Diagenesis and pore-pressure-induced dim spots on offshore French Guiana’s stiff reservoirs

Abstract Overpressure mechanisms typically involve transferring some load from the rock framework to the pores, which reduces velocities and preserves the porosity of over-pressured rocks. Despite efforts to improve well-drilling safety through understanding and predicting these mechanisms, there is little information on how they influence amplitude variation with offset (AVO) analysis. We model the overpressure effects on elastic properties by introducing a pressure-dependent rock physics model for shales and sands. With data sets at multiple scales (plugs, logs, and seismic), we show how differential diagenesis and composition impact the pressure dependency and thus the seismic analysis on the French Guiana and the offshore Amapá basins. We perform AVO analysis using synthetic seismic data generated from our rock physics model and compare it to the field data through 3D seismic interpretation. The modeling of our equatorial margin data set shows that high pore-pressure settings cause a reduction of zero offset and gradient amplitudes and potentially a shift between class I and class II AVO. In our field 3D seismic interpretation, however, we attribute the normal impedance amplitude reduction to differential diagenesis, reducing the rate of quartz cementation, preserving rock’s porosity and bulk density.