Geochemical characteristics and significance of aromatic hydrocarbons in the Upper Carboniferous mud shales from the Ermahu Sag, Wuwei Basin

The Carboniferous is a potential oil-bearing layer in Wuwei Basin, west China. The research on the related parameters of source rocks is relatively weak, which directly restricts the oil-gas exploration in the basin. A detailed analysis of aromatic compounds in the Upper Carboniferous mud shales from Wudi 1 well in the Ermahu sag, nestled in the northern of the Wuwei basin, has been executed utilizing gas chromatography-mass spectrometry (GC-MS) and other geochemical analytical techniques. The comprehensive assessment reveals that the extractable soluble organic matter within these mud shales is distinctly characterized by a prominent abundance of compounds, such as,phenanthrenes, naphthalenes, and fluorenes. Additionally, compounds such as 1,3,6-trimethylnaphthalene (1,3,6-TMN), 2,3,6-trimethylnaphthalene (2,3,6-TMN), 1,3,5,6-tetramethylnaphthalene (1,3,5,6-TeMN), and 1,3,6,7-tetramethylnaphthalene (1,3,6,7-TeMN) are also notably prevalent. Conversely, biphenyl, flexor, benzothiophene (DBT), and dibenzofuran occur in lesser concentrations, while trace quantities of 1,2,7-trimethylnaphthalene (1,2,7-TMN), 1,2,5-trimethylnaphthalene (1,2,5-TMN), and 1,2,5,6-tetramethylnaphthalene (1,2,5,6-TeMN) are detectable. The compounds parameters, encompassing alkylnaphthalenes, alkylphenanthrenes and alkyldibenzothiophenes, suggest that the mud shales originated from a transitional marine-continental sedimentary environment, marked by fluctuating oxidation-reduction conditions. Aquatic organisms serve as the primary biological source, with a substantial contribution from terrestrial organic matter. Furthermore, the mud shales indicate a relatively high maturity stage. According to the abundance, type, and maturity of organic matter in the Upper Carboniferous mud shales of this region, oil and gas exploration should prioritize the search for natural gas, with liquid petroleum as a secondary focus.