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