Solvent extraction efficiency of an Eocene-aged organic-rich lacustrine shale

Abstract

To study nano-scale inorganic pore systems in shale plays, the removal of soluble organic matter (SOM) using solvent extraction is an important pre-treatment step to reduce the in-situ petroleum blockage effect in shale oil samples. However, factors controlling the extraction efficiency are not well-understood, making the reliability and consistency of this extraction process questionable. In this work, an oil window mature organic-rich lacustrine shale in the Bohai Bay Basin of East China was prepared in six different sample sizes (from 1 cm cubes down to <75 μm powder), extracted for different times (1 day–25 days), and analysed for chemical composition to assess the extraction efficiency and behaviour, in order to arrive at a consistent workflow for sample pre-treatment for pore structure characterization. In general, the smaller the sample is used and the longer the sample is extracted, the higher the extract yield is obtained. The maximum extraction efficiency (based on the amount of extract after 25 days for the powder sample) is highly correlated with the sample size, indicating that there would always be residual soluble material in larger-sized samples even after extraction for a month. Molecular examination of the extract reveals that the early extract is dominated by saturated hydrocarbons, and these in turn are enriched in n-alkanes, while the saturated hydrocarbon content declines relative to aromatic and polar fractions, and the saturates are more dominated by branched and cyclic hydrocarbons, as extraction time increases. Such a knowledge about the extraction time- and sample size-dependencies of the extraction rate are fundamentally important in designing the solvent extraction protocol prior to carrying out nano-petrophysical studies.