'The petrophysical interpretation of turbiditic reservoirs assumes that the small grain fraction (clay minerals and particles) can be distributed within the rock in 3 different ways (laminar, dispersed and structural). These different types of clay distribution, or texture, affect the bulk and elastic properties of sand/clay mixtures differently. In this study, The Thomas-Stieber approach to petrophysical analysis of thin beds and the Dvorkin and Gutierrez Sand/Shale rock physics models are applied to predict compressional (Vp Dvorkin and Gutierrez have derived Sand/Shale models to describe bimodal grain mixing for laminated and dispersed shale/sand mixtures to predict Vp Fluids are introduced in each of the end members using Gassmann's fluid substitution. Laminated sand and shale grains are mixed using the Reuss average equation, while dispersed shale grains are mixed using the Hashin-Strikman lower bound model. The output is the bulk and shear moduli for a fluid saturated dispersed or laminar shale/sand mixture. From these elastic moduli and assuming a bulk density measurement, the shear and compressional wave velocities can be calculated.
We have applied this approach to a selection of laminated turbidite sands from different basins. The results show the technique has considerable potential but also significant limitations. These are explored and alternative techniques considered to better constrain the predictive capability for compressional and shear velocities in thin beds sediments.
Fluid substitution is used routinely to estimate the effect of different fluid saturations on the elastic properties of rock formations. Through this approach, often based on Gassman's equations, it is possible to improve well ties and confidence in modelling and seismic inversion. In this study, The Thomas-Stieber (1975) approach to petrophysical analysis of thin beds and the Dvorkin and Gutierrez (2001) Sand/Shale rock physics models are applied to predict compressional (Vp
The thin-beds dealt with in this model are typically the D layer of the Bouma sequence (Bouma, 1962); unit D is typically formed of thinly parallel laminated sand-shale sequences.
Although other components such as feldspar and shell fragments occur, sand grains and hence the mineral properties are considered to be pure quartz.) and shear (Vs) velocities in thin beds. We report the results of applying this technique to a number of thin-bedded datasets and explore the limitations to this approach.
and Vs. Assuming that the Thomas-Stieber approach gives an ideal porosity value for each of the pure sand and pure shale end members, these porosity values are then used as input to the rock physics model when modelling the bulk and shear moduli of the sand and shale. The bulk and shear moduli of the each of the dry end-members are calculated using the Hertz-Mindlin contact theory equations, which require grain coordination number, critical porosity of small and large grains, shear modulus of grain material, bulk modulus of the grain material (or Poisson's ratio for the grain material) and the effective pressure as inputs.
) and shear (Vs) velocities in thin beds.
