Oil Shales are usually fine-grained sedimentary rocks containing relatively large amounts of organic matter from which significant quantities of shale oil and combustible gas can be extracted by destructive distillation. The product thus generated is known as synthetic crude or more simply, syncrude. Included in most definitions of oil shale, either stated or implied, is the potential for the profitable extraction of shale oil and combustible gas or for burning as a fuel. An oil shale, which has a very high proportion of organic matter in relation to mineral matter, is categorized as a coal.
Oil shales range in age from Cambrian to Tertiary and occur in many parts of the world. Total world resources of oil shale are conservatively estimated at 2.6 trillion barrels. However, petroleum-based crude oil is cheaper to produce than shale oil because of the additional costs of mining and extracting the energy from oil shale. Because of these higher costs, only a few deposits of oil shale are currently being exploited in China, Brazil, and Estonia. However, with the continuing decline of petroleum supplies, accompanied by increasing costs of petroleum, oil shale presents opportunities for supplying some of the fossil energy needs of the world in the years ahead.
The Indian Scenario :
North-East India is endowed with rich deposits of coal. The coal is found in the Barail Formation of Tertiary age. Carbonaceous shale occurs interbedded with the coal. The presence of coal and shale has been recorded in wells drilled for hydrocarbons by ONGC and OIL. These formations outcrop on the surface towards the south of the oil fields in a region called the Belt of Schuppen. Studies have indicated that these coals and carbonaceous shale constitute the principal source rocks that have generated the hydrocarbons produced from the region.
The favourable characteristics of Assam coal for conversion to liquid fuels have been known for a long time. CFRI (Dhanbad) had carried out a feasibility study on this subject and submitted a report as far back as in 1968. Commonly, the assessment of the yield of hydrocarbons from coal or oil shale is based on pyrolysis or heating under controlled conditions.
In the late 1980s, Oil India Limited and Robertson Research Inc. (UK) had analyzed a large number of rock samples obtained from oil wells, outcrops and coal mines in connection with hydrocarbon exploration in the region. The Rock-Eval yields for the coal and carbonaceous shale indicate prolific hydrocarbon potential for Barail Coals, of the order of 280 kg of hydrocarbons per ton of rock. The Barail Series oil shale gave a maximum yield of about 80 kg hydrocarbons per ton of rock. This compares favourably with some of the yield values obtained from other oil shale deposits in the world.
In order to assess the viability of syncrude generation from the Assam coal, OIL established a pilot plant for the extraction of oil from the coals of Assam with technology from the USA.
The studies on Oil Shale prospects in India has been planned in three phases:
Phase-I: This comprised of evaluation of the resource potential of the shale in selected areas of North east part of India. This study was completed in 2009. An estimate of 396 MMT of oil upto the depth of 500 meters was estimated in three blocks.
Phase-II: The following activities are envisaged: a. Techno economic feasibility b. An assessment of environmental impact arising out of oil shale exploitation.
Phase-III: Exploitation of the oil shale resource based on outcome of Phase-II study.