Obtaining Grain Density of Carbonate Rocks: Core Plugs and Powder Samples

Abstract Rock matrix grain density is a basic measurement of routine core analysis used as an input for log quality control and interpretation. It has been reported repeatedly in the literature that laboratory measured grain density of carbonate rocks is systematically lower than the commonly defaulted values. Three hypotheses were formulated as to the root causes of this observation; the cores may not have been fully cleaned and dried, the cores could contain other minerals other than pure carbonates, and there may be presence of isolated/unconnected porosity. The objective of this study is to design an experimental study try to resolve this challenge in routine core analysis. In this study, we measured the basic rock properties, including porosity and grain density, of 24 outcrop carbonates core samples. The measured porosity and grain density of the samples ranged between 15%- 33% and 2.670- 2.830 g/cm3, respectively. To investigate the effect of drying, samples were subjected to extensive high temperature vacuum drying before measurement of grain density. To evaluate the effect of isolated pores, thin sections and scanning electron microscope (SEM) images were acquired and analyzed for porosity types. Finally, the samples were crushed to powder and powder grain density and mineralogy by using X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) measurements were performed. Results indicate that drying the samples at up to 190°C did not change the grain density. This illustrates that the difference between the measured grain density and that often defaulted is not due to incomplete drying. Mineralogical analysis using XRD and FTIR confirmed the presence of other minerals with lower grain densities, such as quartz and clays. Synthetic grain densities calculated from mineralogy results are higher than those measured from core plugs but still fall short of the expected often defaulted values. Thus, rock mineral impurities may only play part of the role. Finally, thin section and SEM images showed the presence of inaccessible isolated pores. If exists, it will lower the matrix density. The results and findings of this study should help to resolve a long-standing issue in core analysis and lead to a better understanding of why the measured grain density of carbonate rocks tends to be lower than textbook values, attributing to rock mineral impurities and existence of isolated pores.