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)