Pore fluid and porosity mapping from seismic

Abstract

We use rock physics to map pore fluid and porosity from seismic data in a vertical section between two wells. First, well log data are used to establish an effective-medium model that links the impedance to pore fluid and porosity. Next, stacked seismic data are used to produce P-wave impedance inversion. Finally, the rock physics transform is applied to the impedance section to identify pore fluid and produce a porosity section. For decades, the main use of seismic data has been to delineate sedimentary bodies and tectonic features in the subsurface. The mission of exploring inside the geological body is a relatively recent development. Mapping porosity, lithology and other reservoir bulk properties inside the geological body has become possible due to the recent dramatic improvement in seismic acquisition, imaging and inversion quality, as well as the accompanying advances in rock physics. Rock physics provides transforms between a reservoir's elastic properties and its bulk properties and conditions, including porosity, lithology, pore fluid and pore pressure. Such transforms are known as trends. Trends are built from controlled experiments where both the elastic and bulk properties of rock are measured on the same samples under the same conditions. The most commonly used source of such experimental data in modern rock physics is the borehole measurement. For example, an empirical impedance–porosity trend developed from sonic, density and porosity curves can be applied to a seismic acoustic impedance volume in order to map porosity in 3D. However, it is always advantageous not only to find an empirical trend but also to understand the physical laws that determine the trend or, in other words, find an appropriate effective-medium model. Such rationalization of an empirical trend by effective-medium modelling generalizes the trend, determines the domains of its applicability, and thus reduces the risk of using the trend outside the immediate data range. Here, we illustrate the rational-rock-physics approach by mapping porosity in a large producing gas/oil reservoir. Well data are used to establish a transform from impedance to porosity, based on rock-physics theory. This transform is then applied to a vertical impedance section obtained from stacked seismic data via inversion. The reservoir under examination consists of relatively soft sands. As a result, the acoustic impedance of the gas-saturated sand is much lower than that of the oil- and water-saturated sand. This large impedance difference allows us to identify the pore fluid from P-wave data only, without using offset information. As a result, we map both pore fluid and porosity, using only stacked seismic data.