The theme of the 1986 SPWLA Symposium is "Integrating the Earth Sciences." This theme aptly depicts the role of logging in unifying geological, geophysical and reservoir data. Pore volume formation evaluation is itself an integration of data from many sources including logs, cores, samples, production history, pressure analysis and the analysis of produced fluids. The hostile borehole environment where logging measurements are made must also be considered in any evaluation, thus understanding of the drilling process is also an important part of the evaluation. The six members of the panel on difficult log analysis "Challenges in Log Analysis," were asked to address what they consider unresolved problems in formation evaluation. Their examples are specific; however, they cover an important group of generic problems. We will have the opportunity for the audience to suggest additional problems during the question and answer session. It would have taken a large panel to cover all problems. One addition might have been the invasion of fresh mud into formations having high resistivities, beyond the ability of induction logs to measure Rt, where the high Rxo/Rt ratio makes laterologs also inappropriate. Adding salt to the mud then running laterologs would solve this problem if it were anticipated; unfortunately, we cannot always control drilling practices which is one of our problems. Another problem, covered by panel members, is unknown variations in Rw. This problem seems worthy of an additional example. Figure 1 is a three-well cross-section from a small Dakota Sandstone Field in Northwest Colorado. The Dakota, in this field, is an excellent reservoir which would probably not be difficult to interpret if Rw were known. Drillstem tests in the field recovered waters of about 1.5 ohmm, 5 ohmm and 8 ohmm. These variable water resistivities could not be correlated from well to well. This course-grained sandstone contains kaolinite which is associated with anomalously high SP deflections in this area. The clay apparently increases the SP which is not as we would normally expect (clay would normally be expected to reduce the SP). The SP thus cannot be used to differentiate the variations in water resistivity. There is little resistivity difference between water-productive and oil-productive intervals. Porosity was not a problem as there was an excellent correlation between travel time and core porosity. The problems covered by the panel, while not all encompassing, emphasize the problems of integrating all data and understanding the affect the borehole environment has on logging measurements. This seems worthy of note. I believe that it can be concluded from these papers that those who undertake formation evaluation without successfully integrating data from all available sources are likely to be saddled with inferior answers. Joe Spalding's example should remind us that we cannot always control the logging environment and that this should be considered before any recommendation is made on the basis of logging data. Roberto Peveraro reinforces this conclusion. He shows the extreme variation in logging measurements which can result from the borehole environment, but also shows that high quality data is not necessarily easy to interpret. Roberto also shows how lack of knowledge about the basic response of logging tools, in this example the neutron log, can cause serious interpretation problems.

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