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.
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.