Acoustic properties of Neogene carbonates and siliciclastics from the subsurface of the Florida Keys: implications for seismic reflectivity

Abstract

Results of measurements of compressional-wave velocity and shear-wave velocity have been compared with other rock properties, XRD and thin-section analyses for over fifty minicores from two cores drilled through sedimentary rocks on Stock Island and Long Key (Florida Keys). The comparisons indicate that sonic velocity in siliciclastics and carbonates is mainly controlled by three factors: (1) porosity and primary pore type, (2) quartz content and (3) dolomite content. In the case of two Pliocene prograding formations, the combination of depositional fabric and diagenetic alterations produced patterns and values of almost identical impedance for fine-grained carbonates and siliciclastics. Consequently, seismic sections can not be used to distinguish between the prograding quartz sands and the prograding fine-grained carbonates. Compressional and shear-wave velocities in the limestones are strongly controlled by total porosity and the primary pore type. Generally, high-porosity rocks have lower velocities than low-porosity rocks, but pore types can significantly influence velocities. Overall, an increasing percentage of dolomite in carbonate rocks results in an increase in velocity. Some dolomitic fabrics deviate from this trend. Fabric preserving, mimetic dolomites yield the highest velocities, whereas samples that are of purely sucrosic dolomites produce lower velocities, which, however, are still higher in average than non- or only partly dolomitized samples. Quartz sandstones and quartz-bearing limestones have generally low and uniform velocities. The low velocities of the sandstones containing > 80% quartz is a result of their primary interparticle porosity. Due to the low diagenetic potential of quartz grains, a high quartz content inhibits major diagenetic alterations, which would result in cementation and higher velocities. Synthetic seismograms and nearby seismic sections reveal two major seismic facies as a result of the observed velocity distribution: (1) carbonates of the Stock Island Formation and siliciclastics of the Long Key Formation are both characterized by low seismic reflectivity or even seismic transparency. This seismic facies is caused by a very similar, uniform, vertical distribution of low velocities with almost identical impedance values in the two formations. Thus, the transition or an interfingering between the time-equivalent siliciclastic Long Key Formation and the carbonate Stock Island Formation might not be detectable on a seismic section. (2) The over- and underlying shallow-marine carbonates, in contrast, have higher velocities with a wider range of velocities resulting in a succession of high-amplitude reflections.