Factors influencing acoustic properties of carbonate rocks: Examples from middle Jurassic carbonates, Central Saudi Arabia

Abstract

The study aims to identify lithofacies, diagenetic properties, porosity and permeability, compressional and shear wave velocity, rock strength and dynamic elastic moduli of carbonate rocks. All these aspects are used together to delineate the major factors that influences acoustic and geomechanical properties of carbonate rocks of Jurassic Dhruma Formation. A total of sixty carbonate rock samples were collected, petrographic thin sections and SEM images were prepared, core plugs and small cubes were prepared for porosity, permeability, acoustic wave velocities and point load tests, Powder samples were prepared to understand the mineralogical control on these properties. Nine lithofacies were found and minor lithofacies controls on acoustic and geomechanical properties were figured out. Porosity and permeability showed poor to moderate correlations. P-wave and S-wave velocities are poorly correlated with porosity and better correlated with permeability (compared with porosity). Poor correlation between Poisson's ratio and porosity and moderate correlation between dynamic Young's modulus and porosity were found. Two failure patterns of induced fractures were found when applying point load: regular discrete (straight) pattern related to samples of higher strength index and Young's modulus, irregular patterns were found in low strength index and Young's modulus, these induced fractures are compared with natural fractures that are observed in the outcrop. Four petrographic classes were found based on pore type and diagenetic classifications control on acoustic properties: Class 1 is characterized by moldic and intraparticle pore types with limited microporosity. It shows the weakest correlation; Class 2 is characterized by intensive micro-fracturing. This class shows the best wave velocity versus porosity correlation, class 3 is intensively cemented, and Class 4 is characterized by microporosity. Vertical semi-variograms were measured for P and S-wave velocity as well as porosity. A spherical model was found for P and S-wave velocities with zero nugget and range values equal to 3 m. Hole effect reflecting the control of cyclicity was found. Porosity semi-variogram shows exponential modeling with zero nugget and range value equal to 3 m. This difference in semivariogram between velocity and porosity supports weak and moderate correlations between them.