Abstract
<p>High lateral variability and frequent reversals with depth of petrophysical properties in shallow-water carbonates illustrate the effects of depositional and post-depositional environments. Knowledge of the initial properties and early diagenesis provide insights into end-member characteristics. This has been investigated through the measurement of petrophysical properties in unconsolidated sediments and their lithified counterparts from modern depositional environments on the Great Bahama Bank and Florida platform.Unconsolidated sediments of the Great Bahama Bank and Florida platform demonstrate a marked spatial homogeneity on a scale of 10 m--1000 m which correlates to boundaries of discrete depositional environments. Petrophysical properties of characteristic sediments have a small range, which can sometimes be uniquely associated with a depositional environment. Despite large differences in physical characteristics among representative sediment facies, acoustic velocity and velocity responses to extrinsic factors are very uniform relative to modern lithified counterparts.Acoustic velocity (Vp, Vs) of unconsolidated carbonate sediments responds strongly to confining pressure, mid 50%--100% velocity increases are common as confining pressures are brought to 40 MPa. Elevated velocities continue past 80 MPa and persist during hysteresis, despite pervasive grain destruction. Artificial lithification of sediments by compaction alone provides a negligible increase in acoustic velocity measured at near-surface pressures.Experiments with grain and matrix mixtures of natural carbonate sediment components demonstrate a critical ratio of grain to matrix which represents a porosity minimum and acoustic velocity maximum. This critical ratio is 10% fine-grained matrix for ooid grain mixtures and 20%--30% for skeletal grain mixtures. Saturation of unconsolidated wackestone to grainstone carbonate sediment facies elevates Vp and Vp:Vs ratios disproportionately at confining pressures of <20 MPa.Petrophysical properties of lithified carbonates from modern depositional environments, including acoustic velocity, demonstrate a large variability and extensive heterogeneity in fabric development at the cm scale, in contrast to their sediment counterparts. Unique association with a depositional environment is difficult with many properties. Meteoric diagenesis of shallow subsurface carbonates in Florida can result in an overall reduction in inter/intra-granular porosity, widespread cementation, and remineralization. High variability in many petrophysical properties obscures the initial characteristics of source sediments.</p>