Interpretation of borehole gravity surveys in a native-sulfur deposit, Culberson County, Texas

Abstract

Borehole gravity surveys to determine formation densities were conducted by the U. S. Geological Survey in four exploratory wells penetrating the native-sulfur deposit at the Duval Culberson property, Culberson County, Texas. The borehole gravity meter has a depth of penetration comparable to the recovery radius of the Frasch process for mining sulfur, which is an advantage for evaluating the sulfur content of heterogeneous deposits. Mineralized zones are treated as three-component systems--bioepigenetic limestone, interstitial sulfur, and water-filled pores. A ternary diagram relating combinations of these components to borehole gravity densities, together with empirical data relating sulfur content and water-filled porosity, are used to estimate sulfur volumes from the borehole gravity data. Estimated sulfur contents range from 0 to 33 percent. Comparisons with the amount of sulfur observed in core and cuttings from the four study wells and from surrounding wells show both the positive benefits of a greater range of investigation and the negative effects of relying solely on formation density as a measure of sulfur volume. Estimates of sulfur content based on borehole gravity data can supplement conventional analyses of core, as well as standard geophysical surveys of boreholes.