"Cost effective, efficient production of hydrocarbons depends on accurate reservoir description and reserves estimates, both of which depend on the application of the relevant petrophysical model. The thin bedded nature of deep water turbidities makes petrophysical evaluation of these sand-shale sequences challenging, especially when shale can exists as discrete beds and as dispersed and structural shale. To predict the reservoir performance it is essential to understand the nature of the shale distribution in the reservoir. The Thomas-Steiber cross plot is a common method used with gamma ray and porosity data to determine shale distribution. To our knowledge no equivalent method exists based on formation anisotropy measurements.
Laminated sand-shale reservoirs are characterized by a macroscopic anisotropy. Multi-component induction instruments and crossed-dipole shear-wave acoustic tools provide a direct measurement of macroscopic formation anisotropy and offer an additionally information to characterize shale distribution. In this paper, we explore both theoretically and with real data the computation of macroscopic anisotropy from elastic properties derived from logging tool data.
We have developed a model that allows a forward calculation of elastic wave velocities and anisotropy ratios as a function of sand-porosity, laminated and dispersed shale content, elastic properties of the sand matrix and the shale. Our calculations clearly demonstrate:-the different influences porosity and shale content have on the compressional and shear wave velocities-that the shear wave velocities depend on the shale distribution and that the difference in shear wave polarization can be related to the shale distribution-that compressional wave velocities are not very sensitive to shale distribution, perhaps explaining the success of the Castagna?s "mudrock-line" in explaining the velocity in terms of simple volumetric volume of shale measures. Our model can also be applied to electrical resistivity and hydraulic permeability.
We have applied the analysis to data from deep water depositional environments and found with the shear-wave based method that we are able to discriminate between laminated and dispersed shaly zones and provide an estimate of the sand reservoir properties. Crossplots of various elastic properties and shale content from nuclear logs demonstrate the separation of the two basic shale types in a manner similar to the classical Thomas-Steiber approach. In one example, the comparison of shear-wave anisotropy and resistivity anisotropy demonstrates the controlling influence of the ratio of the ?microscopic? properties (shear modulus, conductivity of the sand and (laminated shale components."
