A number of previously unknown or less common sterane and terpane hydrocarbons in crude oils of the Nepa-Botuoba anteclise (NBA) have been found by gas chromatography-mass spectrometry. The former include pregnane structures demethylated at C10 (principal fragment ion at m/z 203) and C26-C29 tricyclic secosteranes (m/z 219). By the m/z 123 mass chromatogram, a number of bicyclic sesquiterpanes with the drimane or tricyclic norisopimarane structure have been identified for the first time in NBA oils. The origin of these terpenoid hydrocarbons is usually associated with the involvement of higher plant debris in sedimentary source rocks. Their presence in Neoproterozoic oils suggests other precursors of these hydrocarbons. Like 10-desmethylpregnanes, 12- and 13-monomethylalkanes, which are unique biomarkers for Neoproterozoic oils of the Siberian platform, have been found in Vendian and Lower Cambrian oils of the Arabian platform and provide evidence for a close similarity between biotas that existed in these ancient basins.
