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. 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. This study provides a comparison of older carbonate samples from different basins and different geological 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, 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 vs. Ordovician age rocks in the Michigan Basin). While the empirical Wyllie time average equation 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 nano-scale porosity, it is observed that the Wyllie time average equation should be applied as an upper data boundary. In addition, this study has shown that the relationship to dominant pore type is less direct than in a macropore system where it can be assumed that the dominant pore type also has the greatest effect on the effective permeability.
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