The Ayoluengo oil and gas field (North Burgos, Spain) discovered in 7964 was originally developed mainly using pre 3D technologies: a certain well gridding and well to well correlations. 24 years later■ with 51 wells drilledan accurate estimation of remnant reserves was required. The structural complexities of the reservoir formed by numerous lenticular sands (43), highly fractured and partitioned by multitude of faults -multiplying the number of individual reservoirs to more than 300- the diversity of gas-oil-water contacts and the wide range of oil densities (20-39° API), forced to the recording of a 3D seismic survey before the target could be accomplished. The interpretation of the new seismic combined and integrated with the well data created the basis for the new reserves estimation