ABUNDANCE OF SOURCE ROCKS FOR OIL AND GAS WORLDWIDE

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Abstract
Source rock abundance through geological time cannot be measured in absolute quantitative terms. The source rocks of giant fields, which are known in most cases, can serve as an appropriate yardstick for a percentage allocation of source rocks to geological time intervals, based on ultimately recoverable reserves. This yardstick is used to relate source rock percentages per time interval to in situ reserves and resources of oil and gas worldwide, both conventional and unconventional. Necessary corrections are made. On this basis it appears that the Cretaceous is the most important oil‐generating period (49%), while the Tertiary is the most important gas‐generating period (76%). For total hydrocarbons in terms of oil equivalents, the most important hydrocarbon‐generating intervals are the Tertiary (69%), followed by the Cretaceous (25%), the Jurassic (4%) and the Triassic to Paleozoic (2%). The percentages refer to source‐rock age and not to time of generation. All estimates have to be regarded with much caution. They represent an approach to the problem rather than a solution. Major uncertainties still exist. It must be stressed that the problem of source rock abundance should be seen in a wider hydrocarbon habitat context, especially in the light of retention. The important role of bacterial gas and gas hydrates is emphasized.