Maturation-induced modification of organic matter in shales: Implications for geological CO2 storage

Abstract

Shales have low to ultra-low porosity and permeability, which makes Bakken shales an attractive candidate for CO2 storage or as a caprock. The organic material within source rocks consists of extremely complex macromolecules with a host of other minor components. In this study, we experimentally investigated how the artificially-induced thermal maturation of organic matter in shales will impact geological CO2 storage (GCS). We artificially induced Bakken shale samples to thermal maturity (Tmax) for an exposure period of 0–48 hr, with incremental Tmax. Firstly, we conducted source rock analyses to evaluate and analyze the modified geochemical properties due to artificially-induced thermal maturation. Secondly, we utilized X-ray diffraction to assess the mineralogical changes in the immature and mature samples. Subsequently, we conducted scanning electron microscopy (SEM) for microstructural characterization of the morphological modifications and distribution of organic matter. Our post-treatment geochemical results showed a decrease (-42%) in the total organic carbon content (TOC) and a decrease (-26%) in quartz content, and these alterations continued with an increasing Tmax. The results of the microstructural analyses also suggest changes in the pore structure of the shale samples. Further, we assessed the implications of the modified geochemical and microstructural properties for GCS. The observed modifications in the properties of Bakken Shales due to the impact of artificially-induced thermal maturation of organic matter may improve its suitability as a caprock in conventional trapping scenarios, or as a CO2 storage unit.