Porosity evolution in oil-prone source rocks

Abstract

The origin of porosity and mechanisms of fluid flow in the presence of organic matter and clay minerals in source rocks are poorly understood. Burial and maturation of the source rock modify or create the pore systems in these rocks. Kerogen decomposition and consequent shrinkage may change the load bearing state of the minerals and organic matter and affect pore system since early stages of maturation. Geochemical evidence confirms that the hydrocarbon expulsion process (i.e. primary migration) is not 100% efficient. Expulsion of hydrocarbon is mainly driven by (1) pressure increase in the source rock due to solid kerogen conversion and volume increase and (2) continued compaction of the sediment. Converted organic matter is partly retained in the source rock diverse framework constituents. Quantitative measurement of and determining producibility of the retained hydrocarbon in the source rock is to date highly debated. The source rock hydrocarbon storage capacity is controlled by pore-hosting particles, pore system topology and rock–fluid interactions.The presence of organic matter and clay minerals affect log responses by generally overestimating porosity, because the low density kerogen is not accounted for, and together with low resistivity caused by presence of clay minerals can result in erroneous saturation calculations; thus, accurate reserve estimation often is challenged if the impact of low organic matter density is not explicitly addressed.In order to understand porosity evolution and the interaction of organic byproducts (i.e. bitumen and pyrobitumen) with rock minerals during thermal maturation, one must study source rock samples with different maturities. For this reason, ten Bakken Shale samples with varying maturity and mineralogy were selected in this study. Pore size distributions (PSD), specific surface areas (SSA) and geochemical characteristics of the samples were measured in native state and after successive solvent extraction.The PSD and SSA measured after each extraction shows recovery of the pore system with successive cleaning. Most significant was the recovery of kerogen-hosted pores with removal of soluble, oil-like organic material. Using successive extractions we are able to determine the evolution of organic matter porosity through maturation which is otherwise not feasible using visual techniques or other conventional laboratory procedures.