Oil shale is a sedimentary rock containing kerogen, a solid, combustible organic material. Kerogen is a precursor that requires extensive processing to convert it into usable liquid fuel, often called shale oil or syncrude. The extraction and conversion process raises questions about its environmental cost, particularly the release of carbon dioxide (CO2). Determining the total CO2 footprint requires analyzing the entire life cycle, from extraction to the final combustion of the resulting fuel. The energy required to unlock the kerogen is a major source of its overall emissions profile.
The Energy-Intensive Path to Oil Shale Emissions
The substantial carbon footprint of oil shale production stems from the large amount of energy required to process the rock before fuel is created. Unlike conventional petroleum, the kerogen in oil shale must be heated to high temperatures (pyrolysis or retorting) to break down the solid organic matter into shale oil and gas. Emissions are generated from both the final burning of the syncrude and the upstream preparation activities.
The retorting stage is intensely energy-demanding, requiring the rock to be heated to temperatures often exceeding 450°C. This thermal requirement places a significant burden on the overall energy balance of the process. The heat of retorting can range widely (e.g., 240 MJ/t to 880 MJ/t of shale), depending on the rock’s characteristics. The energy source used to supply this heat directly influences the final carbon intensity.
The two main methods for heat application are ex-situ and in-situ conversion. Ex-situ processing involves mining the shale and heating it in surface retorts, requiring energy for crushing and material handling. In-situ methods heat the kerogen underground, demanding large energy input for heating elements and thermal barriers. Both approaches require large quantities of power, defining oil shale as an inherently emission-heavy resource compared to less processed fossil fuels.
Quantifying the Carbon Dioxide Footprint
Quantifying the CO2 footprint of oil shale syncrude requires a full life-cycle assessment, tracing emissions from extraction through combustion. Liquid fuels derived from oil shale have full-fuel-cycle CO2 equivalent (CO2e) emissions estimated to be significantly higher than those from conventional petroleum. The life-cycle intensity of oil shale fuels, such as diesel, typically ranges from 110 to 160 grams of CO2 equivalent per megajoule (gCO2eq/MJ). This range reflects variability in the source rock and the conversion technology used.
Emissions are distributed across three distinct life-cycle stages, with final combustion being the largest contributor. Combustion of the finished transportation fuels accounts for 50% to 65% of the overall life-cycle footprint. The industrial retorting stage, which heats the rock to convert the kerogen, contributes a substantial 25% to 40% of the total CO2e emissions.
The remaining 5% to 15% of the footprint comes from upgrading and refining the raw shale oil into usable syncrude. This upgrading process is necessary because raw shale oil is often heavy and high in contaminants, requiring energy-intensive steps like hydrotreating. Upstream emissions, generated before the fuel reaches a vehicle’s tank, make up a large share of the total environmental burden.
Oil Shale Emissions Compared to Conventional Fuels
Comparing the life-cycle emissions of oil shale to conventional crude oil reveals a clear difference in carbon intensity. Assessments show that emissions from oil shale derived fuels are 25% to 75% higher than those from traditional liquid fuels. For context, the well-to-wheel life-cycle carbon intensity for conventional petroleum-based gasoline ranges from approximately 97 to 128 gCO2eq/MJ. Conventional petroleum-based diesel typically has a life-cycle intensity of around 95 gCO2e/MJ.
Oil shale syncrude, with its intensity range of 110 to 160 gCO2eq/MJ, consistently ranks higher on the emission spectrum. This difference is primarily due to the intense energy penalty incurred during the retorting and upgrading processes. Oil shale falls into a category of higher-carbon-intensity resources, similar to or exceeding some tar sands-derived fuels.
The process results in a carbon footprint that is between 1.2 and 1.75 times greater than that of conventional crude oil feedstocks. This means oil shale requires a larger energy input and releases a greater volume of greenhouse gases per unit of energy delivered. The high processing energy required for the solid rock matrix is the largest factor driving this comparison.
Key Factors Driving Emission Variability
The quantitative figures for oil shale emissions are presented as ranges because the final carbon intensity depends on several operational and geological factors.
Grade of Shale
One significant variable is the grade of the shale, which is the concentration of kerogen within the rock. Lower-grade shales require processing more rock material for the same amount of oil, demanding more energy and resulting in higher emissions per barrel of syncrude produced.
Retorting Technology and Mineral Composition
The specific retorting technology employed also introduces variability into the final emission number. Different ex-situ technologies, such as those using gaseous versus solid heat carriers, have varying thermal efficiencies and energy requirements. Another factor is the mineral composition, particularly the presence of carbonate minerals like dolomite or calcite. When heated during retorting, these carbonates decompose and directly release additional CO2, separate from the fuel combustion.
Energy Source for Processing
The source of the energy used to power the retorting facility is a major determinant of the overall footprint. If the facility relies on high-carbon-intensity power sources, such as coal or natural gas, upstream emissions will be substantially higher. Conversely, if the heat and power are supplied by low-carbon sources, or through efficient utilization of non-condensable gases produced during retorting, the total life-cycle emission figure can be reduced.