Rock physics models are used to estimate the geo-technical properties such as the elastic moduli from the porosity and mineralogy datasets. If the velocity measurement is available the same rock physics model can be used to predict the saturation of gas or gas-hydrate based on the difference between the observed and modeled velocities. Several rock physics models are developed to predict the elastic moduli of shallow unconsolidated marine sediments as a function of porosity, mineralogy, effective pressure, and pore-fluid compressibility. Each of these theories has its own sets of assumptions and some site specific empirical constant. In this paper, we are going to discuss a couple of rock physics model and demonstrate with the help of well log examples from the Krishna-Godavari (KG) and Mahanadi basins the applicability and limitations of such models. In this process, we will also establish the rock physics model that is applicable to the eastern continental margin of India.
Rock physics modeling is performed for a variety of reasons, most commonly to estimate the saturation of oil/gas/water/hydrates and to construct missing acoustic and elastic logs (VP and VS and density). The first model is Effective Medium Model (EMM; Dvorkin et al., 1999) which assumes that the elastic moduli lie between the moduli of dry sediment at the critical porosity and zero porosity. The fluid-saturated moduli are estimated using Gassmann's equation. The main limitation of this model is that it requires a lot of apriori information and is computationally involved. The second model is Wood's model (Wood, 1941) which assumes that the elastic moduli of the sediment are an isostress average of the solid and fluid phase bulk moduli and shear modulus is zero. This model relates velocity to porosity for sediments that are in a state of suspension, i.e. having no frame stiffness.