Hidden Potential: The Impact of Seismic Tuning on Gas Volumes Estimation: Case Study of a Tertiary Reservoir, Onshore Niger Delta

Abstract

Seismic amplitude anomalies are widely used as direct hydrocarbon indicators (DHIs) to infer fluid contacts where well control is sparse, yet seismic tuning can bias amplitude-derived fluid contacts and volumetric estimates towards conservatism. We analyze the M1000 gas reservoir in the Bonn-Ario field (onshore Niger Delta) to quantify how tuning-driven effects produce an apparent seismic gas-water contact (GWC) that underestimates the true gas column. Using forward modelling, amplitude variation with angle (AVA) analysis and wedge models, we show that amplitude “shut-off” on the seismic corresponds to a tuning roll-off rather than the physical termination of reservoir gas; tuning effects produced a seismic-derived GWC that would reduce gas initially in-place (GIIP) by ~15% relative to a well log-constrained contact. Wedge modelling indicated that amplitude dimming becomes dominant below ~35 ft of gas column for the local wavelet and velocity. The seismic response effectively fails to detect a gas column thinner than ~35 ft. Analogous examples from the Niger Delta and other basins confirm that this mechanism is widespread. We demonstrate practical mitigation: routine forward-modelling, AVA calibration and wedge modelling to identify tuned intervals and avoid systematic under-booking of volumes. The study highlights that amplitude maps should be treated as λ /4-contour (resolution) indicators, rather than fluid contact proxies, unless verified by wedge modelling, AVA analysis and well calibration. Emerging technologies such as broadband seismic acquisition, machine learning and multi-attribute analysis provide additional toolbox for mitigating tuning-related pay and volumetric mis-estimations.