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

Micro-scale sealed vessel (MSSV) pyrolysis experiments have been conducted at temperatures of 150, 100, 250, 300,330 and 350 degrees C for various tim es 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 alky lthiophenes usually encountered in the flash pyrolysates of sulphur-rich ke rogens are also produced at much lower pyrolysis temperatures (i.e. as low as 150 degrees C) as the major (apart from hydrogen sulfide) sulphur-contai ning pyrolysis products. MSSV pyrolysis of a long-chain alkylthiophene and an alkylbenzene indicated that at 300 degrees C for 72 h no beta-cleavage l eading to generation of LMW alkylated thiophenes and benzene occurs. In com bination with the substantial production of LMW alkylthiophenes with a line ar carbon skeleton at these conditions, this indicated that these thiophene s are predominantly formed by thermal degradation of multiple (poly)sulfide -bound linear C-5-C-7 skeletons. which probably mainly originate from sulph urisation of carbohydrates during early diagenesis. LMW alkylthiophenes wit h linear carbon skeletons seem to be unstable at MSSV pyrolysis temperature s of greater than or equal to 330 degrees C either due to thermal degradati on or to methyl transfer reactions. LMW alkylthiophenes with a branched car bon skeleton most likely derive from both multiple (poly)sulfide-bound bran ched C-5-C-7 skeletons and alkylthiophene units present in the kerogen. (C) 1998 Elsevier Science Ltd. All rights reserved.