Improving Our Understanding of Porosity in Source Rock Reservoirs through Advanced Imaging Techniques

Abstract

Summary URTeC 1619700 Understanding porosity and its development is essential when characterizing source-rock reservoir quality. Using a field-emission scanning electron microscope (FESEM) on argon-ion polished shale samples, images show that most of the porosity occurs within organic matter (OM) and that the pore geometry and characteristics vary with increasing thermal maturity. Comparison of the OM in shales from numerous plays reveals that the OM is heterogeneous. High magnification FESEM images (>30,000X) show the OM is composed of spherical to cylindrical units, 7-45 nm in diameter dispersed in an organic matrix, where the porosity has developed within this arrangement. These subunits and pore structures are evident in rocks from low to high thermal maturity (e.g., %Ro equivalent 0.6 to >3). The common presence of OM with this texture in open pores and between clay platelets in larger, open, stress-protected pore bodies supports an interpretation that the OM was formed within intercrystalline space as a “colloidal-like” system. Observations suggest that with the onset of thermal maturation and degradation of the OM, molecular self-assembly took place through noncovalent interactions to form a colloidal-like system. Large pores and high organic porosity do not occur in all OM within a sample and OM with no significant visible porosity does occur in high-stress locations and in thin sheets between mineral grains. The presence of minerals within the OM may have an effect on visible organic porosity size and distribution.