SULFUR 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 t he Vena del Gesso Basin (northern Italy) containing immature (R-0 = 0. 25%) S-rich organic matter was artificially matured by hydrous pyrolys is at temperatures from 160 to 330 degrees C for 72 h to study the dia genetic fate of n-C-37 and n-C-38 di- and tri-unsaturated methyl and e thyl ketones (alkenones) biosynthesised by several prymnesiophyte alga e. During early diagenesis, the alkenones are incorporated into the ke rogen by both sulphur and oxygen cross-linking as indicated by chemica l degradation experiments with the kerogen of the unheated sample. Hea ting at temperatures between 160 and 260 degrees C, which still repres ents 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-ch ain ketones, C-37 and C-38 mid-chain 2,5-di-n-alkylthiophenes, C-37 an d C-38 1,2-di-alkylbenzenes, and C-37 and C-38 n-alkanes. With increas ing 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 rela tively labile. These results suggest that in natural situations satura ted ketones with an n-C-37 and n-C-38 skeleton can be expected as well as the corresponding hydrocarbons. Copyright (C) 1997 Elsevier Scienc e Ltd.