Factors controlling sonic velocity in dolostones

Although the impact of rock texture and crystal size on porosity and permeability of dolostones is well documented, the impact of such factors on the sonic velocity of these sedimentary rocks remains poorly understood. In this study, 100 samples from five formations outcropping along the Arabian Plate, were analyzed to investigate the main factors controlling the sonic velocity in dolostones. The study integrates results obtained using different techniques including: thin-section petrography, SEM, XRD, digital image analysis, porosity and permeability measurements, velocity measurements, in addition to rock physics modeling. Results indicate that porosity is a major factor in controlling the sonic velocity in dolostone where the porosity-velocity relationship shows a determination coefficient (R2) of 0.82. The scatter in the porosity-velocity relationship can be mainly attributed to variations in the dolostone texture (fabric preserving, FP, versus non-fabric preserving, NFP) and the associated pore types. FP dolostones (characterized by macro interparticle, moldic and shelter pores inherited from the precursor limestone) show higher velocities than NFP dolostones (dominated by intercrystalline pores) at a given porosity. The experimental porosity-velocity data, and the rock texture and pore types observations are consistent with rock-physics modeling incorporating variable equivalent pore aspect ratio (EPAR) values in the differential effective medium (DEM) theory. FP dolostones display higher EPAR values (>0.15) compared to NFP dolostones; a criterion that can be used to differentiate between the two dolostone textures. When classifying data based on texture (FP versus NFP), we observe strong porosity-impedance relationship (R2 > 0.95) for both the studied samples, and literature datasets (regional and global) following a consistent distinct trend for each texture. This implies a universality of the porosity-impedance trends observed here, which when combined with the presented rock physics analysis, can inform the modeling and quantitative interpretation of sonic logs and seismic data in dolostone strata.