The intramontane Upper Magdalena Basin of Colombia has been actively explored for hydrocarbons and is reaching a mature level of activity. However it has been long suspected that important oil accumulations still remain trapped in structures presently located beneath overthrusted basement rocks. Several attempts in the past were unsuccessful in testing this play concept. The basin has been subjected to several phases of active compression since at least Oligocene times, and it is encroached on both sides by inward verging thrust faults that carry basement rocks of Jurassic and Paleozoic age on top of a Mesozoic-Cenozoic sequence. During the last year, the HOCOL-TEPMA consortium drilled the deepest well to date in the basin to test the sub-basement concept. The Iskana 1A well drilled over 9000 feet of Paleozoic basement rocks and reached a final depth of 17588 feet. The objective Lower Cretaceous sandstone of the Caballos Formation was found oil-bearing close to the prognosed depth. The well encountered a lower section with heavy oil (19º API) and an upper section that produced 29º API oil at noncommercial rates. Iskana proves for the first time the validity of the subthrust play in the area. However it is still uncertain if sealing against basement rocks is an effective trapping mechanism since the structure possibly has some fourway dip closure. Similar structures remain untested along the trend, but important technical challenges need to be solved in order to make this play economically viable.


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