Geologic Interpretation of Well Logs

A manual about the principles of geophysical well log correlation, together with examples of lithologic interpretation and expressions of stratigraphic and structural features which can sometimes be recognized on well logs.

The purpose of this manual is to compile under one cover the principles of well log correlation, together with examples of lithologic interpretation and expressions of stratigraphic and structural features which can sometimes be recognized on well logs.

For the past 35 years, geologists and engineers have correlated electric and radioactivity logs. During the first 20 years, this correlation consisted of a comparison of log curves checked by paleontologic dating and lithologic infonnation from cores and cuttings. Very little consideration was given to the electrochemical and petrophysical fundamentals and relationships that determine the amplitudes and shapes of the various curves. Quantitative analysis was relegated to the log analyst, usually an engineer, whose primary interests were porosity and hydrocarbon saturation.

Advancing technology during the last 15 years has provided a multiplicity of electric, acoustic, and radioactivity logs which have greatly enhanced both qualitative and quantitative interpretation, but for all except the expert log analyst, correlation and geologic interpretation seem to be more difficult than with the simpler appearing logs of the past. Modern drilling fluids and drilling practices have also contributed to the confusion. Geologists and petroleum engineers must therefore acquire a basic understanding of petrophysics and quantitative log interpretation. These subjects are now well covered by Company manuals by Mr. J. E. Walstrom, by Chevron Research reports, and by several published texts. Geologic interpretation and correlation of modern logs are not adequately covered; hence the need for a supplemental manual designed primari1y for the exploration geologist.