Controls on Sonic Velocity in Dolostones

Dolostones represent one of the major hydrocarbon reservoirs in the world. Understanding the elastic behavior of these units is crucial for hydrocarbon exploration and/or development. In this study, 100 samples from five formations within the Arabian Platform, were used to examine the main controlling factors on the sonic velocity of dolostones. A combination of field and laboratory analyses were conducted on the collected samples including; thin-section petrography, SEM, XRD, digital image analysis, porosity and permeability measurements, velocity measurements, and rock physics modeling. The studied samples have a wide range of porosity (1- 45%, averaging 18.5 %), and permeability (0.01 – 2000 mD, averaging 196 mD). Compressional V_P and shear wave V_S velocity ranges from 3.0 - 6.7 km/s, and 1.6 – 3.7 km/s, respectively. In general, porosity-velocity trajectory is showing a negative relationship with a coefficient of determination R² of 0.82. However, some samples are deviated from this trendline due to their inherited and diagenetic parameters. These parameters include texture, mineralogy, pore type, and crystal size. Fabric-preserving dolostones have, relatively, higher velocities than non-fabric preserving dolostones. Although 95% of the studied samples are dominated by dolomite, samples with higher content of calcite and quartz, have lower velocities. Moldic and vuggy pore-dominated samples have, relatively, higher velocities than samples dominated by intercrystalline pores and microporosity. For non-fabric preserving dolostones, samples with larger crystals show higher velocities than samples with smaller crystals. Using equivalent pore aspect ratio (EPAR), a clear distinction between permeable (>10 mD) and tight (< 10 mD) samples can observed, where most of the permeable samples have high EPAR values, while the tight samples have low EPAR values. The result of this study might significantly help in the interpretation and understanding of the sonic logs and seismic data from dolostone strata.