Source facies of the Paleozoic petroleum systems in the Tabei uplift, Tarim Basin, NW China: implications from aryl isoprenoids in crude oils

Aryl isoprenoids have been detected for the first time in crude oils from Paleozoic petroleum systems in the Tabei uplift, Tarim Basin, northwestern China. The principal compounds possess the 1-alkyl-2,3,6-trimethyl substitution pattern characteristic of diaromatic carotenoids found in the Chlorobiaceae family of photosynthetic sulfur bacteria, with a predominance Of C-13 - C-23 homologues in these samples. Flash pyrolysates of the asphaltene fractions isolated from crude oils show an unusual abundance of 1,2,3,4-tetramethylbenzene, an indicator for a significant contribution of diaromatic carotenoids to the source kerogen of these oils. The wide distribution of these compounds in crude oils reveals that penetration of the photic zone by sulfidic waters during deposition enabled prolific growth of photosynthetic green sulphur bacteria (Chlorobiaceae). This suggests that the source rocks for the Paleozoic petroleum systems in the Tabei uplift were likely deposited under euxinic conditions with sulfate and sulfide-rich water bodies, which doesn't support previously published conclusions of a Middle-Upper Ordovician source that marks the slope facies at the margins of structural uplifts. (C) 2003 Elsevier Science Ltd. All rights reserved.