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USGS1980

Analysis of borehole geophysical data in an evaporite sequence at Salt Valley, Utah

Interpretation of the lithology of an interbed sequence (thin-bedded sediments between thick layers of salt) is often difficult due to the presence of dolomitic sandstones and halite inclusions. The most useful well logging measurements in an evaporite sequence are gamma-ray, neutron-neutron, density, and acoustic velocity. The high resistivity of salt and the low resistivity of commonly used drilling fluids (brine muds) make it difficult to obtain measurements of electrical properties in an evaporite sequence. Tests of a single-coil induction probe at Salt Valley show good resolution of the interbed lithologies and may be useful in determining moisture in the halite. Complex structural features (faulting and folding) are difficult to interpret with individual well logs. Interpretation of complex folding can be aided by plotting the percent frequency of occurrence of well log response values for the interbeds. Limbs of a fold that intersect the drill hole at different dip angles yield similar distribution patterns on these percent-frequency plots.

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