Extended poroelastic impedance

Abstract

Linearized approximations to the P-wave reflectivity as a function of the incidence angle (called amplitude variation with offset) involve the extraction of band-limited reflectivity terms that are a function of changes in the elastic constants of the earth across each lithologic interface. The most common of these extracted reflectivities are the intercept and gradient, usually labeled [Formula: see text] and [Formula: see text], respectively. The extended elastic impedance (EEI) method uses a rotation angle [Formula: see text] to map [Formula: see text] and [Formula: see text] into a new reflectivity corresponding to a particular elastic parameter. The success of EEI depends on finding an optimum value for the angle [Formula: see text]. This value is usually calculated by correlating the EEI result over a range of [Formula: see text] angles with various elastic parameters and then finding the best correlation coefficient. We have developed a new approach for the interpretation of the EEI method, which incorporates the Biot-Gassmann poroelastic theory and attaches a physical meaning to the [Formula: see text] angle. We call this method extended poroelastic impedance (EPI). The main advantage of the EPI method is that the [Formula: see text] angle is now interpreted as a parameter that is dependent on the dry-rock properties of the reservoir, rather than a parameter whose value is estimated empirically. The method is evaluated by numerical and synthetic seismic examples and by application to field data from a gas sand reservoir.

An integrated study for seismic structural interpretation and reservoir estimation of Sawan gas field, Lower Indus Basin, Pakistan

“…This increase in P-impedance indicates the compaction of lithology and a decrease in porosity. The increase in Vp/Vs ratio with low P-impedance indicates the presence of hydrocarbon [ 52 ]. The blue color points are falling in the shale area where Vp/Vs ratio and P-impedance values are high.…”

“…The cross plot of P-impedance and S-impedance is used to identify the fluids variation trends [ 52 ], as shown in Fig. 30 .…”

“…The data points encircled by a blue circle are indicating brine sandstone. The high Mu-rho and Lambda-rho data, encircled by black color are shales representing points [ 52 ]. In terms of significance, each of these parameters (AI-SI, AI - Vp/Vs, lanbda-rho vs. Mu-rho) provides unique information about the subsurface and can be used in different ways to interpret geophysical data.…”

“…34 showing low P-impedance and high Vp/Vs ratio are indicating hydrocarbon presence. The pale green and golden color data points showing a high Vp/Vs ratio and high P-impedance fall between brine sandstone and gas sandstone area [ 52 ]. The parrot green and turquoise color data points indicate the compacted lithology with low Vp/Vs ratio and high P-impedance.…”