Carbon isotopic compositions of 1,2,3,4-tetramethylbenzene in marine oil asphaltenes from the Tarim Basin: Evidence for the source formed in a strongly reducing environment

Abstract

Although 1-alkyl-2,3,6-trimethylbenzenes and a high relative amount of 1,2,3,4-tetramethylbenzene have been detected in marine oils and oil asphaltenes from Tabei uplift in the Tarim Basin, their biological sources are not determined. This paper deals with the molecular characteristics of typical marine oil asphaltenes from Tabei and Tazhong uplift in the Tarim Basin and the stable carbon isotopic signatures of individual compounds in the pyrolysates of these asphaltenes using flash pyrolysis-gas chromatograph-mass spectrometer (PY-GC-MS) and gas chromatograph-stable isotope ratio mass spectrometer (GC-C-IRMS), respectively. Relatively abundant 1,2,3,4-tetramethylbenzene is detected in the pyrolysates of these marine oil asphaltenes from the Tarim Basin. δ 13 C values of 1,2,3,4-tetramethylbenzene in the pyrolysates of oil asphaltenes vary from −19.6‰ to −24.0‰, while those of n-alkanes in the pyrolysates show a range from −33.2‰ to −35.1‰. The 1,2,3,4-tetramethylbenzene in the pyrolysates of oil asphaltenes proves to be significantly enriched in 13 C relative to n-alkanes in the pyrolysates and oil asphaltenes by 10.8‰-15.2‰ and 8.4‰-13.4‰, respectively. This result indicates a contribution from photosynthetic green sulfur bacteria Chlorobiaceae to relatively abundant 1,2,3,4-tetramethylbenzene in marine oil asphaltenes from the Tarim Basin. Hence, it can be speculated that the source of most marine oil asphaltenes from the Tarim Basin was formed in a strongly reducing water body enriched in H 2 S under euxinic conditions.

Carbon and hydrogen isotopic compositions of n -alkanes as a tool in petroleum exploration

“…One suite of oils also contains 25 nor-hopanes and features similar, relatively low δ 13 C values (<-33.5‰) in the saturate fraction n-alkanes and in the asphaltene-released n-alkanes. Moreover, the asphaltenes do not contain biodegraded residues (Jia et al 2008). This led to the conclusion that a biodegraded fluid was charged later by a non-biodegraded fluid.…”

Isotopic compositions and biomarkers in crude oils from the Tarim Basin: Oil maturity and oil mixing

“…13 C values of n-alkanes released by asphaltenes in some of these oils (DH4C, HD11C, LN58T and TZ401C) have been reported to range from À34‰ to À35‰ (Jia et al, 2008), values that agree well with those in the oils. Such a consistency between d 13 C values of n-alkanes released by asphaltenes and those of the corresponding oils suggests that asphaltenes could have the same source as n-alkanes in the oils (Odden et al, 2002).…”

“…The occurrence and formation of marine oils and reservoir bitumen in this basin attracted many geochemists' attention in the past decade due to the long, complex and deep burial geological history of oils Li et al, 2000Li et al, , 2010Zhang et al, 2000Zhang et al, , 2011Sun et al, 2003;Jia et al, 2008Jia et al, , 2010Cai et al, 2009;Pan and Liu, 2009;Yu et al, 2011Yu et al, , 2012. The two old marine oil source units identified in the basin are the Cambrian-Lower Ordovician and Middle-Upper Ordovician strata Zhang et al, 2000).…”

“…Biomarkers have been used as the main tool for the study of those oils and reservoir bitumens, and recently, compound specific carbon isotope compositions were employed (Jia et al, 2008(Jia et al, , 2010Li et al, 2010;Wang et al, 2010;Yu et al, 2012). By combining these methods, new information about oil genesis was acquired.…”

De-convoluting crude oil mixtures from Palaeozoic reservoirs in the Tabei Uplift, Tarim Basin, China

“…First, such basins are characterized by multiple reservoirs that are widely distributed both horizontally and vertically. The occurrence and formation of marine oils and reservoir bitumen in the Tarim Basin attracted attention in the past decade due to the long, complex and deep burial history of hydrocarbons Li et al, 2000Li et al, , 2010Zhang et al, 2000Zhang et al, , 2004Sun et al, 2003;Jia et al, 2008Jia et al, , 2010Cai et al, 2009a,b;Pan and Liu, 2009;Yu et al, 2011). Second, such basins contain a range of petroleum types, including dry gas, wet gas, condensate, light oil, heavy oil and solid bitumen.…”