The velocity-deviation log: A tool to predict pore type and permeability trends in carbonate drill holes from sonic and porosity or density logs

The velocity-deviation log, which is calculated by combining the sonic log with the neutron-porosity or density log, provides a tool to obtain downhol e information on the predominant pore type in carbonates. The log can be us ed to trace the downhole distribution of diagenetic processes and to estima te trends in permeability. Laboratory measurements on over 300 discrete carbonate samples reveal that sonic velocity is a function not only of total porosity, but also of the pr edominant pore type. In general, there is an inverse porosity-velocity corr elation, but significant deviations occur from this relationship for certai n pore types. Frame-forming pore types, such as moldic or intrafossil poros ity result in significantly higher velocity values at equal total porositie s than do pore types that are not embedded in a rigid rock frame, such as i nterparticle porosity or microporosity. The results of the laboratory measurements can be applied to expand interpr etations of standard wireline-log data, as shown in this study on two drill holes through Neogene carbonates from the Great Bahama Bank. The velocity- deviation log is calculated by first converting porosity-log data to a synt hetic velocity log using a time-average equation. The difference between th e real sonic log and the synthetic sonic log can then be plotted as a veloc ity-deviation log. Because deviations are the result of the variability of velocity at a certain porosity, the deviation log reflects the different ro ck-physical signatures of the different pore types. Positive velocity devia tions mark zones where velocity is higher than expected from the porosity v alues, such as zones where frame-forming pore types dominate. Zero deviatio ns show intervals where the rock lacks a rigid frame, such as in carbonates with high interparticle porosity or microporosity Negative deviations mark zones in which sonic log velocities are unusually low caused, for instance , by a cavernous bore-hole wall, fracturing, or possibly by a high content of free gas. By tracing the velocity deviations continuously downhole, one can identify diagenetic zones that are characterized by these different por e types. In addition, this method can be used to observe permeability trend s because pore types influence the permeability of the rock.