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As part of the Regional Aquifer Systems Analysis of the Upper Colorado River Basin, the porosity in selected Paleozoic rocks was estimated from borehole geophysical logs of oil-and-gas exploratory boreholes. Assumptions were made concerning the geophysical properties of selected sandstone, limestone, and dolomite rocks in order to calculate porosity from sonic, neutron, and bulk-density logs. Departures from conditions requisite for porosity estimations from each type of log were identified; the most common departure was the presence of shale in the formation rock. Intervals containing shale lenses, natural gas, and fractures were discarded from the analysis; however, no accounting was made for interstitial shale. Geophysically estimated porosity values were compared with laboratoryporosity values of core samples from 19 formation intervals in 17 boreholes, and a general correspondence was established. Porosity then was calculated from the sonic, neutron, and bulk-density logs from 99 formation intervals in63 boreholes in which all 3 geophysical logs had been run, but in which no laboratory core analysis had been done. The grand mean geophysically estimated porosity of the sandstones that comprise the aquifers in upper Paleozoic rocks ranges from 1.0 to 16.2 percent. The grand mean geophysically estimated porosity of the carbonate rocks that comprise aquifers in the middle Paleozoic rocks ranges from 0.4 to 10.5 percent. Values of porosity estimated by this method of analysis for shaley formations are more indicative of total porosity than of effective porosity.

Geophysically estimated porosity of selected Paleozoic rocks in the Upper Colorado River basin, Colorado, Utah, Wyoming, and Arizona