Comparative analysis and geological significance of kerogen isolated using open-system (palynological) versus chemically and volumetrically conservative closed-system methods

Abstract

Characterization of kerogen, the insoluble organic fraction of sedimentary rocks, is of key importance in investigations of the geochemistry of ancient depositional systems, paleo climates, basin thermal history, and petroleum generation systematics. Furthermore, exploitation of organic-rich oil shales as unconventional petroleum resources requires understanding of the physical chemistry of the kerogen conversion reactions and the coking behavior. These issues can be effectively addressed only if pure, representative kerogen is obtained from the rock matrix using chemically and volumetrically conservative methods that preclude losses, alteration, or fractionation of the organic matter. Isolation of representative and pure kerogen concentrates from the rock matrix is difficult. Many methods have been used over the years with various degrees of success. Most are based on traditional palynological processing approaches in open systems that commonly yield fractionated products, generally contaminated with pyrite and insoluble neo-fluorides. A few are based on complex sealed-system procedures that purport to conserve the entire kerogen with no fractionation, no alteration and no contamination. This study focuses on a comparative analysis of kerogens isolated with the conventional open-system approach (HCl/HF/heavy liquid separation) and kerogens concentrated with a fully automated, conservative, closed-system method (HCl/HF/CrCl 2) under an inert N 2 atmosphere. Source-rock samples of various richness, kerogen types, ages, depositional environments, lithologies and thermal histories were selected for kerogen isolation using the two approaches. Kerogen recovery efficiency, progress of the processes and quality of the isolated kerogens were monitored via mass balance calculations, organic elemental analysis and X-ray diffraction analysis. The inherent inadequacy in the open-system approach for geochemical research and the improved features of the chemically and volumetrically conservative method are documented.