Origin of low-molecular-weight alkylthiophenes in pyrolysates of sulfur-rich kerogens as revealed by micro-scale sealed vessel pyrolysis

Abstract

Micro-scale sealed vessel (MSSV) pyrolysis experiments have been conducted at temperatures of 150, 200, 250, 300, 330 and 350¡ãC for various times on a thermally immature Type II-S kerogen from the Maastrichtian Jurf ed Darawish Oil Shale (Jordan) in order to study the origin of low-molecular-weight (LMW) alkylthiophenes. These experiments indicated that the LMW alkylthiophenes usually encountered in the flash pyrolysates of sulphur-rich kerogens are also produced at much lower pyrolysis temperatures (i.e. as low as 150¡ãC) as the major (apart from hydrogen sulfide) sulphur-containing pyrolysis products. MSSV pyrolysis of a long-chain alkylthiophene and an alkylbenzene indicated that at 300¡ãC for 72 h no -cleavage leading to generation of LMW alkylated thiophenes and benzene occurs. In combination with the substantial production of LMW alkylthiophenes with a linear carbon skeleton at these conditions, this indicated that these thiophenes are predominantly formed by thermal degradation of multiple (poly)sulfide-bound linear C5¨CC7 skeletons, which probably mainly originate from sulphurisation of carbohydrates during early diagenesis. LMW alkylthiophenes with linear carbon skeletons seem to be unstable at MSSV pyrolysis temperatures of ¡Ý330¡ãC either due to thermal degradation or to methyl transfer reactions. LMW alkylthiophenes with a branched carbon skeleton most likely derive from both multiple (poly