Distribution and isotopic signature of 2-alkyl-1,3,4-trimethylbenzenes in the Lower Paleozoic source rocks and oils of Tarim Basin: Implications for the oil-source correlation

Abstract

Series of 2-alkyl-1,3,4-trimethylbenzenes (ATMBs) were detected in most of crude oils and source rocks collected from various strata and locations of the Tarim Basin. They appeared to have heavy carbon isotopic signatures ( δ 13 C, up to ∼ −16‰) compared to those hydrocarbons from oxygenic phototrophic organisms, indicating that the unequivocal source of green sulfur bacteria (GSB) and photic zone euxinia (PZE) existed in the original environment. Considering the high paleoproductivity, the PZE may have enhanced the preservation of organic matter, which triggered the formation of extremely organic-rich source rocks with TOC up to 29.8% for the Lower Cambrian Yuertus Formation (Є 1 y ). The coexistence of ATMBs and the diagnostic products from secondary alterations (e.g., abundant 25-norhopanes, thiadiamondoids, and diamondoids) indicated a stronger ability of anti-second-alterations. Combined with the results of quantitatively de-convoluting mixed oil, the oil-source correlation based on ATMBs from a large number of Lower Paleozoic samples of the Tarim Basin suggested that the abundant deep crude oil resources contained a dominant contribution (>50%) from the Є 1 y source rocks. Therefore, the ATMBs, as diagnostic biomarkers indicating unequivocal precursors under special habitat conditions, might provide important insights for the exploration of deep Lower Paleozoic crude oils in the Tarim Basin.