Pyrolytically Derived Polycyclic Aromatic Hydrocarbons in Marine Oils from the Tarim Basin, NW China

Abstract

Polycyclic aromatic hydrocarbons (PAHs) in oils from the cratonic region of the Tarim Basin have been characterized by gas chromatography–mass spectroscopy to investigate their origins and geochemical applications. PAHs dominated by unsubstituted components together with pyrolytically derived components such as anthracene, fluoranthene, benzoanthracene, benzofluoranthenes and benzopyrenes occur in some oils represented by those from well TD2. The widespread spatial and temporal occurrence of igneous activity within the Palaeozoic succession in the Tarim Basin makes the derivation of the latter PAHs from pyrolytic sources probable. Exposure to high temperatures results in the pyrolysis of the oil reservoir, adding high-temperature aromatic artifacts to the oil. Geochemical measurements show the wide occurrence of mixing processes between pyrolytically derived PAHs and diagenetically derived hydrocarbons. Good preservation of pyrolytic PAHs in oils from well TD2 and a few other wells is because they are less affected by late charge and dilution processes. However, these pyrolytically derived PAHs in conjunction with the bulk carbon isotopic compositions have been commonly misinterpreted in oil-source correlations. Only oils from well TD2 and a few other wells have been correlated to the Cambrian source rocks because of the wrong selection of end members. When the effects from the rapid heating of organic material by magmatic intrusions and/or hydrothermal fluid activity are properly accounted for, confusion in oil-source correlations may potentially be resolved and the contribution from the Cambrian strata can appropriately be assessed.