Types of Geological Wells

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The subsurface is a central element of modern energy systems, due to its capacity to contain and preserve different types of natural fluids. Hydrocarbons, water, gas, and hydrogen can accumulate in certain specific geological formations, whose properties allow for their exploitation or storage.

A geological reservoir refers precisely to a porous and permeable underground formation capable of retaining and transmitting fluids (gas or liquids), beneath an impermeable layer called a cap rock. This concept underpins numerous energy applications, ranging from CO₂ storage to geothermal energy, as well as the exploration of natural hydrogen.

What makes this concept particularly remarkable is its versatility: the same reservoir can successively serve multiple purposes depending on the industrial context and the stage of its lifecycle. The energy transition thus confers new value on geological formations that were previously exploited solely for hydrocarbons.

What is a geological reservoir?

A geological reservoir does not correspond to an empty cavity in the subsurface, but to a structured natural system composed of several complementary elements that allow for the accumulation and circulation of fluids at depth.

  • The reservoir rock is a porous and permeable geological formation capable of containing and allowing fluids to circulate, whether water, gas, oil, hydrogen (H₂), or CO₂. In the case of geothermal systems, it also enables heat transfer. It constitutes the primary volume in which fluids are stored.

  • The cap rock, also known as the caprock, is an impermeable layer located above the reservoir rock. It plays a key role in preventing fluids from migrating back to the surface. This confinement mechanism is essential in many energy contexts, whether for hydrocarbons, CO₂, or, more recently, natural hydrogen — a gas of geological origin whose exploration is attracting growing interest. The U.S. Department of Energy (U.S. DOE) highlights that these hydrogen accumulations form precisely in environments where the gas has migrated before being trapped beneath such a formation [4], illustrating the central role of this geological barrier.

  • The geological trap corresponds to the structural configuration of rock layers that allows fluids to be contained by limiting their lateral or vertical migration. It results from the geometry of the subsurface and complements the action of the cap rock in the trapping process.

Within a regulatory framework, the Pipeline and Hazardous Materials Safety Administration (PHMSA) distinguishes three main categories of structures used for underground natural gas storage: depleted hydrocarbon reservoirs, aquifers, and salt caverns [2]. This classification shows that the concept of a geological reservoir applies to a diversity of contexts, well beyond hydrocarbon production alone.

Figure 1: Different types of underground gas storage systems

The two fundamental properties: porosity and permeability

For a rock formation to function as a reservoir, it must exhibit certain essential physical properties, which are systematically evaluated in geoscience. Among these, porosity and permeability are decisive for understanding fluid behaviour at depth.

  • Porosity corresponds to the fraction of the total volume of a rock occupied by voids, called pores. These spaces may be of primary origin (linked to the formation of the rock) or secondary (resulting from processes such as dissolution or fracturing). The level of porosity directly determines the storage capacity of a reservoir: the higher it is, the greater the quantity of fluid that can be contained.

  • Permeability, for its part, describes the ability of a fluid to circulate within the rock, depending on the degree of connectivity between pores. A rock may thus exhibit significant porosity while still limiting fluid circulation if its pores are isolated or poorly connected. Conversely, good interconnection of the pore network allows for more efficient circulation, which is essential for production or injection operations.

The combination of these two properties determines whether a reservoir is exploitable, whether for resource production or long-term storage. Their analysis thus constitutes a key step in the evaluation and development of subsurface projects.

The main types of geological reservoirs

Geological reservoirs are not all alike. Their lithological nature (i.e. the type of rock that composes them), their depth, and the fluids they contain determine their potential use. Several major families can be distinguished, each of particular interest in the context of the energy transition.

Table I : Types of geological reservoirs and their uses

Reservoir type Simplified description Fluid(s) concerned Link with the energy transition
Depleted oil or gas reservoir Former producing formation from which hydrocarbons have been extracted CO₂, natural gas, H₂ Possible conversion into CCUS or H₂ storage site
Deep saline aquifer Porous formation saturated with non-potable salt water, at great depth CO₂ Priority site for geological CO₂ storage
Salt cavern Artificial cavity carved out of salt layers Natural gas, H₂ Fast-cycle storage (seasonal or daily)
Geothermal reservoir Permeable formation containing hot water or steam Heat / hot fluid Production of decarbonised electricity or heat
Natural hydrogen reservoir Underground trapping zone where geological H₂ has accumulated Natural hydrogen (H₂) Active exploration for low-carbon H₂ production

Depleted reservoirs and saline aquifers: the most relevant for storage

Among these categories, depleted hydrocarbon reservoirs and deep saline aquifers attract particular attention for the geological storage of CO₂. Their characteristics and differences merit closer examination.

  • Deep saline aquifers are porous formations saturated with very salty, non-potable water, located at great depth. They currently represent one of the main options envisaged for the geological storage of CO₂, notably since the first industrial projects such as Sleipner in the North Sea, launched in 1996 [6]. Their storage efficiency factor generally falls between 2 and 20% [6], meaning that only a fraction of the available pore volume is effectively mobilised for storage.

  • Depleted gas fields, by contrast, can achieve storage efficiency factors of up to 80% [6], making them highly competitive candidates for carbon capture and storage (CCS) projects. According to the IPCC, depleted hydrocarbon reservoirs, coal formations, and, in particular, deep saline aquifers can all be considered for CO₂ sequestration [1].

  • The Federal Energy Regulatory Commission (FERC) in the United States officially classifies storage sites into three categories: depleted hydrocarbon fields, aquifer fields, and salt caverns [3], a taxonomy that well reflects the diversity of options available at an industrial scale.

Further reading: "Why geological CO₂ storage is one of the key components of the energy transition"

Geothermal and natural hydrogen reservoirs

Two other types of reservoirs are occupying an increasingly prominent place in discussions related to the energy transition: geothermal reservoirs and those associated with natural hydrogen. Still relatively absent from public debate, they nonetheless attract sustained interest in scientific and industrial circles.

  • Geothermal reservoirs correspond to deep permeable formations containing fluids naturally heated by the Earth's thermal gradient. Depending on geological conditions (temperature, pressure, nature of the rocks), these systems can be exploited to produce heat directly or to generate electricity via geothermal power plants. Their operation generally relies on the circulation of fluids within the reservoir rock, either naturally (hydrothermal systems) or in a stimulated manner in the case of enhanced geothermal reservoirs. These resources are present in a variety of geological contexts, making them an energy option adaptable to different territories.

  • In the case of natural hydrogen, exploration efforts focus on underground accumulations of dihydrogen (H₂) of geological origin, formed through various natural processes. As with other fluids, this gas can migrate through rock formations before being trapped beneath a cap rock, thereby creating conditions favourable to its accumulation. Although this field is still in the exploration phase, initial estimates suggest potentially significant volumes on a global scale, ranging between 10³ and 10¹⁰ million tonnes according to a study published in Science Advances in 2024 [7].

Abandoned reservoirs: an overlooked asset of the energy transition

A depleted geological reservoir is not necessarily a useless piece of infrastructure. The energy transition opens the way for their repurposing, transforming former industrial liabilities into valuable resources for energy storage or carbon capture.

What distinguishes a depleted field from an unexplored saline aquifer is precisely the wealth of documentation that accompanies it. A reservoir whose production has ended already has a well-documented geology: drilling data, subsurface mapping, production history. This information considerably reduces the risk and cost of a new development project [5].

The IEA Greenhouse Gas R&D Programme (IEAGHG) has published a comparative analysis between depleted reservoirs and saline aquifers for CO₂ storage [5], highlighting that the pre-existing geological characterisation of depleted fields constitutes a significant advantage when developing a CCS project. This explains why, according to the University of Liverpool study (2024), several major CCS projects are currently planned in depleted gas fields, notably due to their potentially higher storage efficiency compared to aquifers [6].

One reservoir, multiple value cycles

Beyond CO₂ storage, a reservoir can pass through several phases of use over time. This concept of multi-use valorisation is well established in the natural gas sector: a portion of the volume of a storage reservoir is reserved as "cushion gas", a permanent volume of gas maintained in the reservoir to preserve the operational capacity of the site [8].

Thus, a reservoir can successively pass through several phases depending on the evolution of technologies and energy needs. The IPCC confirms in this regard that deep geological formations can serve simultaneously or successively for both production and storage [1].

Conclusion

The geological reservoir is not a concept confined to the oil industry's past. The available data remind us of the real and lasting value these formations represent: depleted reservoirs can, under favourable conditions, achieve high efficiency factors, sometimes estimated at up to 80% [6], while the global subsurface is estimated to contain between 10³ and 10¹⁰ million tonnes of natural hydrogen [7], a resource whose exploration relies entirely on the understanding of these same geological structures.

In this context, the energy transition is reinventing the role of the subsurface. Whether for storing CO₂, producing heat, or accessing natural hydrogen, the detailed characterisation of geological reservoirs is becoming a central geoscientific competency for the decades ahead.

Understanding geological reservoirs means understanding the scientific foundations of several rapidly growing energy sectors. This bridge concept between geology and energy lies at the heart of the challenges of exploration and sustainable subsurface valorisation — challenges that Squatex follows closely.

Would you like to learn more about these topics? Follow Squatex on LinkedIn to be informed of upcoming articles. You can also explore the subject further by reading our articles on geothermal energy, CCUS, or natural hydrogen.

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