Determination of TOC in Oil Shale Application Note · TOC Solid Analyzer

Introduction

Oil shale is fine-grain sedimentary rock containing significant amounts of organic material in the form of kerogen and/or bitumen that can be used to produce the liquid and gaseous hydrocarbons. Oil shale is defined by its properties characterized by the low solubility of its organic content in low-boiling organic solvents and the generation of liquid (shale oil) or gaseous (oil shale gas) organic products by destructive distillation (retorting). Oil shale needs to be differentiated from bitumen-impregnated rocks (oil sands and petroleum reservoir rocks), humic coals and carbonaceous shale. While oil sands do originate from the biodegradation of oil, heat and pressure have not transformed the kerogen in oil shale into petroleum yet.

In addition to oil production, oil shale can also directly be burned in furnaces as a low-grade fuel for power generation and district heating. However, oil shale mining and combustion leads to significantly higher CO2 emissions compared to other fossil energies.

In order to determine the geological source rock quality for oil and gas production purposes, the total organic carbon (TOC) concentration is a critical parameter. Oil shale is typically constituted of at least 67% clay minerals, carbonate minerals and less than 1/3 of organic matter. A TOC value of 0.5% by weight is considered the minimum for a suitable source rock, whereas concentrations above 2% are already classified as good quality for oil production usage.

According to DIN EN 15936 (EN 13137 or ISO 10694 provide identical method definitions) two methods of TOC determination can be applied, the difference and the direct procedure. For the difference method two independent measurements are to be performed, once a combustion of the untreated sample for the determination of TC (Total Carbon) and in a second step the determination of the CO2 which is released by acid treatment of the sample, representing the TIC (Total Inorganic Carbon). The TOC content is then calculated by subtraction: TC – TIC = TOC. The direct TOC method requires only one measurement step plus a sample pretreatment step (acidification) for TIC removal.