Sulphur and oxygen sequestration of n-C-37 and n-C-38 unsaturated ketones in an immature kerogen and the release of their carbon skeletons during early stages of thermal maturation

Sedimentary rock from the Gessoso-solfifera Formation (Messinian) in the Vena del Gesso Basin (northern Italy) containing immature (R-0 = 0.25%) S-rich organic matter was artificially matured by hydrous pyrolysis at temperatures from 160 to 330 degrees C for 72 h to study the diagenetic fate of n-C-37 and n-C-38 di- and tri-unsaturated methyl and ethyl ketones (alkenones) biosynthesised by several prymnesiophyte algae. During early diagenesis, the alkenones are incorporated into the kerogen by both sulphur and oxygen cross-linking as indicated by chemical degradation experiments with the kerogen of the unheated sample. Heating at temperatures between 160 and 260 degrees C, which still represents early stages of thermal maturation, produces large amounts (up to 1 mg/g TOC) of S-bound, O-bound, and both S- and O-bound n-C-37 and n-C-38 skeletons, saturated n-C-37 and n-C-38 methyl, ethyl, and mid-chain ketones, C-37 and C-38 mid-chain 2,5-di-n-alkylthiophenes, C-37 and C-38 1,2-di-alkylbenzenes, and C-37 and C-38 n-alkanes. With increasing thermal maturation, three forms of the n-C-37 and n-C-38 skeletons are relatively stable (saturated hydrocarbons, 1,2-di-n-alkylbenzenes and saturated ketones), whereas the S- and O-bound skeletons are relatively labile. These results suggest that in natural situations saturated ketones with an n-C-37 and n-C-38 skeleton can be expected as well as the corresponding hydrocarbons. Copyright (C) 1997 Elsevier Science Ltd.