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Shaly Sand Gas Reservoir Characterization: A Successful Case Study in the Lower Magdalena Basin, Colombia


The overall goal of this study is to identify and delineate new prospective areas in a shaly sand gas field located in the Lower Magdalena Basin, Colombia, based on rock physics and geophysical analysis. Detailed petrophysical and rock physics analyses and modeling were performed in order to understand the relationship between petrophysical and elastic properties at the reservoir level. This phase generated a robust well log data set used as input to a low frequency model used as the starting point of seismic inversion. Rock physics diagnostics and fluid substitution analysis showed the fluid effects in the reservoir on seismic response. Wet shaly sands have similar acoustic impedance and Poisson's Ratio to shales, but gas shaly sands have a very low Poisson's ratio. The gas shaly sands show clear separation from the other facies in the Poisson's Ratio vs. P-impedance domain. Next, pre-stack gather conditioning was performed in order to improve the seismic data quality (Singleton, 2009) in advance of pre-stack inversion. Conditioned well log and pre-stack seismic data were used for the simultaneous seismic inversion. P-impedance, S-impedance and Poisson's Ratio were the most useful attributes computed in this phase. Calibration between rock physics results and seismic attributes permitted identification of the 3D distribution of zones with higher a probability of gas saturation. From this study, two new locations were drilled in the field, both penetrating the target reservoir as discoveries. This successful result validated the workflow used and was used to update the geological, geophysical and petrophysical model for future exploration and exploitation plans in the area.