Simultaneous model-based inversion for lithology, porosity, and fluid

Abstract

The three main variables that determine the elastic response of sediment within a clastic environment are porosity, mineralogy, and pore fluid. The number of the elastic variables offered by seismic data does not exceed two (e.g., the impedance and Poisson?s ratio). Therefore, resolving seismic data for reservoir properties is formally unattainable, even if a perfect elastic rock physics model exists. We solve this problem by utilising an additional petrophysical link, the one between the clay content and total porosity, which is often observed in a dispersed-shale environment. First, we establish a rock physics model. Next, we identify the reservoir through a combination of impedance and Poisson?s ratio. Then, within the reservoir, we invert the P-wave impedance for both the total porosity and clay content by using two relations: (a) the rock physics model that links the P-wave impedance to the total porosity and clay content; and (b) the petrophysical model that links the clay content to total porosity. The results are accurate when the method is applied directly to well-log data. Most importantly, we obtain accurate reservoir property estimates by using the method with upscaled well-log curves, which demonstrates its relevance to real seismic data.