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Twenty-one layer maps are presented showing source rock distribution worldwide from the Precambrian to the Pleistocene. Data at hand are inadequate to display source rock distribution in meaningful quantitative terms (e.g., initial volume and type of organic matter for geological time intervals). The common procedure, therefore, is to relate source rock importance in time to measurable yardsticks such as ultimately recoverable hydrocarbon reserves alleged to have been generated from specific source rock intervals. Although source rock type, thickness, richness, and wide regional distribution are vital parameters to account for the hydrocarbon richness of a specific basin, other factors such as size of drainage area, level of organic metamorphism, migrational aspects, trap storage capacity, timing, and retention are of equal importance. Present concepts of source rock distribution and abundance in time are subject to revision when the sources of unconventional hydrocarbon resources such as heavy oil, asphalt, tar, and gas hydrates are considered. In addition, the source rock potential of undrilled areas and traps must also be incorporated in any worldwide material balance estimate. The hypothesis is advanced that the organic matter of Oligocene to Pleistocene sources has generated the largest portion of the world's hydrocarbon resources in terms of energy equivalents. The bulk of these resources is shallow bacterial gas trapped in the form of gas hydrates in permafrost areas and in shallow layers in the deeper parts of the oceans.
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