between rock grains. Hydrocarbon is lighter than water, therefore when oil and gas escape from the source rock and encounter porous and permeable rocks (also known as reservoir rocks), such as sandstones and limestones, buoyancy forces the oil and gas upwards through the pore spaces. When oil and gas reach another impermeable layer that blocks the upward migration, they will move laterally along the layer boundary towards a trap-like structure where they begin to accumulate. Traps are normally created by folds and faults, and antiform is the most common natural trap. This type of trap is called the conventional oil and gas reservoir.
To produce hydrocarbon, a vertical well is drilled straight from the surface to this highly concentrated region. The International Energy Agency’s (IEA) defines conventional oil as “a category that includes crude oil – and natural gas and its condensates.”
Figure 1. A cartoon demonstration of oil and gas reservoir geology and trap environment. The bright orange-coloured layer is the source rock, the yellow dotted layers are reservoir rocks (typically sandstones and limestones with high porosity and permeability level), and the peach-coloured layers are caprock with low porosity and permeability so that oil and gas cannot escape. The cartoon shows two different trapping environments: fault on the bottom left and antiform at the top.
(https://i.pinimg.com/736x/eb/33/1e/eb331eeb5eb5fa28a4015aea20fab4ed–oilfield-life-oil-industry.jpg)
When the world thought that we had hit the peak of oil and gas production in the 2000s and that we had to focus on developing alternative renewable energies, newly developed technology to extract unconventional reservoirs made the production of shale gas in the US jumped from 1% in 2000 to over 20% by 2010. (https://www.chathamhouse.org/publications/papers/view/185311) This rapid growth, predicted to by the US government’s Energy Information Administration, is going to continue that 46% of the US’ natural gas supply will be provided by shale gas. There is no doubt that the unconventional oil and gas exploration will continue to grow globally with the growing technology.
Unconventional drilling produces hydrocarbons directly from source rock layers or tight rocks (poor quality rock layers that contain migrated oil and gas) through horizontal wells. Although there is still an on-going debate on the precise definition of unconventional oil, in this essay we use the definition made by the IEA World Energy Outlook (WEO) in its 2011 report: “[u]nconventional oil include[d] extra-heavy oil, natural bitumen (oil sands), kerogen oil, liquids and gases arising from chemical processing of natural gas (GTL), coal-to-liquids (CTL) and additives.(https://www.iea.org/publications/freepublications/publication/WEO2011_WEB.pdf) Unconventional oil and gas are harder and more expensive to exploit, however, due to the increasing demand of oil and the continuous shift from coal to natural gas in electricity production, companies and governments
