oil shale

oil shale ‘Oil shale’ is a collective term for a diverse and complex group of rocks that are immature oil and gas source rocks and potential fossil fuels. Oil shales contain insoluble solid organic matter called ‘kerogen’, derived primarily from algae. Few oil shales contain any oil. Many are not in fact shales, but are carbonates with varying proportions of clay minerals, quartz, and feldspars admixed with the kerogen.

Unlike oil, natural gas, and coal, most oil shales cannot be used directly as a fuel, but must be processed to obtain synthetic oil or gas. Their attraction is that retorting (heating) at c. 480 °C yields oil products that are similar to conventional liquid petroleums; their principal disadvantage is their lack of economic viability compared with other fossil fuels.

Use of oil shales

Oil-shale deposits occur in more than fifty countries, and some have been exploited widely. Only one country, Estonia, currently has an energy economy dominated by oil shale because of the lack of any other fossil fuels. The Russian Federation and China also produce small quantities of shale oil.

Many oil-shale plants opened between 1830 and 1860: for example, Young's plant in Scotland, which from 1851 until closure produced 120–30 gallons of oil per ton from an algal coal (torbanite) on retorting. But attempts to maintain economically viable oil-shale industries collapsed because the costs of mining and processing oil shales were too great to compete with those of conventionally produced petroleum. The pattern of production from the Scottish plants over nearly a century would be typical for many producers, although United Kingdom tax concessions probably extended the life of the Scottish plants before their closure. The oil crises, particularly the second, in the 1970s led to renewed interest in oil shales and were responsible for much of the modern-day understanding of these deposits.

Classification and composition of oil shales

For many years there was no definition of ‘oil shale’, other than that it was a rock buried at shallow depth which yielded oil in commercial quantities when pyrolysed. Before the advent of ‘flow’ petroleum, yield cut-off values varied and were set as low as 25 litres/metric tonne, which would correspond to a kerogen content of 5 per cent in the shale. Commercial practice felt safer with a cut-off value of approximately 40 litres/tonne because at this level the shale would yield more energy as shale oil than the energy required to process the rock by retorting. Nowadays, with the low cost of other fossil fuels, no cut-off levels would be ‘commercial’ unless there were special circumstances (as, for example, in Estonia, which has no other fossil fuels available).

Any modern classification of oil shales has to take account of both coals and petroleum-source rocks, since both these groups overlap with oil shales. The subdivision of oil shales, outside of a purely chemical classification, was greatly aided by the development of reflected fluorescence microscopy. The derivations of the end members are:bituminite: from degraded cyanobacterial mats, red and brown algae, giving amorphous material with some cryptic structures;lamalginite: probably from benthonic algae referable to the Cyanophyceae (blue-green algae) or to the Chlorophyceae (green algae), forming elongate anastomosing strands; andtelalginite: discrete, colonial, or unicellular forms derived from the Chlorophyceae.Chemically, telalginite has the highest hydrogen/carbon atomic (H/C) ratio and the highest yield of oil, lamalginite is lower on both counts and bituminite the lowest. These oil-shale types have different environments of deposition (freshwater through brackish, saline to marine), which control the thickness and distribution of the resulting deposits. Thus, telosites (which include the specifically named deposits of tasminite, kukersite, and torbanite) are thin and discontinuous, have high H/C atomic ratios and high yields of oil, but the deposits are rarely substantial enough to have commercial potential. Lamosites are more widespread in occurrence and one sub-group, the lacosites, typical of the well-known American Green River and Australian Rundle deposits, yield large quantities of oil, making them attractive for future commercial exploitation. ‘Pure’ bitosites are rare, but some of the most important marine oil shales and petroleum source rocks contain a mixture of bituminite and lamalginite. Bituminite alone has a low specific yield of oil and a high yield of residual carbon, but many bituminite-rich shales are thick and widespread. When intercalated with rocks containing other forms of alginite, they may therefore be commercially attractive.

Oil-shale reserves

World-wide resources of shale oil in place are estimated to be in excess of 10 × 10¹² barrels, but the level and quality of available information on reserves vary considerably for different countries. Reasonably comprehensive data are available for several countries (Table 1). The resource base of Australia is enormous, with many schemes projected for exploitation; Morocco has large resources, but has made relatively little progress so far in planning for exploitation. Jordan is another country with extensive deposits, these shales being virtually the only fossil-fuel resource of the country; fortunately they are of quite good quality.

Notable omissions from Table 1 are Brazil, China, Estonia, and the United States of America, all of which have extensive reserves of oil shales but for which less information is available. Of the countries omitted from Table 1, Brazil was reported in 1996 as possessing 0.4 billion tonnes measured and 8.1 billion tonnes estimated of shale oil. Little information is available for China, but output of shale oil is said to be about 60 000 tonnes/year. Total deposits in Estonia, the only indigenous energy source for that country, amount to 5.9 billion tonnes; oil output in 1996 was 343 000 tonnes, of which 184 000 tonnes were exported, the greater part to Denmark. There is again a vast resource in the United States; ‘in-place’ shale oil in the Green River Formation amounts to 1700 billion tonnes. Recoverable shale oil from the marine black shales of the eastern United States is said to exceed 400 billion barrels.

The future for oil shales

Petroleum, natural gas, and coal are finite fossil-fuel resources. As world population increases and conventional fossil fuels diminish in amount, oil shales may eventually have to provide a substantial part of world energy. At present the costs of mining and processing oil shale are too high to compete with conventional petroleum production, but oil shales may have a brighter future. First, there is an immense resource base; second, the price of crude oil will inevitably rise as supplies decline; third, the geology of many of the major oil-shale deposits is now well understood; and fourth, many oil shales contain potentially valuable by-products (minerals and metals) that can be exploited together with the extractable hydrocarbons.

Table 1 Reserves of oil shales in selected countries with substantial deposits (at end 1996)

Country	Proved	Proved	Average	Estimated
	amount	recoverable	yield	additional
in place	reserves	of oil	reserves
(million	(million	(kg oil/	(million
tonnes)	tonnes)	tonne)	tonnes)
(shale)	(oil)		(oil)
Data from World Energy Conference Survey of Energy Resources (1998).
Morocco	58 800	3083	60	300
Thailand	18 600	1700	43	–
Turkey	1 600	20	50	10
Ukraine	2 674	300	126	6 200
Israel	15 360	600	62	–
Jordan	40 000	4000	100	20 000
Australia	69 200	3651	53	32 374

If the future for oil shales is to improve, an essential requirement is that recovery of the oil should become more cost-effective and less environmentally damaging. The simplest method of oil-shale recovery is by opencast excavation or, where the overburden is too thick, by deep-mining methods. The environmental consequences of both methods are serious, as are those from retorting and refining. The problems are widely recognized, but not easily resolved. Attempts have been made to develop potentially less costly methods of recovery in situ by circulating hot hydrocarbon gases through oil-shale deposits, but because of technological problems the costs of retorting in situ are as great as for conventional surface retorting after excavation. Other technologies, for example catalytic processing and radio-frequency heating, may well develop if interest in oil shales revives because of economic and political circumstances. Three principal factors will determine whether oil shale is to have a significant impact on the energy market in the future: reduction in the costs of producing oil from oil shale; the price of competing fuels, including the costs of methane and synthetic fuels made from coal; and the costs of dealing with environmental issues (oil and water pollution) related to shale-oil production, which currently pose horrific problems for countries such as Estonia and the Russian Federation that are producing shale oil.

Duncan Murchison