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The exceptionally low geothermal gradient (~2 °C/100 m) in the Tarim Basin, notably in the Shuntuoguole (STGL) area with depths exceeding 7000 m, has catalyzed the discovery of ultra-deep liquid hydrocarbons. Yet, the genesis of these hydrocarbons remains elusive, presenting significant challenges to further exploration and development endeavors. To address this quandary, an extensive analysis involving biomarker assessments and compound-specific δ2H and δ13C isotopic evaluations of n-alkanes was conducted, unveiling the secondary alterations and origins of these crude oils. For the first time, an approximately horizontal distribution of compound-specific δ2H and δ13C in n-alkanes was observed. Through the integration of diagnostic biomarker compounds, it was elucidated that the STGL crude oils did not undergo significant biodegradation, thermal alteration, or thermal sulfate reduction. Biomarker- and compound-specific C/H isotopic correlations suggest that the STGL crude oil predominantly originates from the Lower Cambrian calcareous shale, positing it as a potential end-member oil for this lithofacial source rock. Furthermore, the STGL crude oil exhibits similar biomarker configurations (notably abundant C28 triaromatic sterones, C23 tricyclic terpenoids, and aryl isoprenoids, with minimal 4-methylstane and gammacerane presence) to those oils previously considered as mixed from the Tazhong and Tabei Uplifts within the Tarim Basin. This similarity suggests that these previously deemed mixed oils may also derive from the Lower Cambrian calcareous shale. Consequently, more focused investigations into the Lower Cambrian calcareous shale are imperative to bolster refined deep and ultra-deep petroleum exploration efforts within the Tarim Basin in forthcoming studies. [ABSTRACT FROM AUTHOR]

Forty-six crude oil samples were selected from the Ordovician in the northwestern part of the Tahe oilfield for detailed molecular geochemical and isotopic analysis, including group compositions, carbon-hydrogen isotopes and gas chromatograms of saturated hydrocarbons, as well as the characteristics of terpane, sterane and other biomarkers, indicating that crude oils are of the same origin from different districts in the Tahe oilfield and were derived from the same source kitchen (or oil source formation), i.e., mainly stemming from marine hydrocarbons. Detailed studies of oil physical properties of 25-honpane revealed that such oils have heavy or thick oil qualities due to biodegradation. Comprehensive assessment in terms of five maturity parameters shows that the oils from the Ordovician with R values varying from 0.80% to 1.59% are widely distributed in the northwest of the Tahe oilfield. [ABSTRACT FROM AUTHOR]

The geochemical characteristics of natural gases discovered in the Bozhong Depression are systematically described in this paper. The natural gases are composed mainly of hydrocarbon gases. Natural gases occurring in the Paleogene and older reservoirs are wet gases, whereas those in the Neogene reservoirs are dry gases. Methane and ethane in the gases are significantly different in carbon isotopic composition. The methane carbon isotopic composition of the gases in structure BZ28-1 and the ethane carbon isotopic composition of the gases in structure QHD30-1 are characterized by the heaviest values, respectively. The natural gases are in the mature to highly mature stages. The hydrocarbon gases are of organic origin and can be classified as oil-type gases, coal-derived gases and mixed gases with the third one accounting for the major portion. [ABSTRACT FROM AUTHOR]

Oil samples from the Bongor Basin, SW Chad have been geochemically characterized to investigate the biodegradation influence on alkylphenanthrenes. Concentrations of C0–3-alkylphenanthrenes (C0–3Ps) increase markedly after level 6 biodegradation due to the removal of other vulnerable components, decrease sharply after level 7 biodegradation and approach to absence at level 8. Phenanthrene appears to have higher ability to resist biodegradation than C1–3Ps at certain biodegradation levels (≤level 7) due to demethylation, which has been inferred as a possible reaction process during biodegradation of the aromatic hydrocarbons. The enrichment of non-alkylated phenanthrene in biodegraded oils makes biodegradation assessment complicated on the basis of alkylphenanthrene distributions. Individual isomers in alkylphenanthrenes exhibit variable ability to resist biodegradation influence. While certain isomers do show higher ability to resist biodegradation than others, no uniform biodegradation sequence can be established. Meanwhile, the biodegradation susceptibility between hopanes and alkylphenanthrenes varies greatly in different samples. The biodegradation systematics of alkylphenanthrenes proves to be highly complex, which may be indicative of the multiple charges and mixing during biodegradation. [ABSTRACT FROM AUTHOR]

The Tuofutai Bulge in central Tabei of northern Tarim Basin provides insights into the complex geochemical processes of deep oil accumulations. Fifteen crude oil samples collected from deep Ordovician reservoirs were tested and analyzed with light hydrocarbons (LHs) and diamondoids to evaluate the oil family, secondary alteration, and mixing processes. Results show that the oil samples share similar distribution characteristics of LHs and diamondoids and contain low concentrations of methyldiamantanes, suggesting that they belong to one group and have not been significantly altered by thermal cracking. Geochemistry of LHs and diamondoids indicate that the thermochemical sulfate reduction is limited and negligible, whereas some samples are severely altered by biodegradation. The phase fractionation is mainly related to the leakage of light hydrocarbon caused by geological activities such as tectonic uplift and fault activity, and some samples show weak evaporative fractionation. The biodegradation causes the increase in the values of methyladamantane to adamantane (MAs/A) and the concentrations of (1 + 2)-methyladamantane and (3 + 4)-methyldiamantane. The thermal maturity of oil calculated on the basis of LHs and diamondoids is generally higher than that calculated using saturated and aromatic fractions. The difference of various maturity parameters related to saturated hydrocarbon, aromatic hydrocarbon, LHs and diamondoids records multiple oil charging and mixing processes. • Light hydrocarbons and diamondoids of the deep oils was revealed. • The deep oil suffered limited thermal cracking and weak evaporative fractionation. • Significant biodegradation was revealed in some oil samples. • Effect of biodegradation on diamondoid parameter was revealed. • Different molecular maturity indicators reflect multiple charging and mixing. [ABSTRACT FROM AUTHOR]