Shale gas are unconventional deposits which appear as continuous accumulations of natural gas spread out on large sedimentary units, stretched all over a region. The volume of rock is usually saturated with hydrocarbons and the contact between gas and water is not a useful data to define the deposit. Gas exploited in this type of deposit is contained in a sequence of fine-grained rock where the Shale predominates.

Natural gas is stocked in the layer in two ways :

• adsorbed gas : gas is fixed to the surface of ions or molecules. The portion of adsorbed gas varies between 20% (Barnett Shale) and 85% (Lewis Shale).
  


• free gas : gas is contained in the matrix porosity (interbedded layers of silt or sandstone in the shale) and in the natural network of fractures.

Contrary to conventional deposits, Shale gas act as a source that generates natural gas and also as reservoir rock for the stocking of gas. The origin of gas can be biogenic (by actions of bacteria) or thermogenic.

Shale gas have been known for more than a hundred years, but most of them were not developed because the techniques up to now did not allow a profitability of the projects. On the other hand, nowadays there is an enthusiasm for this type of deposit due to many factors : stocks of natural gas are diminishing, prices are higher, and mainly, progressing technology ensures economical exploitation of this resource.

Explosion of Production in the United States

Explosion of gas production from the Shale in the United States started at the beginning of years 2000; it can be mainly explained because of the improvement of wells fracturation techniques and with the use of horizontal drillings. For example, in 2006, we counted more than 6,200 natural gas production wells in the Barnett Shale, located in Texas. In 1999, there were only 523 wells in operation while in 1989, 10 years earlier, there were only 47 wells producing gas from the Shales.