Characterization of Pab Formation using Rockphysics Analysis and 3D Seismic Inversion in Zamzama Field, Lower Indus Basin, Pakistan

Abstract

For hydrocarbon prospecting, seismic reflection is a widely used indirect geophysical method. The main goal is to characterize physical parameters and evaluate hydrocarbon reservoirs. Because the Zamzama gas field is located in the Lower Indus Basin and has a compressional regime, the structural interpretation is a thrusted anticline. The major purpose of this research is to improve reservoir characteristics estimation in the sands of the Pab Sandstone formation in the Zamzama Gas field in the Lower Indus Basin. Seismic interpretation, petrophysical analysis, seismic inversion, and rock physics techniques are mainly used to achieve the objectives. Acoustic logs were used to create a Synthetic Seismogram at the well location. Horizons of interest are identified after a successful cross correlation of synthetic and seismic traces at the well location. Khadro formation, Pab Sandstone, and Fort Munro Formation are the marked horizons. In the study area, Pab Sandstone acts as a reservoir. To see the depth variations of these reflectors in the study area, Two way time and depth contour maps of selected horizons were created. Subsurface structures are also confirmed with time and depth contour maps. Petrophysical study of well log data is used to evaluate reservoir parameters. The Pab formation has been identified as a reservoir that satisfies all the criteria for a reservoir at Zamzama-02. The Pab formation has the potential to produce effective porosity greater than 8%, shale volume less than 30%, water saturation 30% to 60% and high hydrocarbon saturation 40% to 70%, according to petrophysical results. Using well Zamzama-02 Cross plotting of neutron against density porosity and density vs effective porosity confirms the reservoir lithology. These studies are carried out at various reservoir levels, indicating that the reservoir primary lithology is sand. The seismic inversion technique (Model Based Inversion) resolves the seismic in the acoustic impedance model. Porosity was determined and extrapolated. High impedance value above and below the Pab sandstone indicating the existence of shale or another high impedance rocks Porosity maps were distributed across the study region. Impedance and porosity have an inverse relationship, which means that Pab Sandstone has a low impedance but a high porosity, confirming the petrophysical and rock physics results. In terms of elastic moduli, rock physics modelling confirms the zone of interest, calculates reservoir parameters to obtain a better knowledge of reservoir properties, and also confirms the result of petrophysical analysis. As the acoustic impedance and VpVs ratio decrease, it indicates the existence of gas sand. As VpVs ratio and acoustic impedance increase indicate presence of brine sand. Decrease in acoustic impedance and increase in VpVs ratio indication of shale.