Maturation Impact on Polyaromatic Hydrocarbons and Organosulfur Compounds in the Carboniferous Keluke Formation from Qaidam Basin, NW China

Abstract

A maturity sequence from the Keluke Formation of the Upper Carboniferous marine–continental transitional depositional environment in the Qaidam basin, NW China has been geochemically characterized by bulk and molecular compositions, especially the behavior of polyaromatic hydrocarbons and organosulfur compounds. Some commonly used maturity parameters such as the methylnaphthalene ratio (MNR), the dimethylnaphthalene ratio (DNR), the methylphenanthrene ratio (MPR), methylphenanthrene index-3 (MPI-3), dimethylphenanthrene index-2 (DMPI-2), the methyldibenzothiophene ratio (MDR) and dimethyldibenzothiophene (DMDBT) ratios (4,6-/1,6- + 1,8- + l,4-DMDBT and 2,4-/1,6- + 1,8- + l,4-DMDBT), Rock-Eval T max, and measured vitrinite reflectance (%R o) increase gradually with burial depth, whereas others such as the trimethylnaphthalene ratio (TMNr), the tetramethylnaphthalene ratio (TeMNr), MPI-1, and DMPI-1 show no correlation with these maturity indicators. The calculated equivalent R o values from MPR and MDR based on empirical correlation reported in the literature overestimate the maturity level. The degree of alkylation plays a dominant role in molecular compositional variation and maturity parameter validity, which is in turn controlled by the nature of organic input, depositional environment, and lithology rather than solely controlled by maturation. Dealkylation of alkylnaphthalenes at R o ∼ 1.0% removes most thermally unstable isomers from C3- and C4 homologues, which makes parameters based on them lose sensitivity. The proportion of phenanthrene varies greatly in the marine–continental transitional depositional system and the involvement of phenanthrene in the formulation makes MPI-1 and DMPI-1 fail to reflect the maturity level. Overestimation of the maturity level based on the degree of isomerization is caused by a high catalytic effect in the marine–continental transitional depositional system, which facilitates isomerization and dealkylation processes.

Experimental Investigation of Spontaneous Water Imbibition into Methane-Saturated Shales under Different Methane Pressures

“…The three shale cores used in this study were collected from the Hurleg Formation of the Upper Carboniferous in the eastern Qaidam Basin, China. Regional geological surveys and organic geochemical analysis indicate that the Carboniferous units have good potential for hydrocarbon production and thus exhibit good development prospects. − …”

Novel Thermal Maturity Parameters Derived from Alkylbiphenyls and Alkyldiphenylmethanes

“…Core samples were pulverized by swing mill, and extractable organic matter (EOM) of source rock was extracted by dichloromethane/methanol (93:7, v/v) for 72 h in a Soxhlet apparatus. The separation methods of saturated and aromatic hydrocarbons, resins, and asphaltenes are the same as those in the study by Li et al Gas chromatography–mass spectrometry (GC–MS) analysis was performed on an Agilent 5975C MSD system interfaced to an Agilent 7890A gas chromatograph. GC–MS settings are the same as those in the study by Li et al Identification of all MBP, EBP, DMBP, and alkyldiphenylmethanes were based on comparison of their relative retention time to reported literature data. ,,, The structures and numbering of alkylbiphenyl (I) and alkyldiphenylmethane (II) are shown in Appendix A of the Supporting Information.…”

“…The separation methods of saturated and aromatic hydrocarbons, resins, and asphaltenes are the same as those in the study by Li et al Gas chromatography–mass spectrometry (GC–MS) analysis was performed on an Agilent 5975C MSD system interfaced to an Agilent 7890A gas chromatograph. GC–MS settings are the same as those in the study by Li et al Identification of all MBP, EBP, DMBP, and alkyldiphenylmethanes were based on comparison of their relative retention time to reported literature data. ,,, The structures and numbering of alkylbiphenyl (I) and alkyldiphenylmethane (II) are shown in Appendix A of the Supporting Information. Phenanthrene- d 10 , an internal standard, was used to calculate absolute concentrations of alkylbiphenyls and alkyldiphenylmethanes, with a response factor of 1.0.…”

“…As a result of the collision between the Indian and Eurasian plates, the Qaidam basin has experienced significant uplift and erosion since Oligocene, contributing to the deviation of depth at present and historical periods. In the study area, carbonate and coal-bearing depositions are developed in the Carboniferous strata with a marine–continental transitional facies sequence. , The Lower Carboniferous contains Chuanshangou, Chengqianggou, and Huaitoutala formations, while the Upper Carboniferous includes the Keluke and Zabusagaxiu formations. The Keluke formation, including marl, dark-colored shale, carbonaceous shale, and coal, is the main organic-rich source rock in the Carboniferous strata, with a known thickness of up to 202.1 m ,− (Figure b).…”

Correlation of Maturity Parameters Derived from Methylphenanthrenes and Methyldibenzothiophenes in the Carboniferous Source Rocks from Qaidam Basin, NW China

“…depth/R o is consistent with maturity governing thermal stability difference between of 3-, 2-MP and 9-, 1-MP. Therefore, the involvement of phenanthrene makes MPI-1 invalid for maturity correlation [53].…”