Spectral Decomposition Applied to Reservoir Management: A Brazilian Pre-Salt Case Study

Abstract

Detecting 4D anomalies in pre-salt carbonate reservoirs can be a major challenge for seismic interpretation due to low impedance contrasts and complex fluid substitution effects, as those observed in Mero field located at Santos Basin, Brazil. To overcome this task, the applied methodology comprised the use of Generalized Spectral Decomposition (GSD) jointly with a 4D seismic inversion to analyze frequency, phase and the acoustic components of the 4D signal near an injector well, where frequencies (9 Hz, 18 Hz, and 26 Hz) and phase rotations (±180º, ±90º, 0º) were evaluated. The results indicate that, although the individual spectral decomposition components do not highlight gas anomalies in the oil zone, the use of a combined attribute response — involving the specific 18 Hz spectral component rotated by -90 degrees, together with 4D Acoustic Impedance (IP4D) — shows good correlation with the variation in gas saturation observed in the vicinity of the well. This methodology, also enhanced by geobody generation to isolate anomalies, improves the detection of subtle variations associated with water-alternating-gas (WAG) injection cycles in complex carbonate settings, thereby supporting long-term reservoir management.