted and used to generate energy. However, tremendous energy is needed to extract these compounds, making this rock appealing only in regions where other sources of oil for energy have been exhausted.

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Mary McMahon
By Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a All The Science researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

Discussion Comments
By nony — On Jun 29, 2011

@PelesTears - I am all for wind and solar, but they are not powering transportation vehicles-at least not on a mass scale. Shale still offers the most immediate and cost effective solution for our energy problems.

As for the logistics of exploration, the technology has been in use for some time now. Fracking has been used for oil exploration for decades -it’s not exactly new.

To date, thousands of wells have been drilled in Pennsylvania and Texas to extract shale from a formation known as the Marcellus Shale-one of the largest shale formations in the United States, and one which could provide the United States with its energy needs for decades to come.

By Alchemy — On Jun 26, 2011

I have a question I hope someone could answer for me. What is Bakken shale and how is it different from any other shale? I need to write a paper on Bakken shale for a geology class, and I was honestly dozing off during that lecture.

By istria — On Jun 26, 2011

The shale gas discussion is often argued in terms of shale geology and resource consumption, but the truth is, it does little to reduce carbon emissions or fossil fuel dependency. While global warming is so last decade, climate change is real. While I am not writing to argue the causes of climate change, I am attempting to argue a precautionary approach to climate change.

In my opinion, all subsidies that go to shale gas exploration are a waste of resources. We should be pumping money into domestic energy resources that not only create high paying science jobs, but also high paying manufacturing jobs. Why roll the dice on climate change when we can improve the economy and improve our future?

By Amphibious54 — On Jun 26, 2011

@PelesTears- You highlighted one of my biggest issues with shale oil production. Hydraulic fracturing uses so much water. Most people do not realize, but an even bigger crisis than the energy crisis is the water crisis. You have probably heard that one sixth of the world's population does not have access to clean water. What you may not realize is this number is growing, and the water crisis is beginning to affect industrialized nations.

Oil fracking in Texas has been big business of late, but it is coming to a point where there is not enough water to keep the industry afloat. I read in the Economist recently that oil and gas exploration companies are offering farmers and agribusinesses 72 cents per barrel of water (42 gallons), and farmers are turning them down.

I live in Arizona (the driest state), and I pay 1.50 per 750 gallons of water. The article stated the average well uses 13 million gallons of water per year, enough for a few hundred people. Multiply that by the thousands of shale oil wells in Texas alone, and you can see the pending problem.

Here is a little food for thought...hydrologists are the highest paid scientists next to petroleum engineers. That says something about how important water resources are. A well could have all the gas in the world, but it is useless if you do not have the physical resources to extract it.

By PelesTears — On Jun 26, 2011

@Everetra- I would have to take the opposite standpoint. While the nation's shale reserves have potential as an abundant fuel source, the technology to extract that fuel is not that advanced. Furthermore, not only environmentalists are worried about shale gas exploration.

To extract these resources, gas exploration companies use hydraulic fracturing, which is only economical when prices of gas are high. Hydraulic fracturing, or fracking, requires pumping fluid, usually potable water, into the ground to fracture shale formations. After the rock is split, a slurry of sand or other permeable materials is pumped into the fractured slabs to keep them open. Finally, more water is pumped into the well to flush out the gas or oil.

The process is very energy intensive because you have to heat and pump the fluid deep into the ground. When you combine the energy expenditure to extract the fuel, with the efficiency losses associated with transporting and burning the fuel, alternatives like wind and solar are much more economical, stable, and environmentally friendly.

By everetra — On Jun 26, 2011

While I am a strong believer in alternative fuels I also believe we need to mine all the resources we have at home, if we’re going to become truly energy independent. To this end, shale gas is a rich store of untapped fuel source which we could use to solve the energy crisis.

The fact is that shale is a lot cleaner than coal, it is abundant, it is untapped, and over the past decade the costs of extracting shale have become lower thanks to advances in technology.

The only thing holding it back now is political will. Environmentalists have concerns about potential damage to the environment, and they may have a point when it comes to worst case scenarios.

However we should still be able to come to some compromise that would protect the environment while ensuring we have access to this great fuel resource.

Mary McMahon
Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a...

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