Improved Reservoir Characterization Of Pab Sandstone By Integrating Petrophysics, Seismic Inversion, And Rock Physical Analysis Of Zamzama Gas Field, Lower Indus Basin, Pakistan

Abstract

Seismic reflection is an indirect geophysical technique that is frequently employed for oil exploration. The evaluation of the hydrocarbon reservoir and the characterization of the physical reservoir parameters are the main goals. Zamzama gas field is located in the Lower Indus Basin, which is under the influence of compressional regime. The Pab Formation of Late Cretaceous age which is composed of major sandstone but minor mudstone and shale components. Due to thin layering of shale beds it is not characterized by conventional method. Using borehole data of Zamzama 03 reservoir zone is identified by petrophysical analysis. According to petrophysical data, the Pab Formation has a reasonable effective porosity of 10%, a small amount of shale of 21%, and a hydrocarbon saturation of 70–80. For wells Zamzma-03, Rock Physics confirms the results of Petrophysical study and calculates reservoir rock physics parameters to obtain a good knowledge of reservoir attributes. As the Acoustic Impedance and Vp/Vs ratio decrease, it indicates the existence of reservoir zone. From facies analysis the cross plots of Acoustic Impedance and Vp/Vs ratio is plotted, a decrease in P-impedance and Vp/Vs ratio indicated a large amount of sands. Brine sand appears as a second type of sand in the cross plots due to its high Acoustic Impedance and Vp/Vs ratio. Presence of shale shows in cross plots but these values different from the earlier situation for both sand types, Shale can be found where the P-impedance is low but the Vp/Vs ratio is high. Seismic interpretation is performed to delineate the subsurface structure. On a Synthetic Seismogram, Pab and Fort Munro are two horizons that are marked. Two faults are marked by distortions or discontinuities with an easterly-verging N-S thrust fault. Time and depth contour maps also validates subsurface structures. The reservoir zone is confirmed by performing three types of post stack inversions. Model based (MBI) inversion, Sparse Spike Inversion (SSI), and band-limited inversion (BLI) are performed on seismic data. MBI has a correlation coefficient of 0.99 and a reservoir-level error of 0.05. SSI's correlation coefficient is (0.97) and its error is (0.024), while BLI's correlation coefficient is (0.97) and it likewise falls within acceptable ranges (0.90). All inversion results provide good results at reservoir level, but MBI performs the best followed by SSI and BLI.