A comparison of the relationship between measured acoustic response and porosity in carbonates across different geologic periods, depositional basins, and with variable mineral composition

Abstract

Recent work has shown that there is a predictable inverse relationship between laboratory-measured sonic velocity response and porosity in carbonates, which can be reasonably approximated using the empirical Wyllie time-average equation (WTA). The relationship was initially identified in late Cretaceous to Cenozoic age samples collected from the Great Bahama Bank and the Maiella Platform, an exhumed Cretaceous carbonate platform in Italy. We have compared older carbonate samples from different basins and different geologic ages to determine the applicability of this relationship and subsequent correlations to key petrophysical properties to other carbonate basins and other geologic time periods. The data set used for the comparison shows this relationship to be relatively consistent in other depositional basins (Michigan Basin, Paradox Basin) and with samples from older geologic periods (Pennsylvanian, Ordovician, and Mississippian). However, this basic relationship is also observed to vary significantly within a reservoir system and within a depositional basin in samples from different geologic periods (e.g., Silurian- versus Ordovician-age rocks in the Michigan Basin). Although the empirical WTA can generally be applied as a first-order estimate across a wide range of sample ages in carbonates, limited data suggest the relationship between velocity and porosity to be moderately more complex. For instance, in unconventional carbonate reservoirs characterized by predominantly micro- to nanoscale porosity, it is observed that the WTA should be applied as an upper data boundary. In addition, this study has shown that the relationship to the dominant pore type is less direct than in a macropore system in which it can be assumed that the dominant pore type also has the greatest effect on the effective permeability.