Integrating Rock Physics Templates for Enhanced Seismic Interpretation: A Comprehensive Analysis of Influencing Factors on Frequency-Dependent Velocity of Tight Carbonate Formations

ABSTRACT: This study conducts an in-depth rock physics analysis of the Bakken and Three Forks formations within the Williston Basin, emphasizing the role of fluid types, saturation levels, pore structure, and frequency on defining rock properties. The objective is to enhance seismic interpretation and reservoir characterization by developing rock physics templates (RPTs) for these specific formations. We explore how pore structure heterogeneity influences velocity sensitivity over a broad frequency range and varied fluid types. By integrating both Gassmann and Biot theories for fluid substitution and considering frequency-dependent velocity via Geertsma-Smit approximation, the study models fluid properties, and their interaction with the rock matrix, supported by case studies from the Bakken formation. Findings from this study reveal that gas-water systems are more frequency-dependent compared to oil-water systems. Moreover, the frequency-dependent velocity modeling revealed that combining pore structure with varying porosity and permeability levels can significantly influence the velocity-frequency relationship. The research further delineates the complex interplay between the Vp/Vs ratio and acoustic impedance, pore morphology, and fluid saturation. Notably, crack-like pores are linked with larger Vp/Vs ratios, while interparticle and vuggy pores show a more homogeneous stiffness distribution. The proposal of geology-dependent RPTs emphasize the need to consider lithological and diagenetic variations for accurate seismic interpretations. 1. INTRODUCTION The detection of fluids through seismic reflections is a critical aspect of hydrocarbon exploration, marked by distinct indicators such as bright spots, dim outs, phase reversals, and flat spots (Smith Smith Smith et al., 2003). This is especially challenging for gas due to its compressibility and lower density compared to other fluids, which can lead to abnormal frequency-dependent velocity dispersion and attenuation (Malki et al., 2022a). The complexity is further heightened in carbonate reservoirs compared to siliciclastics, due to their heterogeneous pore structures, which intricately affect the velocity response to various fluid types and frequency amplitudes (Anselmetti Malki et al., 2023a). This sensitivity is pronounced in fractured reservoirs, where the coexistence of diverse pore structures necessitates sophisticated rock physics modeling to accurately predict P- (Vp) and S-wave (Vs) velocities under varying saturation levels and frequency ranges (Saberi, 2010).