Fluid and Rock Impedances: New Orthogonal Seismic Attributes that Honor Rock Physics

We explore some new rock physics attributes that can be used to screen for reservoir sandstones and hydrocarbon pore-fill from seismic inversion data. We honor the physical properties of the rocks by defining attributes, in general referred to as fluid- and rock-impedances, that comply with calibrated rock physics models. Two different approaches are presented, both of which attempt to separate out the effects of fluids and rock stiffness. The first set of attributes include the fluid saturation sensitive CPEI (Curved pseudo-elastic impedance for saturation), and the rock stiffness/lithology attribute PEIL (Pseudo-elastic impedance for lithology), which are near orthogonal to each other in a Vp/Vs versus acoustic impedance (AI) domain. The second set of attributes includes a porosity attribute derived from shear impedance, and the fluid deviation attribute from a background trend in acoustic impedance. The latter set of attributes shows almost perfect orthogonality for any burial depth. We demonstrate the use of these attributes on well log and seismic inversion data from the Norwegian Sea, and successfully screen out reservoir rocks filled with hydrocarbons.