Concludes that the source of oil in the lower part of a thick section of Tertiary continental clastic deposits overlying crystalline basement rocks in the Velasquez oil-field area, central Magdalena valley, Colombia, may have been Cretaceous marine shales of the Villeta formation which unconformably underlie the Tertiary beds east of the Belta thrust fault. Migration of the oil must have taken place in late Tertiary time but prior to formation of the Belta fault, which has locally deformed beds regarded as Pliocene-Pleistocene in age.

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