Rock (micro) properties that control pore geometry determine macroscopic properties of the porous medium. Establishing the relation between micro properties and macroscopic physical properties of a rock sample is an essential requirement in the understanding of fluid flow behaviour and saturation distribution in porous media and in the production of oil and gas from petroleum bearing reservoirs. If variations in rock types are ignored, laboratory measurements for predicting fluid flow may be misleading. Reservoir rock types (RRT) can be defined on the basis of combined petrophysical properties and geological description. However, in complex carbonate reservoirs it may not be straight forward to establish reservoir rock types due to reservoir heterogeneity, which complicates the task of reservoir description where reservoir properties tend to vary as a function of spatial locations both in vertical and areal directions. These rock property variations necessitate the establishment of accurate and detailed understanding of the geological heterogeneities and their impact on petrophysics and reservoir engineering. In special core analysis programs it is critical that a robust sampling strategy is devised so that samples are selected to be representative of the reservoir and to capture the heterogeneity. Samples provide data that directly go into reservoir models, and hence failure to select representative samples may have major impact on reserves and production calculations. The selection of sampling locations in the core can be best commenced on full diameter cores by high definition dual energy X-ray CT scanning, sedimentological description and conventional core analysis. In this presentation the dual energy X-ray CT scanning is introduced with special applications in carbonate and shale formations. From the X-ray data, produced at two different energy levels, continuous whole core scans are calibrated to produce bulk density (BD) and effective atomic number (Zeff) at half-millimetre spacing along core length. The Zeff parameter is related to the Photo Electric Factor (PEF), commonly used in well bore logging. At this resolution, 1 foot of core scanning produces around 700 CT images. The images show important geological features including fractures, vugs, bedding planes, laminations, fossils, bioturbation etc. The high-resolution X-ray computed BD and Zeff values can be used to interpret porosity, mineralogy and organic content (in shale). It will be shown that dual energy data logs and imaging can provide effective early geological information about the core while it is still in the barrel. This information gives lithology log along the core length that can match detailed geological descriptions conventionally acquired on slabbed cores. Such high resolution images and numerical data help in the geological description of the core, and eventually lead to more robust and representative sample selection. The technique provides unique capability for ensuring that the number of drilled samples is representative to the cored lithofacies by statistically linking plugs to the full diameter core. This is an upscaling feature of the methodology which proved to be of great importance in carbonate and shale formations around the globe.