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Oil shale is one of the most abundant and underrated forms of solid organic fuel, with global resources that exceed the proven reserves of conventional crude oil many times over. It is a sedimentary rock in which a mineral matrix is intimately interwoven with organic matter — kerogen — which, upon thermal processing, yields liquid and gaseous hydrocarbons. Understanding the origin, composition and properties of oil shale determines the choice of processing technology and the assessment of the industrial value of this feedstock.

What oil shale is

Oil shale is a dense, finely laminated sedimentary rock containing a substantial amount of organic matter that does not dissolve in conventional organic solvents. Its principal combustible component is kerogen — a high-molecular-weight organic substance disseminated throughout the mineral matrix of the rock. Unlike crude oil and natural gas, the organic matter in oil shale exists in a solid state and cannot be recovered by simple extraction; it requires thermal treatment — pyrolysis.

In essence, oil shale occupies an intermediate position between ordinary sedimentary rocks and the caustobioliths of the coal series, representing a distinct type of fuel-energy and chemical feedstock.

Geological origin and formation

Oil shales formed over geological epochs on the bottoms of ancient marine and lacustrine basins through the accumulation and burial of the remains of planktonic algae, microorganisms and other biomass. The predominance of sapropelic organic material, deposited in a reducing, oxygen-deprived environment, distinguishes them from humic coals formed from the remains of higher land plants.

Through diagenesis and subsequent catagenesis, under moderate temperatures and pressure, the original organic matter was transformed into kerogen. The fine interlayering of organic matter with clayey, carbonate or siliceous mineral material gave the rock its characteristic laminated structure and its ash content.

Composition: kerogen and the mineral matrix

Oil shale consists of two principal parts — an organic and a mineral component. The organic part is represented by kerogen, whose share in industrially valuable shales is typically from 10 to 30 % or more by mass; it is kerogen that, upon heating, decomposes to form shale oil, gas and a solid residue — semi-coke. The mineral part accounts for the rock’s high ash content and consists mainly of clay minerals, carbonates and quartz.

The key components of oil shale include:

Physical and chemical properties

The most important technological characteristics of oil shale are its calorific value, ash content, moisture and oil yield, the latter determined by the standard Fischer assay (low-temperature retorting). The calorific value of commercial shales usually lies in the range of roughly 6 to 16 MJ/kg and is considerably lower than that of coal owing to the high proportion of mineral matter. The oil yield in laboratory retorting serves as the principal indicator of a shale’s value as a feedstock for liquid fuel and typically ranges from a few to 20 % or more on a dry-rock basis.

The high ash content and comparatively low calorific value impose limits on the modes of use: direct combustion is less efficient, so for deep processing the thermal decomposition of kerogen, with separate recovery of liquid, gaseous and solid products, is preferable.

Industrial significance and processing

The industrial value of oil shale stems from the fact that during pyrolysis kerogen is converted into shale oil — a synthetic analogue of crude oil suitable for the production of motor fuels, boiler fuel and a wide range of chemical products. This makes oil shale a strategic alternative to conventional oil and gas for regions holding large reserves of it, and the foundation for the development of a shale-processing industry.

The most effective technology for the energy use of shale has proven to be pyrolysis with a solid heat carrier (SHC, Galoter), in which crushed shale is heated by a hot ash heat carrier. This approach delivers a high yield of shale oil, makes it possible to recover the energy of the semi-coke and to use the feedstock comprehensively, while minimising waste generation.

The English term “shale oil” is ambiguous: it is used both for the oil obtained by thermally processing solid oil shale and for the light crude extracted from low-permeability shale formations — two fundamentally different feedstocks produced by entirely different methods. The first is a product of retorting (pyrolysis) of oil shale; the second, properly called tight oil, is recovered by drilling and hydraulic fracturing. Let us examine the source of the confusion, how each is produced, how their composition and quality differ, and where they are used.

Where the confusion comes from

The root of the misunderstanding lies in the phrase “shale oil” itself. Historically it referred to the oil distilled from oil shale; later, with the rise of fracking, the same loose label was applied to light crude produced from shale source rocks. As a result one term came to cover two distinct concepts, and in popular texts the boundary between them is often blurred.

Yet these products correspond to different geological objects and different technologies. To avoid conflating them, engineers distinguish shale oil — the retort (kerogen) oil derived from solid oil shale — from tight oil, extracted from dense rock.

Shale oil: a product of oil shale processing

Shale oil is produced by the thermal processing of oil shale — a sedimentary rock containing insoluble organic matter known as kerogen. The rock itself holds no liquid hydrocarbons: kerogen is converted into oil only when heated in the absence of oxygen (retorting, pyrolysis) at a temperature of about 480–520 °C, when large organic molecules break down and release a vapour-gas mixture that condenses into liquid oil.

In modern solid heat carrier (SHC) units based on the Galoter technology, heat is transferred to the shale by hot ash, which ensures rapid heating and a high oil yield. In essence, shale oil is a synthetic liquid artificially produced from a solid feedstock rather than extracted from the subsurface in ready form.

Tight oil: light crude from dense rock

Tight oil is ordinary light crude that already exists in the subsurface in liquid form but is trapped in dense, low-permeability rock (often shales and tight reservoirs). Because of the extremely low permeability, such oil will not flow to the well on its own, so it is produced by horizontal drilling combined with hydraulic fracturing: fluid with proppant is pumped into the rock under pressure, creating a network of fractures through which the oil flows into the well.

There is no thermal processing of organic matter here — this is the extraction of already-formed crude, simply from a reservoir inaccessible to conventional methods. It is therefore more accurate to call it tight oil rather than a product of shale processing.

Differences in composition and quality

Because the products have different origins, their properties differ as well. Crude shale oil derived from kerogen is usually heavier and contains more heteroatomic compounds — sulphur and especially nitrogen — as well as unsaturated hydrocarbons and resinous components, so it requires upgrading (hydrotreating) before being converted into motor fuels. Tight oil, by contrast, is light and low-viscosity by nature, with a low sulphur content, and is close to high-quality grades of conventional crude.

Where each product is used

Shale oil is a valuable chemical and energy feedstock. Its fractions are used as components of boiler and marine fuel, as raw material for the production of road and roofing bitumen, and for the recovery of individual chemical products; after hydro-upgrading the light fractions can go into motor fuels. The solid heat carrier technology (Galoter, SHC) makes it possible to process oil shale comprehensively, yielding oil, gas and heat.

Tight oil fits into ordinary oil refining on a par with conventional crude: it yields gasoline, diesel fuel, jet kerosene and petrochemical feedstock. Thus shale oil is closer to specialised chemical-technology products, whereas tight oil is a mass-market fuel feedstock on the global market.