Valleråsen

What is a Sandstone and How is it Formed?

Sandstone is a sedimentary rock composed of sand grains (with a diameter of 0.063–2 mm) that are bound together by a cementing material, such as quartz, calcite, or clay. Sandstone forms in two main stages.

• Sand is transported by water or wind and deposited in lakes, the sea, rivers, or on land (deserts). These sand layers often develop fine sorting patterns, which can provide information about the depositional environment and the direction of transport.

• When the sand layers are compressed due to high pressure from overlying layers (for example, on the sea floor), the sand is transformed into sandstone. Minerals then precipitate in the spaces between the sand grains, “gluing” the grains together. The sandstone found in Grenland is completely cemented. In sandstones where the pore spaces are not entirely filled, there is room for substances such as water, oil, or gas. Such sandstones make excellent reservoir rocks.

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Geological Storage of CO₂ – A Natural Opportunity

For a rock to be suitable for CO₂ storage, it must have high porosity and be easily permeable.

There are three types of storage sites:

• Depleted natural gas or oil fields

• Saline aquifers (saltwater reservoirs)

• Coal seams

The first type is the most important and has been the most thoroughly tested so far.

CO₂ Storage on the Norwegian Continental Shelf

In Norway, CO₂ storage will take place in depleted natural gas or oil fields and in saline aquifers on the continental shelf. The advantage of using such reservoirs is that they have already proven their ability to contain gases and liquids for millions of years. The reservoir rocks are usually sandstone or limestone. Above these, there are so-called cap rocks, which are dense, impermeable layers made of, for example, shale or claystone.

What Happens When CO₂ is Injected into a Former Oil/Gas Reservoir or an Aquifer?

When the oil and gas have been extracted, the cavities (pores) between the grains in the sandstone are filled with saltwater. Compressed CO₂, that is, CO₂ in liquid form, is then pumped into the reservoir. Because compressed CO₂ is lighter than water, it will rise and accumulate at the top of the reservoir, just below the cap rock. Some CO₂ will also dissolve in the saltwater and sink to the bottom of the reservoir, and some CO₂ will react with minerals in the reservoir, either dissolving them or forming new minerals.

For a rock to be able to store CO₂, it must have high porosity and be easily permeable.