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Novel Thermal Maturity Parameters Derived from Alkylbiphenyls and Alkyldiphenylmethanes

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journal contribution
posted on 2019-08-19, 12:42 authored by Qianru Wang, Haiping Huang, Zheng Li, Zongxing Li
A total of 13 Paleogene lacustrine source rocks from well LY1 in the Bohai Bay basin, East China, and 6 Carboniferous marine–continental transitional source rocks from well ZK5-1 in the Qaidam basin, Northwest China, have been analyzed by gas chromatography–mass spectrometry (GC–MS) to investigate the application of alkylbiphenyls and alkyldiphenylmethanes for maturity assessment. The Bohai Bay samples are marginally mature with vitrinite reflectance (Ro) values of below 0.8%, while the Qaidam samples are highly matured with Ro values of above 0.8%. In addition to validate some previous reported maturity parameters based on isomer distributions of methylbiphenyls and dimethylbiphenyls, several novel parameters based on isomer distributions, degree of alkylation, and compound types in alkylbiphenyls and alkyldiphenylmethanes have been provisionally proposed. These new parameters in the studied source rocks show good correlation with known maturity levels. Most maturity parameters are formed on the basis of relative stability of isomer substituent positions. Both meta and para substituents are confirmed to be more stable than ortho substituent counterparts, and various previously recognized maturity parameters, such as 3-/2-methylbiphenyl (MBP), 3,5-/2,5-dimethylbiphenyl (DMBP), and 3,4′-/(2,4- + 2,4′)-DMBP show linear correlation with measured Ro values. Noticeably, our limited data suggest that compounds with para substituents are even more stable than those with meta substituents, as evidenced by increasing ratios of 4-/3-MBP, 4-/3-EBP, 4,4′-/3,3′-DMBP, and 3,4′-/3,3′-DMBP with maturity levels. Ethylbiphenyl (EBP) seems less stable than the methylated isomer, and ratios of 4-MBP/4-EBP and 3-MBP/3-EBP can be used as potential maturity parameters. These two parameters are likely valid only at a high maturity range when the dealkylation process has been initiated. Diphenylmethane (DPM) and its alkylated homologues show overall lower thermal stability than alkylbiphenyls, and ratios of 4-MBP/4-MDPM and 3-MBP/3-MDPM increase with increasing maturity levels as well. The risk for this type of parameter is component concentration because concentrations of alkyldiphenylmethanes are usually below the minimal requirement for reliable parameter calculation. Nevertheless, a continuous increase of these novel parameters in data from the Bohai Bay and Qaidam basins may provide a powerful tool for maturity assessment of different maturated source rocks and oils in petroliferous basins, even though depositional facies may exert some impacts on the thermal evolution behavior of alkylbiphenyls and alkyldiphenylmethanes.

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