Sorting , Porosity , and Elasticity

Grain size distribution (sorting) is determined by deposition. It may affect sediment bulk and elastic properties in a non-linear and non-unique way. The quantification of these relations is important for seismic reservoir characterization, especially in geological settings where sand/shale (bimodal) mixtures are present. Below, effective-medium equations are introduced for calculating the porosity and elastic moduli of such bimodal granular mixtures. These equations can be used for theoretical mixing of sand and shale in dispersed or laminar modes and, ultimately, for seismic forward modeling and reservoir characterization. INTRODUCTION AND PROBLEM FORMULATION Grain size and grain size distribution are basic characteristics of sediment texture (Figure 1). They are determined by the depositional history and affect reservoir quality via two of the most important bulk properties–porosity and permeability. They also affect the elastic properties of the sediment. As a result, seismic attributes can be, in principle, interpreted in terms of texture and porosity. Figure 1. Thin sections of unconsolidated sand showing deteriorating sorting (from left to right). Images courtesy Norsk Hydro. The effect of texture on sediment bulk and elastic properties is not simple. Consider a water-saturated section of a Gulf Coast well at a depth of about 900 m (Figure 2). The