Geochemistry of oil seeps and rock samples of the Early Tertiary section from the Northandean Flank of the Venezuelan Andes

Abstract

An organic geochemical study of oil seeps and Early to Middle Tertiary source rocks of the Northandean Flank (NAF) of the Venezuelan Andes range was performed to establish the possible contribution of terrestrial source rocks to the oil accumulations in the unexplored southern part of the Maracaibo Basin. Source rocks were identified within the Paleocene Barco and Los Cuervos Formations, and the Late Eocene-Oligocene Carbonera Formation. Biomarker distributions indicate a terrigenous organic source for these units. Geochemical modeling performed by Parnaud et al. (1995), indicated Plio-Pleistocene oil generation for the Orocué Group and Middle Miocene to Plio-Pleistocene oil generation for the Carbonera Formation. Geochemical modeling also indicates that the quantity of crude oil expelled by the Carbonera Formation is four times that expelled by the Orocué Group (Barco and Los Cuervos Formations). A northeast to southwest variation from marine to terrestrial oil seeps was identified based on sterane and terpane distributions. The marine oil seeps appear to be derived from the La Luna source rocks. The characteristics of the mixed oils suggest contributions from different sources (La Luna and a terrestrial source rock) most likely the Orocué Group or Carbonera Formation coals. Source facies similar to those in the Los Cuervos and Carbonera Formations coals are most likely the main source rocks of terrestrial oil seeps in the NAF.