Abstract The well logging phase of a coordinated formation evaluation program can improve the quality of basic reservoir data while reducing the need for extensive coring and drill-stem testing. The quantitative data to be determined from log analyses are formation porosity, fluid saturation and formation water resistivity. In order for these data to be obtained successfully, the proper logging devices must be selected, accurate data on the borehole conditions must be obtained and quality control measures regarding logging techniques must be exercised. The use of core analyses and drill-stem test data on a key-well basis to confirm log calculations is a necessary requirement to assure maximum dependability of quantitative log interpretation for reservoir analysis purposes. Introduction Well logging is one of the most useful of the various types of open-hole formation evaluation methods. When properly applied, present techniques of well logging and interpretation can provide substantial reductions in the costs of evaluation, development and exploitation of petroleum reservoirs. The initial benefit from a good logging program is that better well completions can be made. A second and more important benefit is that a reliable calculation of oil in place can be made for reservoir analysis purposes from the location of water contacts, reservoir boundaries and the calculation of formation porosity and water saturation data. Necessary formation evaluation data can be obtained at a much lower cost by using log information to extend key-well core analyses and drill-stem test data for coverage of the entire reservoir instead of using an extensive coring and testing program. A logging program cannot be considered successful and profitable unless the basic reservoir data obtained from the logs are valid and complete. This objective can be reached, however, if the logging program is properly planned and coordinated with the other formation evaluation methods used. It is worthy of note that even the qualitative value of logs is doubtful unless proper planning is used in the selection of the types of logs to be run and unless suitable quality control measures are followed in logging techniques.