posted on 2019-08-19, 12:42authored byQianru 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.