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An introduction to energy resources
An introduction to energy resources

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4.6 The fossil fuel 'bank'

During the period of accumulation of most coal and petroleum, the past few hundred million years, the equivalent of around 1023 J of chemical energy have been 'banked' by Earth processes. As you have seen in Sections 4.3 and 4.4 carbon is added to these reservoirs continually, at a rate today that is equivalent to about 5 × 1017 J yr−1. That rate of growth represents roughly a mere thousandth of present world energy demand.

Question 13

If the consumption of energy from all primary sources in 2002 were 451 EJ, how long would the fossil fuel energy 'bank' last at that rate of consumption?


The fossil fuel energy 'bank' would last (1023 J)/ (4.51 × 1020 J yr−1), or about 220 years, at this rate of consumption.

In fact, the 'bank' contains only about 0.04% of the amount of organic carbon preserved in sedimentary rocks. However, that total store of buried organic carbon is finely dispersed, at an average concentration of about 0.4% in sedimentary rocks. At that level, it can never be exploited, either economically or with an efficiency that yields more energy than it consumes.

Although buried organic carbon is widely distributed, concentrations sufficient to constitute resources are rare and very restricted in space and geological time relative to small amounts of organic carbon that are present in many sediments.