Petrophysical properties and their heterogeneity within sandstone bodies are key parameters in the evaluation of hydrocarbon and geothermal reservoirs. However, common tools applied to constrain the porosity distribution pattern in borehole cores are commonly time consuming and destructive, or suffer from a resolution limited to the meter to decimeter scale. We examine the applicability of nondestructive high-resolution sonic (HRS) logging of well core sections in the ultrasonic frequency range as a method providing porosity proxy data at the centimeter scale in a clastic sedimentary sequence.
The middle Solling Sandstone Member, a gas-bearing reservoir offshore from the Netherlands, is used as a test sample. It consists almost entirely of clean, cross-bedded to massive eolian dune and dry sand-flat deposits that are salt plugged to variable degrees. Plots of HRS logging data versus core-plug porosity values show a positive linear relationship that develops the highest coefficient of determination (R2 = 0.86) in structureless eolian dune sands, most probably caused by the lack of bedding-related anisotropies there. Once calibrated for a particular facies type, this correlation enables the calculation of porosity proxy data from sonic transit time values, acquired at centimetric steps. High-resolution sonic logging thus contributes a reliable and time-efficient, highly spatially resolving quantification of reservoir heterogeneities at centimeter scale and turns out to be a suitable tool for the nondestructive in-situ detection of high-porosity zones in otherwise uniform sandstone successions. Moreover, plots of closely spaced HRS logging–derived porosity proxy data significantly improve interpolation between single, wider spaced core-plug porosity data points.