Bituminous Shale

Bituminous shale is a dark, organic-rich variety of shale containing abundant bitumen and kerogen — the solid organic matter that can be converted into oil and gas. It overlaps strongly with the terms oil shale, black shale, and carbonaceous shale.

Like all shales it is a fine-grained, laminated mudrock, but its high organic content gives it a dark brown to black color, a sometimes oily or tarry character, and the ability to burn. Heating it can release hydrocarbons, which is the basis of the oil-shale industry.

It is both an economic resource and an important source rock for conventional petroleum.

Bituminous shale forms from the accumulation of fine mud mixed with abundant organic matter in quiet, oxygen-poor (anoxic) water — in deep lakes, restricted seas, and stagnant marine basins. The lack of oxygen and burrowers preserves the organic material instead of letting it decay.

As the mud is buried and compacted, the preserved organic matter transforms into kerogen and bitumen, and the rock develops fine lamination and a dark color, often with associated pyrite from sulfur-rich anoxic conditions.

With deeper burial and heating, the kerogen can generate liquid and gaseous hydrocarbons. Classic examples include the Green River Formation oil shales of the western United States and many Paleozoic black shales.

Look for a dark brown to black, fine-grained, finely laminated rock that splits into thin layers (fissile) and may feel slightly greasy or smell of petroleum, especially when broken or warmed.

A diagnostic test is that bituminous and oil shales burn or char with a smoky, oily flame because of their organic content. They are soft enough to scratch with a knife and often contain pyrite. They do not fizz in acid unless carbonate is present.

Distinguish them from coal (which is dominantly carbon and lighter in weight) and from ordinary gray shale (which lacks the organic richness and does not burn).

The principal use of bituminous (oil) shale is as an energy resource: it can be mined and heated (retorted) to release shale oil and combustible gas, and in some regions it is burned directly for power.

It is also the most important petroleum source rock, generating much of the world's conventional oil and gas after deep burial and heating. Some processed shales yield byproducts used in chemicals and construction materials.

It has no gem or metaphysical use. Its development raises environmental concerns over water use, carbon emissions, and waste, which affect where and how it is exploited.