In recent years there has been an increase in unconventional gas production, specifically shale gas in North America to increase the country’s self-sufficiency in energy resources. The definition of unconventional gas as given by Law and Curtis in 2002 as ‘Conventional gas resources are buoyancy-driven deposits, occurring as discrete accumulations in structural and stratigraphic traps, whereas unconventional gas resources are generally not buoyancy-driven accumulations. They are regionally pervasive accumulations, most commonly independent of structural and stratigraphic traps’. Here unconventional gas refers to tight gas, coal bed methane, shale gas and methane hydrates. These are part of a category of frontier and unconventional oil and gas that have been attracting attention recently as conventional resources are becoming exhausted or inaccessible to non-state energy companies’. Frontier resources refer to conventional reserves in challenging locations such as extremely deep, cold and/or very inaccessible regions or are gas deposits which contain acid or sour gas.
The unconventional resources referred to here are nothing new and have been known about for hundreds of years, but have only recently become economically viable and is still not competitive with competitive gas, unless transportation costs are considered.
Coal bed methane (CBM) is methane trapped in coal deposits and is also known as coal seam gas. Most of the methane is adsorbed to the surface.
Tight gas is trapped in ultra-compact reservoirs with a very low porosity and permeability. Therefore, unlike conventional gas, tight gas can’t flow freely.
Shale gas is gas in the ‘source rock’, a clay-rich sedimentary rock with a low permeability, and is either adsorbed in the shale or in a free space in pores of the rock.
What is a source rock?
‘The source rock is the geological layer in which oil and gas are generated. It formed when organic-rich sediments were deposited on the bottom of oceans or lakes, then gradually covered over by additional sediment layers. As they became more deeply buried, the sediments were consolidated into rock, and the organic matter was transformed into oil and gas (oil and natural gas). The oil and gas tended to migrate upward through the pores and cracks of the surrounding rock, sometimes reaching the surface, but some of them were trapped under an impermeable rock barrier and collected beneath this “cap.” With time, the accumulation developed into a petroleum reservoir, the target of conventional oil and gas exploration.
In the case of gas shale, some or all of the gas released during the transformation of the biomatter stayed in place. To be a candidate for gas extraction, source rocks must have reached sufficient maturity to generate the gas, without yet having expelled it’.