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Montana Geological Society: Energy Resources of Montana
WELL LOG ANALYSIS IN THE CRETACEOUS GAS SANDS OF NORTHERN MONTANA
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, along with Gamma Ray and Resistivity logs, the various permeable beds in a well can be accurately evaluated.
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 of the Gamma Ray log. If there are known water-bearing levels and gas-bearing levels within the beds of interest or within the same geological 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* 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 will be shown to illustrate the applications of these two techniques in clean and shaly formations of Northern Montana.
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