Black Shales

As a geologist, when chatting to people I am always surprised when they don't know what the term "black shale" means, considering all of the media coverage on fracking and shale gas.  This post aims to explain what the term means, how they were formed in the past and how black shale formation in the present will reduce the amount of carbon dioxide in the atmosphere.

Black shales are fine-grained rocks, made up of mud-sized particles, with very high levels of TOC (Total Organic Carbon).  TOC refers to the amount of carbon present in a rock sample which is bound in an organic compound.  The organic carbon comes from dead animals, plants and faeces which were incorporated into the mud at the time it was laid down.

If the rock, and organic carbon, have been buried to sufficient depth to "cook" (temperature increases by approximately 1 degree C for every 100 m you go into the Earth ), then the organic carbon can start to form hydrocarbons: oil (less cooking) and gas (more cooking).  If a rock has more than 2% TOC it is said to make a good source rock (4% TOC is a very good source rock).  Black shales frequently have high TOC levels and it's the high TOC content which also gives the rock its black appearance.

Source rocks are rocks which, once cooked, produce hydrocarbons.  Most oil and gas extracted around the world started out in a black shale, the source rock, but the hydrocarbons may have since moved into another rock (a reservoir rock), such as a sandstone.  Where they have moved into a reservoir rock, this is known as a conventional play, and most oil and gas wells extract their hydrocarbons from sandstones.  Where the hydrocarbons are still in the black shale, the technique of "fracking" needs to be employed to extract them because black shales are too fine-grained to allow the hydrocarbons to flow towards the well.  Sandstones have more space between the grains, allowing the hydrocarbons to move.

So why do black shales have such high levels of TOC?

This can be summarised in three words: productivity, preservation and dilution.

To have high TOC levels preserved in a rock, you first need a large amount of organic matter to be produced; that means a lot of life to be living, pooing and dying.  This is known as high productivity.

Once you have your high productivity, you still need that organic matter to be preserved.  That means you need to limit its breakdown by bacteria (also known as rotting).  The best way of doing this is restricting the amount of oxygen available: oxygen is required by bacteria to most efficiently break down organic matter.  With limited oxygen, more organic matter is preserved.

Even if you have then produced a lot of organic matter, and preserved it, your rock will still not have a high TOC if there is a lot of sediment coming in because TOC is a measure of the total percentage of organic matter compared to the amount of other rock.  With a high sedimentation rate, the organic matter will become diluted and the TOC percentage will drop; with a low sedimentation rate, the percentage of organic matter will be higher.

Where do black shales form?

Most black shales in the past formed in shallow seas that covered the continents millions of years ago when relative sea level was higher.  They are found all over the globe and throughout geological time.  They are often associated with limited circulation within the sea, which restricted the amount of oxygen reaching the sea floor.

In the modern they are forming beneath areas of high productivity where bacteria use up the limited oxygen supply whilst breaking down organic matter, before reaching the sea floor.  Examples include areas of upwelling where blooms of plankton and marine organisms feast on the nutrients being brought up from the deep sea by ocean currents, or the out-wash of the Mississippi River which is full of fertilisers, resulting in algal blooms.

By having a bloom of productivity in the upper water column, the sea floor becomes "dead" with no metazoan life able to live there with no oxygen.  However, these areas also become "carbon sinks", where the plankton in the upper water column take in carbon dioxide from the surface water and atmosphere during life, and then, when they die, that carbon is not re-realeased back to the atmosphere because their bodies are not fully broken down by bacteria, due to the limited oxygen.  Instead that carbon is sequestered, or locked, into the sediments, raising the TOC, and possibly one day forming a black shale.

So whilst the burning of hydrocarbons from ancient black shales is increasing the amount of carbon dioxide in the atmosphere, the creation of new black shales in the modern is gradually decreasing the amount of carbon dioxide; it's just unfortunate that the rate of burning is higher than the rate of sequestration.