Отправлено: 18.12.09 08:44. Заголовок: Controls on Sonic Velocity in Carbonates

PAGEOPH, Vol. 141, No. 2/3/4 (1993) 0033-4553/93/040287 3751.50 + 0.20/0
9 1993 Birkh/iuser Verlag, Basel
Controls on Sonic Velocity in Carbonates
FLAVIO S. ANSELMETTI 1 and GREGOR P. EBERLI 2
Abstract--Compressional and shear-wave velocities (Vp and V,) of 210 minicores of carbonates
from different areas and ages were measured under variable confining and pore-fluid pressures. The
lithologies of the samples range from unconsolidated carbonate mud to completely lithified limestones.
The velocity measurements enable us to relate velocity variations in carbonates to factors such as
mineralogy, porosity, pore types and density and to quantify the velocity effects of compaction and other
diagenetic alterations.
Pure carbonate rocks show, unlike siliciclastic or shaly sediments, little direct correlation between
acoustic properties (Vp and V,) with age or burial depth of the sediment so that velocity inversions with
increasing depth are common. Rather, sonic velocity in carbonates is controlled by the combined effect
of depositional lithology and several post-depositional processes, such as cementation or dissolution,
which results in fabrics specific to carbonates. These diagenetic fabrics can be directly correlated to the
sonic velocity of the rocks.
At 8 MPa effective pressure Vp ranges from 1700 to 6500 m/s, and V s ranges from 800 to 3400 m/s.
This range is mainly caused by variations in the amount and type of porosity and not by variations in
mineralogy. In general, the measured velocities show a positive correlation with density and an inverse
correlation with porosity, but departures from the general trends of correlation can be as high as
2500 m/s. These deviations can be explained by the occurrence of different pore types that form during
specific diagenetic phases. Our data set further suggests that commonly used correlations like "Gardner's
Law" (Vp-density) or the "time-average-equation" (Vp-porosity) should be significantly modified towards
higher velocities before being applied to carbonates.
The velocity measurements of unconsolidated carbonate mud at different stages of experimental
compaction show that the velocity increase due to compaction is lower than the observed velocity
increase at decreasing porosities in natural rocks. This discrepancy shows that diagenetic changes that
accompany compaction influence velocity more than solely compaction at increasing overburden
pressure.
The susceptibility of carbonates to diagenetic changes, that occur far more quickly than compaction,
causes a special velocity distribution in carbonates and complicates velocity estimations. By
assigning characteristic velocity patterns to the observed diagenetic processes, we are able to link sonic
velocity to the diagenetic stage of the rock.
Key words: Sonic velocity, carbonates, physical properties, porosity, diagenesis, compaction.
Swiss Federal Institute of Technology ETH, Geologisches Institut, Sonneggstr. 5, CH-8092 Zfirich,
Switzerland.
University of Miami, Rosenstiel School of Marine and Atmospheric Science, MGG, 4600
Rickenbacker Causeway, Miami, FL 33149, U.S.A.