عنوان مقاله [English]
The prestack seismic inversion converts seismic data to the physical properties of a rock such as sonic and shear impedance and density. It provides accurate information for predicting lithology changes and fluid types. In this paper, well logging data is used to construct synthetic seismogram. In the final stage, by transforming offset domain to angle domain and using the well information to the prestack seismic inversion, the extracted petro-physical parameters are discussed. The applied average wavelets of 7 wells in prestack inversion were in the angles of 5-9, 9-13, 13-17, 17-21 and 21-25 degrees. After wavelet extraction, low frequency acoustic impedance models and shear impedance and density were made as one of the inversion inputs. We built these, low frequency initial models using sonic log, shear impedance log and density log from well data.
There are two techniques for doing the pre-stack seismic inversion: simultaneous and elastic inversion. These technics require wavelets and background model. Simultaneous pre-stacking inversion is defined by seismic trace angle, logarithms of P-impedance and S-impedance, and extracted wavelet but Elastic inversion uses a transformation of the Zeoppritz equations
In other words, by performing the prestack inversion, the sonic and shear impedance and density are calculated using above mentioned equations.
In this paper, prestack seismic inversion method was carried out in one of Iranian oil fields in Ghar-Asmari Reservoir and Jahrum Reservoir formations. The results showed that the presence of oil and gas in the Ghar-Asmari zone caused the reduction of the sonic and shear impedance and density. From Ghar-Asmari zone to Jahrum, the amount of the sonic and shear impedance and density increased. Also, the results of sonic impedance cross-plots versus the ratio of sonic-to-shear wave velocity were determined based on porosity variations and water saturation changes. In Ghar-Asmari zone, porosity is high and water saturation is low because of the presence of gas and oil in this section. From Ghar-Asmari zone to Jahrum, water saturation increases and porosity decreases. Hence, using simultaneous inversion, the hydrocarbon reservoir was identified.