Rock Physics Modeling of Shallow Marine Sediments in Eastern Continental Margins of India P. Dewangan 1 , G. Sriram 1 , and T. Ramprasad 1 1 National Institute of Oceanography Dona Paula, Goa-403004, India. ABSTRACT 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 then the same rock physics model can also 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. KEY WORDS: Rock physics, gas hydrates, effective medium model, elastic moduli, NGHP Expedition 01. INTRODUCTION 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 (V P and V S and density). It can also be used to determine the change in elastic properties of rocks due to variations in mineralogy, fracturing or digenesis (compaction, cementation, and dolomitization), the change in fluid type, saturation and pore pressure, and finally, any variation in the reservoir effective stress and temperature. In this paper, we are going to discuss a couple of rock physics theories to predict the velocities of elastic waves from the porosity and mineralogy compositions. 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 moduli for the intermediate porosity are calculated using lower Hashin-Shtrikman bound. 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 iso- stress 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. The EMM model can be extended to estimate the hydrate and free gas concentration in marine sediments from porosity and velocity profiles (Helgerud, 1999). Lee (2006) showed that shear wave velocity plays a prominent role in the estimation of hydrate and gas concentration and is dependent on the compaction factor of marine sediments. We attempt to establish the rock physics model that best fit the observed velocity profiles acquired in the KG and Mahanadi offshore basins and infer the strength and limitation of such rock physics theories. The EMM and wood’s models are applied to the log data collected onboard JOIDES Resolution during NGHP-Expedition- 01 in Krishna Godavari (KG) and Mahanadi offshore basin. Theory Effective Medium Model (Dvorkin et al., 1999): This model assumes that the elastic moduli of the shallow marine sediment lie between the moduli of dry sediment at the critical porosity and the other end point is of zero porosity. The moduli for the intermediate porosity are calculated using lower Hashin Shtrikman bound. The fluid-saturated moduli are calculated using Gassmann’s equation (1951). Porosity below critical: At porosity φ < φc the concentration of the pure solid phase in the rock is (1 −φ) / φc and that of the sphere pack phase is φ / φc. The dry frame bulk moduli K dry and shear moduli G dry is given by following formula ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ + + = − ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ + − + + = − ⎥ ⎥ ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎢ ⎢ ⎣ ⎡ + − + + = − − HM HM HM Hm HM c HM c Dry HM HM C HM HM c Dry G K G K G Z Z Z G Z G G G G K G K K 2 8 9 6 ; / 1 / ; 3 4 3 4 / 1 3 4 / 1 1 φ φ φ φ φ φ φ φ (1) Where 3 1 2 2 2 2 2 3 1 2 2 2 2 2 ) 1 ( 2 ) 1 ( 3 ) 2 ( 5 4 5 , ) 1 ( 18 ) 1 ( ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ − − − − = ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ − − = P G n G P G n K c HM c HM ν π φ ν ν ν π φ (2) K HM and G HM are elastic moduli at critical porosity, ø c is the critical porosity and G and ν are the shear modulus and Poisson’s ratio of the solid phase, respectively, P is the differential pressure; K, G, and ν are the bulk and shear moduli of the solid phase, and its Poisson's ratio, respectively; n is the average number of contacts per grain in the sphere pack. This number is between 7 and 9 (Mavko et al., 1998). The differential pressure is the difference between the lithostatic and hydrostatic pressures and can be expressed as, gD P w b ) ( ρ ρ − = (3) Where ρ b is the bulk density of the sediment; ρw is water density; g is the gravity acceleration; and D is depth below sea floor. The elastic constant of solid phase from those of individual mineral constituents is Author version: Proceedings of the Eighth (2009) ISOPE Ocean Mining Symposium, 2009; 34-36