fax 01-972-952-9435. AbstractEvaluating the complex clastic reservoirs in El Tordillo field of the San Jorge Basin in Argentina using conventional logs is greatly affected by variations in formation water salinity, texture, and lithology (primarily the amount of volcanic tuff material) combined with extreme changes in rock permeability and wide variations in oil viscosity. All of these factors affect most of the conventional logs responses in such a way that traditional log analysis methods may fail to provide proper results and consequently may not achieve appropriate forecasts. Such failures have been proven by the high degree of mismatch between conventional log analysis and test/production results.To address uncertainties in conventional log evaluations, operators may resort to excessive well testing for reservoir characterization and production verification. However, well testing is known to be costly (considering the rig time and the frac jobs used). On the other hand, if the operator does not proceed with well testing, productive zones can be bypassed. That is why an effective log evaluation would be the optimum, cost effective method when it demonstrates agreement with production results.The latest acquisition and interpretation techniques of the magnetic resonance imaging logs (MRIL) have demonstrated promising results in the complex shaly-sand reservoirs of El Tordillo field. The magnetic resonance imaging (MRI) technology run on wireline provides an in-situ evaluation of the reservoir properties vital for producibility predictions. The ability of MRI to estimate the type of fluid in terms of viscosity influences the selection and elimination of zones to be tested based on their mobilities. MRI also helps in properly preparing for the testing procedure by pre-identifying the type of hydrocarbon in a zone and by identifying its reservoir quality in terms of permeability and porosity. Thus, the MRI serves well when determining the need for well testing and in enhancing the effectiveness of any particular well test.This article discusses the effectiveness of applying the MRI logging technology to the characterization of the shalysands of El Tordillo field in both the Comodoro Rivadavia and Mina El Carmen formations. It also argues for the value of MRI technology in minimizing the cost (and associated risk) of well testing by identifying best candidate zones for testing and by providing necessary "prior-to-testing" information on the fluid type (i.e., water, light oil, heavy oil or gas) present in a zone and rock properties, such as porosity and permeability.