Control of pore types and fracture intensity on the P-wave velocity of carbonate rocks

The study objective is to determine the major factors controlling on acoustic and geomechanical properties of carbonate rocks of Jurassic D5 Member of Dhruma Formation. The study integrates petrophysical, petrographic, and geomechanical analysis to characterize different lithofacies, diagenetic properties, shear and compressional wave velocity, rock strength, dynamic elastic moduli, porosity and permeability of carbonate rocks. A total of sixty samples were collected, petrographic thin sections and Scanning Electron Microscopic images were prepared, small cubes and core plugs were prepared for porosity and permeability measurements as well as wave velocities and point load index tests. Porosity and permeability exhibited moderate to poor correlations. P-wave and S-wave velocities are moderately correlated with porosity. However, they showed better correlation with permeability. Poor to moderate direct correlation between dynamic elastic moduli (Poisson ratio and Young modulus) and porosity were found. The lithofacies with large fracture intensity are related to small P-wave velocity values. Four 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, Class 2 is characterized by intensive micro-fracturing, class 3 is intensively cemented, and Class 4 is characterized by microporosity. Micro-fractures pore type samples show the best wave velocity versus porosity correlation followed by densely cemented microporosity and finally moldic and intraparticle show the weakest correlation. 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 values and sill values equal to 3 meters. Hole effect reflecting the control of cyclicity was found. Porosity semi-variogram shows exponential modeling with zero nugget value and sill value equal to three meters. This difference in semivariogram between velocity and porosity supports weak and moderate correlations between them.