**1. Introduction**

 Hydrocarbon reservoirs contain fossil fuels and constitute a major proportion of sources of energy worldwide. In the past, extraction of oil and gas was mainly restricted to conventional reservoirs which underlie a sealing caprock or rock formation with lower permeability and consist of rock and fluid with characteristics that readily allow the flow of oil and gas into wellbores. These reservoirs are easily assessed and contain sufficient pressure such that the external and additional drive necessary to push the hydrocarbon fluids to the surface are not exigent. Conventional reservoirs are recognised by their structural layout, stratification and rock and fluid properties. Typically, they comprise three major parts: a cap rock, a source rock and a reservoir rock. The cap rock is the impermeable rock layer that seals the boundaries of the top and sides and entraps the hydrocarbons within the reservoir. Hydrocarbons are formed in the source rock (normally limestones or shales) which contains kerogen, an insoluble and solid organic matter. The reservoir rock is the permeable and porous layer containing hydrocarbon fluids generated in the source rock. Over a protracted period, oil and gas formed in source rocks migrate to reservoir rocks, a process that is essential for the existence and validity of reservoir rocks.

With the advent in advanced technology and increasing need for more and cleaner energy, oil and gas production has been extended to unconventional reservoirs. Generally, unconventional reservoirs are difficult to produce. They are mainly composed of very tight source rocks containing hydrocarbons that have not migrated to reservoir rocks. These ultra-tight source rocks are termed unconventional reservoirs. Fundamentally, unconventional and conventional reservoirs are differentiated based on the migration of hydrocarbons from source rocks. Conventional reservoirs are rock formations that are recipient of hydrocarbons from source rocks, while unconventional reservoirs are source rocks containing hydrocarbons that cannot be naturally released to reservoir rocks. Nonetheless, the term unconventional reservoirs broadly cover reservoir rocks which are problematic to produce, for instance, tight reservoir rocks (tight sandstones, tight limestone, etc.) and heavy oil reservoir rocks.

Artificial lift is a standard method of instigating flow from the reservoir through the wellbore. This technique decreases the bottomhole pressure (BHP) while increasing the pressure in the reservoir, thereby raising the rate of well production. It is inevitably required at a certain time during the life of an oil/gas field due


**Table 1.** 

*Outline of some fundamental aspects of unconventional oil and gas production.* 

 to diminishing flow rates or for the removal of liquids to enable gas flow. Often, artificial lifts are sufficient for conventional reservoirs as a means of actuating or boosting flow; however, when applied in isolation, the same effects are not obtained in unconventional reservoirs. A vast amount of hydrocarbons are trapped within unconventional reservoirs. These reservoirs possess tremendous economic potential which can only be realised if they are properly stimulated. A range of reservoir stimulation methods are now available which have rendered unconventional reservoirs commercially viable. Some of these methods (e.g. hydraulic fracturing) can be applied to a broad spectrum of reservoirs, whereas others (e.g. some forms of acidisation) have limited applications. Since the boom in production from unconventional reservoirs, great strides in development have been made, with an increasing number of source rocks and depleted reservoir rocks subject to being produced. This has also raised concerns in relation to the impact on atmospheric, aquatic, land and underground environments; climate change; economic viability; technology requirements; health and safety; and sustainability. Ongoing studies are geared towards improving the process of exploiting unconventional reservoirs and increasing value for money while ensuring minimal levels of pollution and contamination to the environment, as well as risk to humans, and flora and fauna. The scope of areas considered in terms of the exploitation of unconventional resources is apparently inexhaustible, especially when viewed from a microscopic perspective. However, these can be harmonised into a more condensed list of themes. Some key aspects regarding the exploration and production of unconventional reservoirs are discussed in this chapter. These are encompassed within the subjects of discourse itemised in **Table 1**.
