Natural Gas Trapped in Its Source Rock
The term “shale gas” refers to “ordinary” natural gas that is characterized by an atypical geologic location. Shale gas is generally found several thousand meters beneath the surface, in the very rock in which it formed.
What is the origin of shale gas?
The origin of shale gas is the same as that of all hydrocarbons - coal, gas and oil. They formed within source rock, the result of the transformation of sediments rich in organic matter taht are deposited at the bottom of oceans and lakes. Over geological time, the sediments gradually became more deeply buried. During this process, they consolidated and subsurface heat and pressure converted the organic matter they contained into hydrocarbons. Most of the hydrocarbons that formed in this manner were gradually expelled from the source rock and migrated up toward the surface. However, in some cases, their migration was blocked by an impermeable rock barrier. The hydrocarbons accumulated beneath this “cap” and eventually formed a conventional oil and/or gas reservoir.
The hydrocarbons that remain in the source rock constitute shale gas and shale oil.
Gas Trapped in Its Source Rock
Shale gas is often trapped very deep beneath the surface (1,500 to 3,000 meters), in the source rock, i.e., the rock in which it, like all hydrocarbons, originally formed. Source rocks began life as argillaceous sedimentary rock rich in organic matter. As it became more deeply buried, the rock underwent changes induced by heat and pressure. It was transformed into rock with a fine-grained, laminated structure, which is typical of the rock classification known as shale. However, source rock is not true shale, so the term shale gas is something of a misnomer.
Because of the compact, impermeable nature of the source rock, the gas is trapped in the tiny pore spaces between the grains and is also tighly "attached" (absorbed onto) the very matrix of the rock - hence the term "source rock". This makes it very difficult to extract.
Gas Found Worldwide
Shale gas can be found in any sedimentary basin – in other words, virtually everywhere in the world. According to current assessments, shale gas is the most prevalent type of “unconventional gas” (a category that also includes tight gas and coalbed methane, or coal seam gas). The International Energy Agency (IEA) estimates reserves at 380 trillion cubic meters, equivalent to half the reserves of conventional gas.
Again, according to the International Energy Agency the world's major potential shale gas resources are found in the United States, Canada, China, Australia, India and numerous European countries, including France and Poland.
North America, which has been developing its shale gas for a number of years, is on the road to gas self-sufficiency. The Energy Information Administration (EIA) anticipates that the United States will be able to halt gas imports in 2020. In Europe, it is still too soon to say whether all or part of the resources can be developed cost-effectively and sustainably. Only the findings of surveys and studies conducted during the exploration can confirm or refute assumptions. Exploration began recently in the United Kingdom, Poland, the Czech Republic, Romania, the Netherlands and Denmark. The process takes between five and six years. In France, the government has revoked the exploration licenses that would have enabled the potential of the sedimentary basin in southeastern part of the country to be appraised.