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Joint Estimation of the Column Height and Reservoir Distribution by Integrating AVO-Inversion and Overpressure-Driven Seismic Velocities


The column heights have been traditionally estimated using geomechanical approaches honoring structural and pore-pressure data, whereas the reservoir distribution is typically addressed by reservoir characterization works. However, in certain gas reservoirs the AVO anomalies can account for both reservoir presence and fluid content, therefore column heights must agree in some way with observed DHI's. Likewise, overpressure can affect seismic velocities creating uncertain seismic responses. With such close relationship we found out that is feasible to solve simultaneously the gas reservoir and the potential column heights. To illustrate this, we applied a workflow to properly image a gas accumulation by integrating seismic, pore-pressure/fracture gradient and rock physics/AVO data in a deep water environment. Modeled AVO response and rock physics analysis allowed discriminating gas sands using the elastic properties. Intercept (I) and Gradient (G) are functions of acoustic impedance (Zp) and Vp/Vs ratio respectively, which were suitable estimated from simultaneous inversion. Thus, we opted for an inversion-guided methodology where the calibrated AVO-inversion results provided the strongest control on the lateral variations of the gas reservoirs. Thus, Zp vs. Vp/Vs ratio cross-plots enabled a 3D mapping of geo-bodies with similar elastic characteristics, from which the lateral extension of seismic net-pay and the column height were properly imaged. Stacked seismic data exhibit strong amplitudes with nice structural conformance; however the actual nature of this DHI and the seal capacity of a gas column down to the flat spot remained unsolved. AVO analysis showed gas-saturated intervals well separated from the background wet-trend in the I-G space, indicative of a Class III AVO confirming the presence of a DHI. Likewise, from seismic interval velocities and pore pressure measurements, the range of plausible column heights were modeled using different GWC, then converted into a range of depth varying fluid contacts as a function of the supported pore pressure consistent with the fracture gradient, the regional stress regime and, particularly, with the seismic inversion results. The outcome of this seismic-based pore-pressure scanning process was reasonable column heights down to the flat spot for the reservoir levels. The method can be effectively used to predict gas accumulations for gas-prone prospects in different overpressure deep water environments.