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The neutron log has been accepted as a standard means of porosity determination for a number of years. The presence of clay or silt in sandstone or carbonate reservoirs has an adverse effect on the quantitative use of the present neutron log. A system responsive to the physical properties of the formation and not to its chemical properties overcomes this limitation. A properly collimated logging tool will accurately measure bulk density of a formation. The intensity of gamma rays (from the source) that have undergone Compton scattering in the formation can be recorded and related to the density of the particular medium. Grain densities of major minerals are known. Fluid density can be considered unity unless gas is the saturating medium. Bulk density from the density log completes the solution of the equation and provides the formation porosity. Success has been obtained with the density log in the Paradox, San Juan and Green River basins of the Rocky Mountain area. Introduction Porosity determination in Cretaceous and Paleozic sand sections of the Rocky Mountain area, as well as Paleozoic carbonate sections of the Paradox basin, has with conventional logging methods been difficult. The radioactive density log has been utilized in a number of wells in the Paradox, San Juan and Green River basins and has given extremely accurate results in a large majority of cases. Since experience is always necessary in order to properly evaluate any new logging device, an attempt has been made to run the radioactive density log through sections representative of typical conditions throughout the area with as many varying conditions of lithology, borehole conditions and drilling methods as possible. This paper dwells predominantly on results obtained in the Paradox zone of Pennsylvanian age in southwestern Utah, the Mesa Verde formation of the San Juan basin and the Frontier sands and Dakota sandstones of the Green River basin of southwestern Wyoming.

Quantitative Evaluation of the Density Log in the Rocky Mountain Area