Crocetane: A potential marker of photic zone euxinia in thermally mature sediments and crude oils of Devonian age

Crocetane, an irregular C20 isoprenoid, in sediments generally associated with gas hydrate settings, is a molecular indicator for the anaerobic oxidation of methane. Green sulfur bacteria (GSB) are the main source for specific aromatic carotenoids (e.g. isorenieratane) in sedimentary environments and are molecular indicators of photic zone euxinia (PZE). The present study comprises a detailed molecular and isotopic study of crocetane and GSB derived carotenoids in Devonian sediments of the Western Canada Sedimentary Basin (WCSB) covering a range of thermal maturities. In addition, a series of oils generated from Devonian source rocks of the basin have been analysed for crocetane. Crocetane was found in ten sediments from the WCSB and in seven Devonian WCSB crude oils. Its abundance was found to increase with thermal maturity, whereas the components derived from C40 derived carotenoids decrease steadily. We were not able to identify 2, 6, 10, 15, 19-pentamethylicosane (PMI) neither in the Duvernay Formation sediments nor in the WCSB crude oils, consistent with previous findings. The preferred proposed natural product precursor for crocetane is thus GSB derived carotenoids. This is corroborated by their similar structural features and the δ13C value of combined crocetane and phytane in these samples. However, contributions from other carotenoids cannot be fully excluded. Crocetane can provide evidence for PZE conditions in highly mature samples and crude oils of Devonian age.