Nuclear Parameters Of Arab-D Carbonates For Enhanced Formation Evaluation

When determining reservoir lithology and porosity using density-neutron cross-plot charts, it is required to know the end points of reservoir rock (sandstone, limestone, and dolostone) sigma, matrix density, and neutron response. If this information is not available, it is a common practice to assume that the reservoir rocks are pure minerals, thus the default values of quartz, calcite, and dolomite are used in log interpretations. Obviously such assumed reservoir parameters are less accurate since reservoir rocks are rarely pure minerals. This is especially important for giant reservoirs since a small error in porosity can be equivalent to a large quantity of hydrocarbon. This study is aimed to reduce this systematic error in reservoir porosity and lithology determination by performing nuclear parameter modeling using laboratory data of reservoir cores. In this study, 99 Arab-D carbonate samples from different wells in a large Saudi Arabia oil field were selected. After completion of conventional core analysis in the laboratory, the samples were crushed into powders for mineralogy and elemental chemistry analyses. Subsample sets of calcite, dolomite, and anhydrite were obtained based on X-ray diffraction (XRD) and Fourier transform infrared (FTIR) mineralogy measurements. All the core and powder data were used for nuclear parameter modeling to obtain parameters of sigma, matrix density, photo-electron factor (Pef), and thermal neutron porosity. Instead of using the default end-points values of pure minerals, the field specific nuclear parameters should be used in routine field log processing. This study demonstrated the importance of field specific core-log calibration for enhanced formation evaluation.