Rock‐physics‐based estimation of critical volume of shale and its effect on seismic and petrophysical properties: a North‐Sea example

Abstract

Knowledge of sand/shale ratio is important for predicting reservoir quality. Estimation of shale content in inter‐well regions from seismic data, however, remains to be a challenging problem. In this paper, a pore aspect ratio parameter is derived from well log based on the theory of poroelasticity to study the inter‐relationships among shale content, pore aspect ratio and acoustic velocity. It is found that this parameter could be used to quantitatively describe the evolution of pore aspect ratio with changes in the volume of shale within clastic reservoir rocks, using a publicly available dataset from the North Sea. The study reveals a good correlation among the pore aspect ratio parameter, volume of shale, and acoustic velocity. The porosity and volume of shale for the studied reservoir range from 2 to 36 percent and from 5 to 43%, respectively. Pore aspect ratio is relatively constant at 0.23 for volume of shale less than 32% with a significant decrease to 0.04 for volume of shale above 32% in the studied reservoir. The point of inflexion at 32% (volume of shale) is defined as the critical volume of shale. Much of the scatters in the velocity‐porosity cross‐plots are observed in the region where volume of shale is above this critical value with a relatively less scattered linear trend for values below it. Quantitative understanding of the dependence of seismic properties on clay content and pore aspect ratio could thus provide physical insights and additional means to delineate sand/shale ratio within shaly clastic reservoirs using seismic data.