AAPG Bulletin, Vol.103, No.5, 1179-1224, 2019
Tectonic evolution of petroleum systems within the onshore Llanos Basin: Insights on the presence of Orinoco heavy oil analogs in Colombia and a comparison with other heavy oil provinces worldwide
Economic accumulations of hydrocarbons in the onshore Llanos Basin of Colombia are characterized by a central zone (Casanare province) with greater than 20 degrees API gravity oils and a southern province with biodegraded, less than 15 degrees API gravity oils. To date, no conceptual model successfully explains this spatial zonation. In this paper, we employ multiple one-dimensional time-temperature models to map the kitchens for three different source rocks and compare maturity levels through the Cenozoic with the presence or absence of reservoir, seal, overburden, and traps in paleogeographic maps of the Llanos Basin. We find that the Llanos Basin Cenozoic petroleum migration and charge may have been governed by a sedimentary-structural evolution tied to the adjacent orogenic belt in which (1) Paleogene stratigraphic traps developed in the south, as favored by a more segmented basement and potentially transpressional stresses; (2) a subsequent Neogene phase with more pervasive east-dipping low-displacement normal fault traps was discovered; and (3) a final Pliocene-present day phase of contractional traps was found in the easternmost foothill areas. When compared with the evolution of several potential kitchens, we suggest that Upper Cretaceous rocks from the Eastern Cordillera are the primary hydrocarbon source in the zone of heavy biodegraded oils to the south, whereas Lower Cretaceous and selected terrigenous Upper Cretaceous source rocks are largely responsible for the younger Neogene contractional traps of the foothills. This evolutionary pattern for the Llanos Basin favors the presence of smaller but numerous hydrocarbon accumulations rather than the broader zones of heavy oils, as found in the Orinoco belt of Venezuela.