Shale itself isn't inherently light, medium, or heavy. Its density varies depending on the specific composition and porosity of the shale.

Here's why:

* Composition: Shale is a sedimentary rock primarily composed of clay minerals, silt, and other fine-grained materials. The specific minerals present and their proportions can affect its density.

* Porosity: The amount of empty space within the shale (porosity) also plays a role. Shale with high porosity will be less dense than shale with low porosity.

Generally:

* Shale with a high clay content tends to be less dense.

* Shale with a high quartz content tends to be denser.

So, to determine if a specific shale is light, medium, or heavy, you need to consider its:

* Mineralogy: The types and proportions of minerals present.

* Porosity: The amount of empty space within the rock.

If you have a specific shale sample, you can measure its density using a laboratory method.