Seismic Structural Interpretation and Reservoir Characterization of Sawan Gas Field, lower Indus Basin, Pakistan (T-1585) (MFN 8318)

Abstract

The information about the subsurface structure, types of fluids present in the reservoir and physical properties of the rocks are important for lead identification. The integrated approach of petrophysical analysis, seismic data interpretation and Gassmann fluid substitution were used for this purpose. Tectonically, the study area of Sawan field is located in the extensional regime. The seismic data set consisting of one strike line and four dip lines along with Sawan-01 and Judge-01 well data was used in this study. The structural interpretation with the help of seismic data indicated the extensional regime with horst and graben structures in the area. The two negative flower structures are cutting the entire cretaceous deposits. The rock unit deposited in Paleocene epoch are dipping in west while earlier deposited formations are dipping in east direction. The time and depth contour maps also indicates the favourable structures for hydrocarbon accumulation. The four possible reservoir zone in Sawan-01 well and two zone in Judge-01 well at B sand and C sand levels are identified on the basis of seismic and well data interpretation. The main lithology of Lower Goru Formation is sandstone with thin beds of shale. The clay types confirms the marine depositional environment for Lower Goru Formation. The water substitution in reservoir at B sand and C sand level indicated the increase in P wave velocity and density. The water substitution effected the shear wave velocity very slightly due to density changes. The cross plots of P-impedance against Vp/Vs ratio differentiate the sandstone with low Pimpedance and low Vp/Vs ratio from shaly sandstone with high values in the reservoir area. The P-impedance and S-impedance cross plot indicates increasing gas saturation with decrease in impedance values. The low values of Lambda-Rho and Mu-Rho indicated the gas sandstone in the cross plot