Mio Pliocene sedimentation and its impact on the petroleum history of the Táchira Depression, Venezuela

Abstract

Even though the first oil field of Venezuela (La petrólia, 1883) was discovered in the Táchira Depression, located at the junction of the Venezuelan Andes with the Cordillera Oriental of Colombia, no liquid hydrocarbon discoveries have been made in the area ever since. However, there are extensive asphalt accumulations impregnating Mio-Pliocene conglomerates, which have been exploited intermittently mostly for road construction. These asphalt accumulations occur in the central eastern portion of the Táchira Depression in the basal part of fluvial conglomerates of the La Copé Fm. that were deposited uncomfortably on top of Cenozoic and Cretaceous rocks. These sediments were sourced and deposited unconformably on the nose and flanks of a large-scale anticline oriented N40° and plunging to the SW (Uribante High) product of the first phase of the Andean orogeny. The conglomerates located on the western part of this high were charged with oil derived from the Cretaceous La Luna Formation that was generating at the time of deposition of the unit. This oil migrated into the conglomerates and moved updip until they became trapped in a major stratigraphic accumulation. The seals of the accumulation were formed by lateral facies changes, overbank shales, or eventually the muddier Upper Member of the La Copé Fm. It is postulated that this stratigraphic trap was later deformed, uplifted and partially eroded, with the asphalt impregnations being the remnants of this paleo-accumulation and not present-day oil seeps as previously postulated.A rough estimate of the total hydrocarbons in place in the outcrops of the La Copé Fm is in the order of 100 MMBO, however the original accumulation was orders of magnitude larger. Since the bridged regional high was the focus of charge in the basin during the peak of oil generation in the Táchira Depression, it is believed that a large portion of the hydrocarbons generated were lost, which may explain the lack of significant discoveries in the area.