Abstract
Sedimentary rock from the Gessoso-solfifera Formation (Messinian) in the Vena del Gesso Basin (northern Italy) containing immature (
R
o = 0.25%) S-rich organic matter was artificially matured by hydrous pyrolysis at temperatures from 160 to 330°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°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-C38 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-
n-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.