A Hidden Reservoir within the Gacheta Formation, Zopilote Field, Llanos Orientales Basin, Colombia (South America)
The Zopilote Field contains two reservoirs, one of them, within de Cretaceous Gacheta Formation, is not easy to identify using standard open hole logs, gamma ray and resistivity. By-passed oil reservoirs in this formation are therefore expected in other parts of the basin. Operators are called to review relevant data in a similar way than is presented here. A full integration of open hole logs with well-site geologist data is extremely important for the identification and delineation of this reservoir. A lithological model based on volume of clay calculations derived from the combination of the density and neutron logs seems to be the most successful tool. Gamma ray logs are strongly affected by mineralogical composition and do not permit the identification of the reservoir. Image logs are powerful tools to identify this reservoir. Porosity derived from density log has been found to fit very well porosity measurements from core data. None of the known saturation equations has been successful to explain production behavior despite resistivity of formation water is known. Resistivities are affected by thin laminations and by mineralogical composition. As a consequence, water saturations are not useful for oil water contact identification, or for net pay counting. An old method of log interpretation has been found useful: Ro (the resistivity of a wet formation) comparison with Rt (the true resistivity of the formation). In this method the only uncertainty is the resistivity of the formation water as porosity is well defined by density log. Rw in the Ro calculation is modified until it fits Rt in most of the sandstone intervals expected to be water bearing. Where Rt is higher than Ro, oil saturation is revealed. But derived Rw does not fit measured Rw, because as pointed out before, resistivities are anomalously reduced. Oil saturations derived from this method are not reliable as they do not fit production behavior, but they are useful for new oil zone detection and oil water contact identification. Capillary and relative permeabilities should be used to find the correct water saturation values for volumetric and other calculations. If an approach like this one is not applied in the evaluation of these reservoirs, conventional petrophysists, those who just apply volume of clay, porosity and water saturation cut-offs to identify hydrocarbon reservoirs, will fail to identify them, adversely affecting reserves auditing and business development.
AAPG Search and Discovery Article #90166©2013 AAPG International Conference & Exhibition, Cartagena, Colombia, 8-11 September 2013