A Mesozoic mature oil field in Mexico's South Region shows a highly complex structural setting, salt tectonic has played a fundamental role in deposition and deformation, complex shaped salt bodies are present inserted within the tertiary and Mesozoic column. Several standard velocity modeling techniques have been applied in the field to depth convert the Cretaceous and Jurassic key stratigraphyc units. The most refined model showed in general 2D depth maps with an approximated prediction error of 200m. To avoid shadow effects during the depth conversion process the interpreter has to extend the respective top and base of anomalous velocity bodies for the whole area of interest, it implies the building of fake horizons and a very time consuming grid merging process. The oversimplification of not considering those velocity anomalies embedded within the key zones implies the failure in the depth prognosis process. The application of an innovative methodology helped to solve this problem. A 3D velocity grid is modeled considering the main structural and stratigraphyc features. Interval velocities from Seismic and from Check Shots are combined and populated. The anomalous velocity bodies are interpreted in the vicinity via manual tracking or seismic amplitude filtering. After being isolated, the bodies are related to different discrete properties as a facie, later the process of appending the connected bodies starts via conditional operations. The result is a new Interval Velocity 3D grid that has considered the complex spatial geometry of high velocity shallow sands, as well tertiary and Mesozoic interspersed salt bodies. This Interval Velocity grid is vertically integrated and mathematically operated to be transformed into the Average Velocity 3D grid we used in the Depth Conversion process. Finally the error in the prognosis process is considerably reduced to a maximum of 100 m for wells with average total depth of 5000m. Introduction Regularly, geoscientists who perform velocity modeling for domain conversion have to deal with areas affected by the presence of anomalous velocity bodies, such as shallow high acoustic impedance sandstones, shallow carbonates, tertiary salt tongues, salt domes, shale domes, over pressured shale…etc. Few times they are lucky to have these features extended in a regional scale, however in most of the cases they cover just one segment of the area to be depth converted. The inclusion of these bodies may be critical to obtain a successful depth conversion process. When one of these anomalous velocity intervals is considered to be part of our velocity model, both top and base of the vertical zone has to be seismically interpreted, not only in the area in which it is spatially located but throughout the whole area to be depth converted. This is performed in order to avoid any shadow effect over deeper horizons. Consequently, once the top and base horizons are completed, the interpreter initiates the painful and laborious task of tracking a faked zero thickness horizon beyond the limits of the body…again, for the whole area of interest. This can be a very time consuming process, in particular when we are dealing with exploratory regions in which the regional extension and the structural setting complexity can be quite considerable. This work presents the application of an innovative methodology which makes use of a 3D geo cellular model to overcome the problem of local velocity anomalies. In this way restricted velocity bodies interpreted following only its top and bottom can be included within a 3D interval velocity grid by applying a series of conditional statements. The final outcome is a 3D solid interval velocity model that considers all the vertical and lateral velocity heterogeneity of the geological column. Of course the complexity of the model will depend on the 3D structural framework a priory built by the interpreters and it will also depend on the time framework available for the project. Fig. 1 and Fig. 2 show a 120km2 3D Seismic Survey of regular quality which corresponds to a mature oil development field and its corresponding location.
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