Characteristics and genesis of marine hydrocarbons in the Halahatang Sag in the northern Tarim Basin

The Halahatang Sag is an important oil and gas unit in the western part of the Northern Tarim Uplift. The Ordovician carbonate reservoir contains abundant petroleum resources with multiple phases, but the variable characteristics and genetic mechanism of the oil and gas are still unclear. Oil-oil and oil-rock correlations indicate that the crude oil has a close genetic relationship with the middle and upper Ordovician source rocks. The comparison of carbon isotopes for individual n-alkanes shows that the Cambrian source rocks might also contribute. Halahatang oils are rich in dibenzothiophenes, and also contain relatively abundant C₂₉-and C₃₀-hopane, which implies a contribution from carbonate source rocks. Based on the Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR MS) analysis, sulfides with low thermal stability have low concentrations, suggesting that TSR alteration is not significant. Source rock maturity is the major factor controlling the genesis of oils with high gas/oil ratios, while biodegradation is the major factor for heavy oil and H₂S formation in the relatively shallow reservoir.