Effects of diagenetic processes on the velocity evolution in carbonate reservoirs

Carbonate rocks are geologically complex due to the diagenetic processes they experience before and after lithification. Diagenetic processes alter their matrix and pore structure leading to the modification in their sonic velocities. Understanding the effect of these diagenetic features on the seismic velocities is crucial to have a reliable image of the subsurface. The dataset used in this study comprises well logs, and core data. Core data were analyzed using different methods (i.e., thin section analysis, X-ray diffraction (XRD) analysis, and scanning electron microscopy (SEM images)) to investigate the presence or absence of different diagenetic processes in each depth interval of the Sarvak formation. In order to minimize porosity effects on velocity variations, we divided all porosity data into five equal porosity classes and performed bar chart analysis in each class. The results indicated that bioturbation (through stiff pore creation and infilling with stiff minerals) and compaction (through pore space volume reduction) increase velocities, but dissolution increased velocities only for the low porosity samples (through moldic and vuggy pores creation) while reduced it in the high porosity samples (through the interconnection of the isolated pores). Furthermore, porosity enhancement (through increasing pore space volume), micritization (through porosity reduction inhibition during compaction), open fracture (through creation of soft pores and cracks), and neomorphism (through the creation of microporosity during compaction) reduce sonic velocities. The study examines the impact of diagenetic processes on sonic velocities in carbonate rocks using data from four wells in southwest Iran.The results reveal new insights into the inverse relationship between micritization, open fracturing, neomorphism, and porosity increase on velocity, compared to the impact of bioturbation and compaction.The study highlights the complex relationship between dissolution and velocity, showing that its impact depends on the porosity of the samples.