"In the Cretaceous section of Colombia free oil has been noted in various parts of the stratigraphic section, but the most striking occurrence is in the Oxytropidoceras zone of the middle Albian. This zone is abundantly fossiliferous, especially in limestone concretions interbedded in dark gray shales. No evidence of oil can be detected on the outside of the concretions, but, when they are broken with a hammer, free oil can be obtained from the hollow central part of the individual chambers of the ammonites."

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