Recent uplift and the origin of hydrodynamic traps in the Llanos Basin of Colombia

Abstract

More than 1.4 Bbls of heavy oil (11–15° API) have been discovered to date in hydrodynamic traps in the southeastern Llanos Basin of Colombia, of which the largest is the Rubiales Field. All these are stratigraphic traps defined by the intersection of the top seal (usually the Miocene Carbonera C6 Unit) with an inclined oil-water contact (OWC) (dipping 0.3° towards the NW). In many cases the accumulations occur where there are minor inflections in the top seal, controlled by the underlying paleo-relief of the Paleozoic.The area of stratigraphic traps coincides with the forebulge of the Llanos Basin. A very striking feature is that the area of the stratigraphic traps, which is found south east of the Meta River is being actively uplifted. This is clearly shown in the different levels of terraces that are present south of the Meta River. The highest present-day terrace has an elevation of near 287 m, (vs. approx. 140 m of the Meta River). All of this is indication that this zone is actively being uplifted (at least 200 m) and eroded in contrast to the rest of the basin that is a zone of deposition.It has been proposed that the tilted oil-water contact is related to hydrodynamic flow from the margin of the basin, or from areas with a higher hydraulic head. However, the orientation of the tilted OWC trend (NE-SW) is almost perpendicular to the hydraulic head contours for the basin but coincides with the present-day trend of active inversion related to right lateral movement along the Algeciras/Rio Meta lineament and the location of the Llanos Basin forebulge. It is proposed that the tilting of the OWC is related to recent uplift in areas where the oil is biodegraded due bacterial activity and did not yet have time to equilibrate.