When you burn gas it delivers energy in the form of heat (energy...

When you burn gas it delivers energy in the form of heat (energy content). Units conventionally used to measure heat are 'Joules', hence when they quote "the size of the resource is estimated to be 3,000 Peta Joules", they mean the gas resource (when refined and delivered for burning as a final retail product) is capable of generating One Peta = 1 million billion joules (energy content).

Of course, if used to make fertilizer it won't be converted to heat but energy content is used as a de facto for quantity of gas (more convenient than expressing it as the number of molecules).

Here is an infographic that explains it...


IMO
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