Humanity distinguishes itself from other species by intellect, innovation, and technology – but we cannot survive and thrive on brainpower alone. We need natural resources to translate our ideas to products that benefit humanity.
To help understand the wide variety of natural resources and their associated issues, benefits, and challenges, I am considering them in four categories:
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- Subsurface energy resources – oil, natural gas, coal, radioactive energy minerals, and geothermal heat
- Other subsurface resources – minerals (critical and non-critical)
- Water and land resources – oceans, lakes, and land
- Resources above ground – solar and wind energy
Today, we look at subsurface energy resources.
How do we define the subsurface?
We can use a really simple definition – anything that is below the surface, beneath our feet, or underground. When talking about subsurface resources, however, we need to be a bit more specific – so we will focus on resources that are buried deep underground.
How deep? Generally, more than 100 metres, and often a lot deeper, up to thousands of metres deep. Groundwater, sand and gravel, and even soils are technically in the subsurface, but groundwater is generally connected to surface water bodies, and we can dig up sand and gravel with shovels and backhoes – no drilling required. So let’s lump the really shallow stuff in with surface (land) resources, and confine our subsurface discussion to the deep resources that require drilling or mining.
Subsurface energy resources
Most of the energy used to power civilization comes from natural resources extracted from the subsurface, including:
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- Oil and gas – also known as petroleum or hydrocarbons.
- Coal – because coal is solid, we can find and extract it right at the surface, but even deposits cropping out at the surface require mining.
- Radioactive energy minerals – primarily uranium, used in various forms to fuel nuclear fission reactors. Thorium also has large-scale energy production potential, but is not yet fully commercial.
- Geothermal energy – the natural heat of the earth. As we drill down beneath the surface, the rocks get hotter. Where rocks get hot enough at shallow enough depths, we can extract the heat for such things as space heating, greenhouses, or industrial processes. Really hot rock can produce steam to turn turbines and generate electricity.
- Hydrogen – there is talk of drilling to produce naturally occurring hydrogen from wells, just like natural gas. It is all talk at the moment – we are not doing it yet.
Figure 1 – Global energy consumption by source
Figure 1 is a breakdown of humanity’s energy consumption by source on an annual basis since 1970. Oil, gas, coal, nuclear, and hydroelectricity dominated the late 20th century. Solar, wind and other renewables have grown rapidly since 2010, but still represent a small portion of our energy consumption. Geothermal is relatively small, and is lumped in with “other renewables”. Natural hydrogen is not on the chart.
So subsurface energy resources are key to humanity’s energy security, and there is nothing more important for human well-being than energy security (Yes, there is an energy transition – and to manage it, we need to understand it).
Consumption of subsurface energy resources has increased rapidly since people first mined coal in the early 1800s. Humanity consumed twice as much subsurface energy in 2023 as we did in 1970. The Energy Institute reported that in 2024 global oil production increased by 0.6%, gas production by 1.2%, and coal by 1% (Statistical Review of World Energy), indicating the growth curves are starting to flatten. It is reasonable to expect that fossil fuel demand growth will plateau at some point in the future as population growth slows, alternative energy supplies are developed, and energy is used more efficiently.
But make no mistake – we will be consuming enormous quantities of subsurface energy resources for decades to come.
Natural resources have limits
Consuming more will be a challenge – because there are only so many barrels of oil, cubic metres of gas, and tonnes of coal and uranium in the global subsurface.
How many? Hard to say, because it is a moving target. The absolute amounts are truly enormous, and we have produced only a tiny percentage so far – but only a fraction of the in-place resources are economically accessible. Most are too dispersed, or in accumulations that are too small to make them worthwhile to drill or mine. You would spend more money (and energy) to extract the resource than you would gain by processing and selling it.
As an example, look at Figure 2, a map of oil-sands development areas and bitumen (ultra-heavy oil) resources in northeastern Alberta. The oil sands extend across an enormous area – officially 142,200 km2. You can drill a well anywhere within the oil-sands areas and discover some amount of heavy oil or bitumen. Add it all up, and there is more than 1 trillion barrels of bitumen in the ground.
But you need to be able to extract a lot of bitumen to justify the cost of either a surface mine (where the resource is at shallow depths) or to pay for high-tech wells where the resource is buried more deeply. In fact, only the most densely packed bitumen resources, coloured red and dark orange on the map, can be exploited. You simply cannot produce enough bitumen from the more sparse areas to pay the bills for development.
Figure 2 – Oil-sands development areas, northeastern Alberta. Colours indicate thickness of ultra-heavy oil (bitumen) deposits, with the hottest colours indicating maximum thickness (from Rahmana et al., 2013).
This introduces the concept of reserves – defined as the amount of resource that can be produced using currently available technology and under current economic conditions. Alberta’s oil-sands reserves are currently estimated at 165 billion barrels – fourth-largest in the world, but only a fraction of the total resource in place (Oil sands 101).
Because the definition of reserves depends on technology and pricing, total reserves recoverable from a particular resource base can increase over time with development of new technologies and rising prices. A great example – in the late 20th century, North America faced an energy security crisis as conventional oil and gas reserves declined and production rates fell. But the development of horizontal drilling and hydraulic-fracturing technologies in the early 2000s enabled industry to access and produce hydrocarbons from shales and other reservoirs formerly deemed uneconomic. Suddenly, we saw a massive increase in oil and gas reserves because technology enabled profitable production of known in-place resources that had until then been dismissed as uneconomic.
Assessing the remaining productive potential of subsurface energy resources is a highly interpretive exercise, relying on expert knowledge and a lot of assumptions. It is easy to find assessments of remaining reserves, or estimates of potential resources in particular areas – but there are still many prospective areas of the world for oil, gas, coal, and uranium that we know little about, and therefore little about their resource potential. And recoverable reserves are always a moving target as technology, demand, and pricing evolve.
So how much oil, gas, coal, or uranium is left? There is a different answer for each, but suffice to say, we are not running out in the next few decades.
What about geothermal energy?
There is an enormous amount of heat in the earth, and we can drill just about anywhere to encounter it. But producing it economically faces the same issue as other subsurface energy resources – you need to be able to extract sufficient energy to justify the cost of drilling the wells.
A global map of surface heat flow shows that geothermal heat is intense in very specific places, particularly along boundaries between tectonic plates and near active or recently active volcanoes (Figure 3). As a result, electricity can be generated from intense geothermal heat close to the surface in places such as the western United States, Iceland, Indonesia, and New Zealand. There are even countries where 20-70% of their total energy demand is met by geothermal (Figure 4). Note, however, that these countries have very modest energy demands and immense geothermal resources.
Figure 3 – Global surface heat flow, showing most intense geothermal heat along tectonic plate boundaries and around volcanic centres. Geothermal energy in deep aquifers: A global assessment of the resource base for direct heat utilization
Canada, on the other hand, produces almost no geothermal electricity and limited geothermal heat because most of the country lies on the cold Canadian Shield. There are geothermal hot spots in remote northern and western areas of the country, and they are being investigated – but if they prove viable, they will cost a lot to develop and tie in.
Geothermal technology, like oil and gas technology, is evolving and making lower-resource (cooler) areas prospective for geothermal development. Closed-loop geothermal drilling (Closed-Loop Geothermal Technology for a 24/7 Carbon-free and Secure Energy Future) and enhanced geothermal systems (Enhanced Geothermal Systems) are both expanding our geothermal reserve base. But compared to other subsurface energy resources, geothermal potential is still relatively small and local.
Figure 4 – Top 10 countries by share of total energy demand provided by renewable energy sources. Iceland, New Zealand, Kenya, and Costa Rica are all situated in areas with high geothermal heat availability, and all have modest total energy demands, so obtain between 20% and 70% of their energy from geothermal. Statistical Review of World Energy
What about geologic hydrogen?
There is media chatter about drilling for naturally occurring hydrogen as a subsurface energy resource, but research by the US Geological Survey shows we are a long way from even defining hydrogen as a resource, let alone extracting it economically (Prospectivity Mapping for Geologic Hydrogen). Realistically, it is not an energy resource today, and is unlikely to ever be significant.
Transporting the resources
Subsurface energy resources dominate the world energy picture today because they are diverse, abundant, and widespread. Some areas are blessed with a disproportionate share of energy resources – such as the Middle East for oil, and Kazakhstan for uranium.
But there is one more key attribute making subsurface energy resources so dominant. Coal, oil, natural gas, and uranium are very energy dense – packing a lot of energy into small packages. We can afford to extract them in one place and ship them as is or with minimal processing to other places where they are used (Figure 5). We are transporting the fuel, not the energy itself.
But geothermal energy? There is no fuel to transport. It is more challenging to transport electricity, and really not possible to move heat energy any distance. We will examine these concepts in more detail when talking about hydro, wind, and solar in a future article.
Figure 5 – 2024 international trade in crude oil. Intricate trade relationships are supported by the high value and energy density of oil. Statistical Review of World Energy
Subsurface energy resources are the key to modern civilization
Even though oil, gas, coal, and nuclear fuels are widely distributed and can be transported readily, access to them is probably humanity’s most critical strategic/geopolitical issue – because they are essential to energy security and prosperity.
Some countries – including Canada and the United States – are incredibly energy rich. They are generally rich and prosperous, and thrive even when their political and policy leadership is lacking. Some countries with few subsurface energy resources (e.g., Japan, South Korea) thrive anyway because they understand the importance of energy and have structured their economies and trading relationships to include multiple sources of coal, oil, gas, and uranium.
But there are many nations, particularly in Africa, with substantial subsurface energy resources that they cannot afford to access, and/or cannot sustain transparent regulatory regimes to support access. They live in energy poverty, and daily existence is a struggle for many of their citizens.
Then there are countries – many in Europe – that have been built on substantial subsurface energy resources but have recently chosen to shut down their nuclear facilities, coal mines, and gas fields in the hope that other energy sources might sustain them. Instead, most are seeing energy security falter, energy prices rise, and heavy industries flee as they can no longer afford the energy they need to run their businesses.
We live in interesting times, and subsurface energy resources will be critical to our future.
