Abstract: Well-Log Analysis in Cretaceous Gas Sands in Northern Montana

E. B. Campen

The fact that the responses of neutron and density logs in gas-bearing formations differ from those in oil- and water-bearing formations has prompted the acceptance of these logs in many areas as standard for the analysis of clean or shaly gas sands. Using these two logs, with gamma-ray and resistivity logs, the various permeable beds in a well can be evaluated accurately.

One approach to gas-sand interpretation, which can be done at the well site, involves a plot of the log data on ordinary graph paper. For each level the difference, apparent porosity from the neutron log minus apparent porosity from the density log, is plotted versus the reading from the gamma-ray log. If there are known water- and gas-bearing levels within the beds of interest or within the same geologic interval, then a liquid line can be determined on the plot. Gas-bearing levels are identified by the location of the corresponding plotted points relative to the liquid line.

A more complete interpretation of the reservoir is provided by the SARABAND (trademark of Schlumberger) computer-oriented technique. Results provided by this analysis include formation porosity, saturation, permeability index, and shale-, clay-, and sand-volume fractions. Experience indicates that good interpretation results are achieved, and that the subsequent completions are therefore improved.

Examples illustrate the applications of these two techniques in clean and shaly formations of northern Montana.

AAPG Search and Discovery Article #90971©1976 AAPG-SEPM Rocky Mountain Sections 25th Annual Meeting, Billings, Montana