Applicability and Limitation of Aromatic Maturity Parameters in High-Maturity Oil from Ultradeep Reservoirs

A substantial reserve of highly mature light oil has been discovered in the Ordovician reservoir of the Shunbei oilfield, Tarim Basin, Northwest China. This study aims to provide an objective assessment of oil maturity in the Shunbei oilfield and establish a reliable maturity evaluation index for oil in the high thermal evolution stage. Eighteen crude oil samples from the no. 4 fault zone (F4) of the Shunbei oilfield were systematically analyzed. Aromatic hydrocarbon, light hydrocarbon components, and diamondoid compositions were analyzed by gas chromatography–mass spectrometry. The depositional environment and the organic matter input of their related source rocks were determined by the corresponding geochemical indicators. The results indicate that the F4 oils have been derived from the same source kitchen and belong to the same oil population. The oil maturity in the study area was evaluated by the maturity parameters relative to aromatics, light hydrocarbons, and diamondoids. The analysis indicates that the results of aromatic parameters for identifying oil maturity contradict those of light hydrocarbon and diamondoid parameters. The equivalent vitrinite reflectance of the oil samples ranges from 1.43% to 1.54%. It is concluded that light hydrocarbon and diamondoid parameters can reliably be used to evaluate the maturity of oil in the high thermal evolution stage. However, most of the aromatic maturity parameters exhibit significant limitations during the high-temperature evolution stage. The maturity parameters of phenanthrene and dibenzothiophene were “reversed” in the high thermal evolution stage due to demethylation and thermal alteration, respectively. Additionally, most naphthalene maturity parameters are affected by thermally induced condensation, rendering them unsuitable for maturity evaluation of the high-temperature evolution stage. Notably, pentamethylnaphthalene ratio [PMNr, PMNr = 1, 2, 4, 6, 7-/(1, 2, 4, 6, 7 + 1, 2, 3, 5, 6)-pentamethylnaphthalene] exhibits a strong positive correlation with gas–oil ratio, n-heptane/methylcyclohexane (nC7/MCH) ratio, and (3 + 4)-methyldiamantane concentrations, indicating their reliability as maturity indicators for highly to overmature oils and source rocks.