Some authors consider that the volume yet-to-find in the Eastern Llanos Basin amounts circa 12 4000 MBO. Two giant fields (Caño Limón and Rubiales), two major (Apiay and Castilla), and over eighty minor fields have been discovered in this region.
The main source rock in this area corresponds to marinecontinental shales from Gachetá Formation, buried under the eastern flank of the Eastern Cordillera (Figure 2). The Gachetá shales contain type II and III kerogen; TOC varies between 1-3% and effective thicknesses between 150 to 300 feet.
Two pulses of migration have been documented: the first one during the upper Eocene-Oligocene, the second one extending from the Miocene to present. Reservoir Rock Sandstones of the Carbonera Formation (C-3, C-5 and C-7 units) and Mirador (Paleogene) are excellent hydrocarbons reservoirs. Besides, some sandy intervals of the Cretaceous sequence are also excellent reservoirs. Their porosity ranges from 10 to 30%.
The regional seal of the basin is the León Formation. The units C-2, C-4, C-6 and C-8 of the Carbonera Formation are recognized as local seals. Cretaceous shales from Gachetá and Guadalupe formations can act as intraformational seals.
So far, exploration has been concentrated on antithetic normal faults. However, the anticlines related to reverse faults and low relief structures, as well as stratigraphic traps that locally represent interesting exploration targets (Figure 3).
