Well-log analysis has made dramatic, if not always steady, progress over the past 50 years. Developments in logging sensors, the physics of porous media, and analysis methods have each contributed to the ability to determine important properties of hydrocarbon reservoirs from well logs. In almost chronological order, major developments are traced, from the identification of formations with electrical logs to the quantitative analysis of porosity and saturation, to today's complex models of rock composition and structure. Historical articles on well logging normally concentrate on the development of logging sensors. The approach presented here is that petrophysical models deserve study in their own right, both to put current understanding in a historical perspective and to demonstrate some of the different creative approaches that are still being used in the solution of log analysis problems. Throughout the historical story, emphasis is placed on the interaction between technology and physics that makes log interpretation as much an art as a science.

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