Abstract: Pore Type and Permeability Predictions in Carbonate Drillholes by Comparing the Velocity and Porosity Logs ("Velocity-Deviation Log")

Flavio S. Anselmetti, Gregor P. Eberli, Leslie A. Melim

The "velocity-deviation log," calculated by combining the velocity and neutron-porosity logs, provides information of the predominant carbonate pore type. Associated diagenetic processes can be traced downhole and estimations on permeability distribution are possible.

Laboratory measurements of sonic velocity and porosity on discrete carbonate samples reveal that sonic velocity is a function not only of total porosity but also of the predominant pore type. In general, there is an inverse porosity-velocity correlation. Significant deviations occur from this relationship for certain pore types that give characteristic elastic properties to the rock. Frame-forming pore types, such as moldic or intraparticle porosity, have significantly higher velocity values at equal total porosities than pore types that are not embedded in a solid rock-frame, such as intercrystalline or microporosity.

The results of the laboratory measurements are applied in interpreting standard wire-line log data of two drillholes through Neogene carbonates from Great Bahama Bank. The best-fit correlation between velocity and porosity, as measured in the laboratory, is used to transform the neutron-porosity log into a synthetic sonic log that displays velocity values which would be expected from the log-porosities. The difference between the real sonic log and the synthetic sonic log can be plotted as a "velocity- deviation log" that reflects the different pore types and their different rock-physical signatures. Positive velocity-deviations mark zones in the drilled section where velocity is higher than expected from the porosity values; these are zones where frame-forming pore types dominate. Ne ative deviations show intervals where the rock lacks a rigid frame, such as in carbonates with high intercrystalline or microporosity. By tracing the velocity- deviations continuously downhole, zones can be identified with different pore types and the related constructive or destructive diagenetic processes. In addition, this method can be used to make permeability predictions, since pore type influences the permeability of the rock.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France