Migration of Oil in Recent Sediments of Pedernales, Venezuela1
Evidence for generation and migration of oil in sediments less than 10,000 years old was found on the flanks of the Pedernales anticline in the delta of the Orinoco River in Venezuela. The gray clay beds contain an average of about 55 parts per million hydrocarbons; the interbedded argillaceous sheet sands that are open to the surface on the flanks of the anticline contain about 40 parts per million. One lenticular sand about 110 feet deep, however, contained free gas and was enriched in hydrocarbons, mostly aromatic, to 160 parts per million. According to carbon-14 age determinations, this sand was deposited about 5,000 years ago.
Measurements of excess hydrostatic pressure made in the holes and on undisturbed cores showed that there is a pressure gradient in the muds upward toward the laterally continuous sands and downward toward the Pleistocene unconformity. Both beds are apparently acting as conduits that permit the escape of fluids to their outcrops along the Pedernales structure. It is inferred from these data that in the lenticular sands, hydrocarbons are being filtered out of the moving stream of water by capillary action. This is not true of the continuous sands that are open to the surface.