Limitations in Reservoir Characterization in Stacked Carbonate Reservoirs, Supported by Seismic Inversion and Modelling in a Mature Oil Field

Abstract Unlike clastic rocks, stacked carbonate rocks present more challenges in reservoir characterization. This study highlights the results of seismic inversion, and the limitations encountered in reservoir characterization, focusing on the main producing carbonate reservoir in a mature offshore oil field. Reservoirs in the study area are shallow water platform carbonates, covering part of inner to outer ramp deposition. Broadly, it is a coarsening upward succession of variable thicknesses of stacked reservoirs with different oil water contacts (OWC's). Two case studies are presented: the first uses a Pre Stack Time Migration (PSTM) seismic volume for porosity prediction through post-stack seismic inversion, while the second employs simultaneous inversion of angle stacks to determine P-wave impedance which was converted into porosity based on rock physics analysis. Both studies partially correlate seismic derived porosities with well based porosities. The study discusses five limiting factors causing discrepancies, including rock properties variations, tuning phenomena, the impact of fluid changes, overburden on seismic response and methodology. The study encompasses wedge modelling, forward modelling, Amplitude variation with Offset (AVO) response, fluid substitution modelling and seismic attribute maps to assist in identifying these limitations. One of the primary challenges in reservoir characterization is managing thin reservoirs that are 5 to 10 feet thick and beyond seismic resolution. For thicker carbonate reservoirs, ranging from 60 to 120 feet, various other features influence the outcomes of reservoir characterization, such as high permeability streaks, relatively tight zones, and regions that are vuggy, fractured, or heterogeneous. Two seismic inversion studies presented show general agreement for porosity prediction in some reservoirs however few discrepancies were noted. Five limiting factors have been identified which individually or as combination cause discrepancies. Seismic response at reservoir is mainly affected by the overburden noise. The wedge model presented effectively determines vertical resolution (86ft with 25% porosity), which appears to change spatially due to porosity variation, indicating that not all reservoir units are seismically resolved. Forward models for several wells demonstrate a strong correlation with surface seismic data and capture changes in response with reservoir thickness and facies, which could significantly affect inversion results. Fluid substitution results confirm that displacing approximately 90% of oil with sea/saline water or gas results in at least 10% change in impedance and therefore on the derived porosities. For some reservoirs, seismic attribute maps show poor correlation with well based porosity map, mainly because of combined effect of reservoir thickness, property and fluid change across the field. The integrated approach presented in this paper emphasizes the seismic inversion study and highlights potential limitations in carbonate reservoir characterization, supported by various models and real datasets. Additionally, this paper explains how modelling results can aid in accurately predicting reservoir properties, ultimately leading to successful well optimization.