Organic matter content and its role in shale porosity development with maturity: Insights from Baltic Basin Silurian shales

Abstract

Porosity, pore size distribution, and surface area are the main petrophysical characteristics indicative of gas storage capacity in shales. This paper investigates the influence of organic matter (OM) content on the evolution of these parameters at different stages of thermal maturity. Ninety-six samples of Silurian shales from the Baltic Basin ranging in maturity from immature to overmature were selected for this study. Porosity evolution was evaluated using N₂ and CO₂ low-pressure gas adsorption. At the immature stage, the samples with high OM content are characterized by lower porosity than the organic-matter-lean samples. At this stage, high OM content leads to the disruption of stiffer mineral framework; mixed organic-mineral framework is more prone to mechanical compaction than mineral framework. At the oil window stage, porosity of OM-rich samples declines due to pore-throat plugging and pore filling by bitumen. At the wet and dry gas generation stage, porosity of OM-rich samples increases mainly due to pore-throats unplugging. Absent or weak correlation between porosity and OM content at the wet and dry gas stages indicates poor development of OM-hosted secondary porosity. High contents of clay minerals in the studied samples fail to provide the rigid mineral framework and pressure shadows necessary for OM-hosted secondary porosity development.