(540i) Multi-Technique Porosity and Pore Size Distribution Comparisons for Source Rocks

Mature source rocks consist of a complex pore space where nanoscale pores are hosted by a heterogeneous matrix with varying minerals and organic components to generate a natural nanocomposite with chemical and textural heterogeneity. Organic and inorganic nanopores in these rocks both store and transport hydrocarbons that are generated during thermal maturation of the rock. Thus, accurate porosity and pore size distribution (PSD) estimation are important to determining reservoir quality and producibility. The amorphous nanometer-sized pores, broad PSD, limited pore connectivity, and sparsely distributed porosity make accurate laboratory measurements of porosity, PSD, and fluid flow challenging. Gas Research Institute (GRI) analysis, mercury injection capillary porosimetry (MICP), and scanning electron microscopy (SEM) imaging are common techniques used in the hydrocarbon industry for characterizing porosity and PSD. However, the porosity and PSD measured by these techniques may be impacted by the type and size of the sample, as well as the sample preparation process. In addition, assumptions made during data interpretation and applicable range of pore sizes may also affect the measured properties. To demonstrate the impact of these variations on mature source rock samples, a multi-method analysis of porosity and PSD is presented. Porosity was measured by all three techniques and PSD was measured with both MICP and LgFOV SEM. The results show that the porosity measured by GRI on crushed and solvent cleaned rock fragments was the highest while that measured by LgFOV SEM (~800 um x 200 um) on small ion milled companion samples was the lowest. MICP was performed on uncleaned core plugs (1” D x 1” H) and crushed samples to a maximum pressure was 60k psi. The MICP-measured porosity on the crushed samples was higher than the porosity of the plug samples, and both were lower than GRI by ~2 porosity units (pu). Finally, the PSD from both MICP sample types was shifted to smaller pore sizes than that from LgFOV SEM. The observed differences in porosity and PSD will be discussed in context of the impact of data interpretation, sample preparation, sample heterogeneity, and induced damage. The results indicate that instead of employing a singular method, a combination of techniques is needed to effectively characterize the nanoporous paths in these highly heterogeneous natural materials. Finally, more effective characterization of the pore structures improves understanding of fluid flow in these rocks toward developing simplified structure-property relationships.