ABSTRACT: Sonic Velocity in Carbonates--A Product of Original Composition and Post-Depositional Porosity Evolution

ANSELMETTI, FLAVIO S., and GREGOR P. EBERLI, University of Miami, Miami, FL

Results from physical properties analyses on discrete samples from Upper Pliocene to recent carbonates of two core borings in the Great Bahama Bank indicate that sonic velocity of carbonates is controlled by two factors: (1) the original composition and (2) the porosity evolution during diagenesis.

In order to understand the relationship between lithology and sonic velocity in carbonates, ultrasonic p- and s-wave velocities were measured on discrete samples under varying confining pressure and constant pore pressure. The measured velocities display a wide range between 2.5 and 6.4 km/s. This range is the result of variations in porosity, texture, and original composition of the rock. In general, porosity is inversely correlated to velocity and is the primary control on the velocity. Deviations from the expected porosity-velocity relation, however, are dependent upon the porosity type. Rocks with the same porosity can produce variable velocities: Carbonates with moldic porosity are able to keep a high velocity despite high porosity, whereas carbonates with interparticle porosity ave lower velocities at comparable porosity values.

Carbonate-mineralogy has only a minor effect on the velocity, but post-depositional diagenetic processes that change mineralogy can alter texture and porosity dramatically and consequently also the velocity. For example, fabric destructive dolomitization leads to a decrease in velocity, while dolomitic cementation produces an increase.

These analyses show that original composition and processes of diagenesis, especially the post-depositional porosity evolution, determine sonic velocity and as a result acoustic impedance in carbonates.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)