What is shale oil?

Shale oil is light oil contained in low-porosity rock, usually shale. Since this rock is not very permeable, the oil cannot easily percolate towards the drilling head to reach the surface.

To produce shale oil (or gas), the rock has to be fractured to create fissures. The most widespread technique is hydraulic fracturing, or " fracking", which involves injecting water at high pressure. This technique is criticized because the way in which the rock is fractured is not fully mastered (seismic risk), and because the water injected has to be mixed with chemicals to improve the efficiency of the process. The exact nature of the products used is a manufacturing secret. However, some of these products are toxic and liable to rise to the surface or pollute groundwater. Moreover, the use of large quantities of water poses a problem in regions already subject to drought.

In addition to water, sand is also injected to prevent the cracks created by fracturing from closing under the pressure of the rocks above.

It is the mastery of horizontal drilling, both technically and in terms of cost, that has made shale oil (and gas) production profitable in recent years. With a single surface rig, it is possible to drill through the thickness of the geological layer containing the hydrocarbons.

Shale oil constantly requires new drilling (and fracturing) campaigns, as and when the deposit is exploited. These campaigns represent major investments that producers must regularly renew.

A: "Conventional" natural gas

B: Conventional oil

C: Shale oil (or gas)

D: Bedrock oil (or gas)

Horizontal (or directional) drilling provides access to oil (or gas) that has not migrated to the reservoir rock.

Rocks with low permeability (shale, sandstone or limestone) need to be fractured for hydrocarbons to rise to the surface.

Source: Wikipedia modified by us

Beware of false friends: we often use the term "shale", even when the original rock is not shale. When we want to distinguish oil from other rocks, we call it " tight oil ", or " light tight oil " (LTO), as this rock has the particularity of being watertight. In French, we call it compact reservoir oil (or gas). Some statistics distinguish between " shale oil " (contained in shale) and " tight oil " (contained in sandstone or limestone). In French, the term that encompasses both notions is pétrole (ou gaz) de roche-mère.