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Abstract

Geologic Problem Solving with Microfossils: A Volume in Honor of Garry D. Jones

SEPM Special Publication No. 93, Copyright © 2009

SEPM (Society for Sedimentary Geology), ISBN 978-1-56576-137-7, p. 29–40.

The two major oil fields in Colombia discovered in the last fifty years are the Caño Limón and Cusiana fields. Caño Limón is located in the eastern region of the unfolded Llanos of Colombia, and Cusiana is located in the leading thrust sheet of the Llanos Foothills. Paleogene strata in both areas were part of a large foreland basin active since the latest Cretaceous. In both cases the main reservoir is a quartz arenite unit, informally called the Mirador formation, that has always been assumed to extend as a continuous Eocene sandstone layer from the Llanos Foothills into the Llanos Basin. However, recent palynological data suggested that this unit is diachronous across the Llanos and Llanos Foothills. Here, we dated 44 sections in the Llanos Basin and Llanos Foothills using a new zonation that is proposed for the region. Biostratigraphic results constrain the age of the Mirador Formation in the Llanos Foothills as early to middle Eocene with no evidence of a biostratigraphic gap with underlying early Eocene strata. In most of the Llanos Basin, including Caño Limón, the quartz arenite unit has an Oligocene age and rests unconformably upon Upper Cretaceous or Paleocene strata. Additionally, there are areas in the Llanos Basin where mudstone, not sandstone, is the dominant facies overlying the unconformity, suggesting that the basal sandstone in the Llanos Basin is not a laterally continuous body of rock. The absence of lower to middle Eocene quartz arenite beds in most of the Llanos Basin can be explained either by bypass or accumulation and subsequent erosion. These results imply a new paleogeography for the time of accumulation of Eocene and Oligocene reservoir units, a different model for basin evolution, and a different fluid-migration history to explain how the Caño Limón and Cusiana oil fields were filled.

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