Oil shale and Shale gas

Oil shales can burn also called as burning rock

Oil shale and shale gas

Oil shale

Oil shales are source rocks, usually mudstones and shales, with a high organic content (TOC), which have not been buried deeply enough to become sufficiently mature for most of the hydrocarbons to be generated. Although they may contain some hydrocarbons they must be heated in an oven (pyrolysis) to 400–500◦C so that most of the petroleum can be generated from the remaining kerogen. Oil shales must therefore be mined near the surface in quarries and then heated in large ovens so that the petroleum can be distilled off. Source rocks may be uplifted close to the surface after deeper burial; it is the temperature history that determines how much of the kerogen is altered to oil and gas. Some source rocks may have been buried to more than 5–6 km (160–170◦C) and they have then generated and expelled most of the hydrocarbons, but some oil and particularly gas may remain. The Upper Cambrian alum shale which is found in the Oslo region is a good example of a rich source rock which has been buried to at least 200◦C (5– 7 km) and lost most of its hydrocarbons during the Caledonian folding in the late Silurian and early Devonian. In Sweden the Upper Cambrian alum shales have not been buried so deeply and therefore contain more oil which can be distilled off by pyrolysis at 400–500◦C. In the Baltic region the lowermost Ordovician shales are even less mature so that more of the kerogen remains and in Estonia this shale is mined for oil on a large scale. Here the reserves are very large (0.6×109 sm3 oil equivalents [o.e.]). Oil shales are used in electric power generation and provide 60% of Estonia’s stationary energy. It is also used for oil production and refining. The waste is 70– 80 Mt. of semi-coke and the mounds exceed 100 m in height. The waste is very fine grained and is alkaline with significant concentrations of sulphides and heavy metals. On a global basis oil shales represent a reserve almost as large as conventional oil. Conservative estimates: 350 Gt shale oil equivalent to 2.6×109 barrels of oil (410×109 sm3 o.e.). This is about equivalent to the estimated world reserves of conventional oil and gas (480 sm3 o.e.). More than 80% of the well known reserves are located in the US but there are probably other regions with oil shales which have not been recorded and evaluated. The Green River Shale in Colorado, Utah and Wyoming is a gigantic source of hydrocarbons. This organic-rich mudstone was deposited in very large lakes during the Eocene and the organic matter was mostly freshwater algae. Oil shales must however be mined and heated in ovens (pyrolysis) to generate the petroleum and the energy for the heating is taken from the burning of the oil shales. The release of CO2 is therefore high also during production. Oil shales may only contain 5–10% organic matter and the volume of waste will then be 10–20 times the oil produced. The waste consists of coke and also smectite formed in the heating process and is very difficult to store. It is also rich in heavy metals, including vanadium and uranium which are typical of black shales. Production of oil from oil shales requires very large amounts of water, so in dry areas the water supply may be a limiting factor. There are therefore considerable environmental problems linked to the exploitation of oil shales as a major source of oil. Some source rocks may have enough permeability to serve as reservoir rocks too. The Miocene Monterey Formation in California is an organic-rich diatomaceous source rock, which is also a reservoir rock. Oil can in this case be produced because of tectonic fracturing which has enhanced the permeability. Source rocks may also be interbedded with thin sandstones or limestones and then only a very short migration is required.

Shale gas

Organic-rich shales which have been buried to depths where most of the oil and gas has been generated and expelled may nevertheless contain considerable amounts of gas. The gas remaining in these shales is present in very small pores and may also be partly adsorbed on remaining organic matter or its residue (coke) and on clay minerals. The shales have been uplifted and may therefore have small extensional fractures, but they must be hydrofractured by water injection to increase the permeability. Barnett Shale is tight shale of Mississippian age in Texas, containing at least 2.5 trillion cubic feet of gas. It is referred to as a tight gas reservoir. Much of the gas is in urban areas such as the Dallas-Forth Worth area. The permeability of the shale matrix is generally very low but there may be thin silty layers and also fractures that increase the effective permeability. Hydraulic fracturing can be carried out to further increase the permeability, and horizontal drilling also helps to produce more gas. The Woodford Shale (Devonian) in Oklahoma can be almost 100 m thick. Devonian tight gas shales include the Middle Devonian Marcellus Shales in the Appalachians.These are now mostly at 1–2 km depth but have previously been buried much deeper (5–6 km or more). The Upper Devonian Bakken Shale is another major producer, particularly in North Dakota, and is part of the Williston Basin. It also extends into Canada. Shale gas is estimated to produce 50% of the gas in North America by 2020. Higher gas prices will also result in increased interest in shale gas in other parts of the world. Shale gas represents very large reserves of hydrocarbons which can be used directly as gas, but can also be converted to diesel fuels for cars and trucks. This gas can also be mixed with heavy oil and tar sand to make regular petrol. Production of gas from shales requires much water for fracturing and produced water may also create environmental problems.

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