Porosity evolution in oil-prone source rocks

Abstract

Summary: The origin of porosity and mechanisms of fluid flow at the presence of organic matter and clay minerals in source rocks is poorly understood. Burial and maturation of the source rock affects all types of pore systems in these fine-grained rocks. Kerogen decomposition and consequent shrinkage change the load bearing state of the minerals and organic matter. Porosity in the kerogen is generated since early stages of maturation but it is only preserved if the kerogen particles are sheltered in a supporting frame of load bearing minerals. Geologic evidence confirms that the expulsion process is not 100% efficient. The expulsion efficiency and its control factors are poorly constrained. Presence of clay minerals and the organic matter affect log responses by falsifying porosity and resistivity measurements. In order to understand porosity evolution and the interaction of byproducts and rock minerals during the course of maturation one must study source rock samples with different range of maturity. In this paper, we studied the evolution of porosity in self-sourcing mudrocks. We measured pore size distributions (PSD) and specific surface areas (SSD) in native state and after successive extraction of the source rocks with solvents of increasing polarities. The PSD and SSA measured after each washing show evolution of the pore system with successive cleaning. Most significant is the recovery of kerogen-hosted pores with removal of hydrocarbons. We found that kerogen-hosted pores were only recovered in the less clay rich samples. This observation confirms the significance of sheltering effect from load bearing minerals. These studies will help us to understand how the log responses must be interpreted at the presence of organic matter and clay minerals and develop practical workflows for integrating geochemical data with well logs for petrophysical characterization of self-sourcing systems.