Geochemistry and origin of the ultra-deep Ordovician oils in the Shunbei field, Tarim Basin, China: Implications on alteration and mixing

Recently, a large quantity of oil was discovered in the carbonate reservoirs of the Ordovician Yijianfang and Yingshan Formations(O2yj-O1-2y) with a burial depth of over 7200 m (21,600 feet) in the Shunbei area, and high-producing wells are present mainly in the vicinity of the SB1 and SB5 strike-slip faults. However, the origins of these oils remain controversial and the accumulation processes were poorly understood. This paper investigated the thermal maturity and estimated the cracking degree of the Shunbei oils, as well as possible oil origins based on quantitative gas chromatogram-mass spectrometry (GC-MS) of saturated and aromatic compounds, GC-MS of whole oil, stable carbon isotopes (δ13C) of n-alkanes, chemical and isotopic compositions of natural gas. In addition, field test data were compiled to characterize the compositional heterogeneity in the Shunbei field. Compared with oils from the SB5 reservoirs, oils in the SB1 reservoirs display relatively low density and viscosity, which indicates a relatively higher maturity for the SB1 oils. This is demonstrated by the saturated, aromatic, light hydrocarbon and diamondoid maturity parameters. The estimated equivalent vitrinite reflectance values from aromatic maturity parameters for the SB1 and SB5 (except for B4) oils were in the range of 0.9–1.53% and 0.77–0.86%, respectively. Aromatic hydrocarbon parameters and δ13C values of n-alkanes reveal that the Shunbei oils share quite similar oil source, possibly originated from the Cambrian source rocks. The concentrations of biomarkers and diamondoid compounds reveal the presence of oil cracking. However, the chemical and isotopic compositions of gas agree well with kerogen primary cracking gas, indicating a limited degree of oil cracking, namely the transformation of normal oil to light oil with no significant amount of gas generated. Low content of hydrogen sulfide and dibenzothiophene/phenanthrene ratio may suggest no or minor influence of thermochemical sulfate reduction (TSR) in the present Ordovician reservoir. Heterogeneities in petroleum compositions and maturity differences in the present reservoirs suggest that faults controlled the petroleum mixing (i.e., mixtures of the similar source with different maturities), and the north-east trending SB1 fault may facilitate the late-stage vertical migration of deep high maturity oil.