Re-Os geochronological evidence for oil generation during the early Oligocene in the South Llanos basin of Colombia: Insights into the Understanding of giant heavy oil accumulations in northwestern South America

The Southern Province of the onshore Llanos Basin of Colombia is characterized by accumulations of heavy-biodegraded oils (<20°API gravity oils) sourced from Upper Cretaceous rocks localized in the adjacent Eastern Cordillera. There are some uncertainties regarding the precise age of oil generation in this sector of the Llanos Basin, however, previous time-temperature models have restricted the timing of oil generation to the Paleogene . Re-Os geochronology in crude oils has shown promising potential to directly determine the age of hydrocarbon generation , migration, or charge in basins with complex tectono-sedimentary settings. Here, we conducted Re-Os isotopic analyses in 29 oil samples from the Castilla, Peguita, and Rubiales oilfields to constrain the timing of hydrocarbons generation in the Southern Province of the Llanos Basin. The Re-Os isotopic data yield an isochron age of 28.9 ± 4.4 Ma, consistent with basin evolution models that support a maximum thermal maturity reached by source rocks of the Eastern Cordillera in the Early Oligocene . This age also seems to precede the onset of exhumation of the Eastern Cordillera at ca. 25 Ma, providing new chronological controls to constrain the timing of cessation of oil generation by Upper Cretaceous rocks. This study highlights the applicability of the Re-Os geochronometer to (1) constrain the temporal evolution of petroleum systems in the Llanos Basin, and (2) evaluate the influence of exhumation of the Eastern Cordillera in the development of petroleum kitchens in the adjacent foreland basin . The new data are instrumental in confirming the evolutionary models of the giant heavy oil accumulations of the South Llanos Basin. • Re-Os geochronology provides new insights on the timing of petroleum generation in northern South America. • The Re-Os new age indicates the peak of oil generation in the Early Oligocene in the South Llanos Basin. • Exhumation of the Eastern Cordillera played a pivotal role on the evolution of petroleum kitchens in the Llanos Basin. • The new age supports expanding the exploratory targets to Paleogene stratigraphic traps in the South Llanos Basin.