AVO FORWARD MODELING AND GASSMANN FLUID SUBSTITUTION ANALYSIS OF BITRISM AREA, LOWER INDUS BASIN, PAKISTAN

Abstract

The study area Bitrism Block is located in the Lower Indus Basin of Pakistan. The producing reservoirs in this area are sandstone (shallow marine) of Lower Goru formation. The present study pertains to the interpretation, analysis and modeling of the productive zones from seismic and well log data. The seismic interpretation shows horst and graben structures bounded by normal faults with probable hydrocarbon accumulation. After time to depth conversion the true structural geometry is obtained in the form of depth sections. The horizons were identified using formation tops from wells and their depths were confirmed through correlation with synthetic seismogram from Density log (RHOB) and Sonic log. Time and depth contour maps of the horizons of interest have been generated to understand the spatial geometry of the structures. The shales present in the Lower Goru Formation acts as a source while the sand present in Lower Goru Formation are the primary reservoirs. Interpretation shows that a thick cover of Upper Goru Formation acts as a seal rock. For the selected zones, petrophysical analysis indicating the reservoir quality. The current study intends to use the amplitude versus offset analysis technique for future hydrocarbon exploration. The AVO analysis is performed on Lower Goru reservoir by using the wavelet software and AVO gathers are generated showing AVO anomalies of class 2. Fluid substitution is an important process which provides a tool for fluid identification and quantification in a reservoir. In the present study, Gassmann fluid substitution analysis is applied for the characterization of reservoir of Bitrism block. Then by using Zoeppritz calculator, reflection coefficient curves as a function of angle of incidence for Zoeppritz and it approximations Shuey 3 term giving a clear indicator of oil and gas based reservoir.