Geochemical characteristics and genetic analysis of crude oils in Mosuowan area, Junggar Basin

Analysis of the biomarkers of saturated hydrocarbon and related compound specific carbon isotopes of crude oils were carried out for the research of origin of oils and controlling factors of oil properties in the Mosuowan area of the Junggar Basin, and mixed-source oil simulation was also carried out to reveal hydrocarbon accumulation in the study area. The carbon isotope of crude oil in the Pen 5 well block is relatively heavy, the Pr/Ph value is higher, between 1.4 and 1.9, and the index of adamantane isomerization is greater, indicating that the oil maturity of the Pen 5 well block is higher than that of the Pencan 2 well and had a more oxidative sedimental environment. The C7 light hydrocarbons of oil of the Mosuowan area are dominated by methylcyclohexane, with a content over 40%, and the distributions of regular steranes are dominated by C29 homologues with a content over 40%. Crude oils in the study area are thus indicated to be generated from terrestrial partial humus source rocks. The carbon isotope compositions and distribution of n-alkanes in the Pen 5 and Pencan 2 well blocks are similar, indicating that they were from the comparable sources, and they are generally similar to the source rocks in the Lower Wuerhe Formation. But the carbon isotope values of n-alkanes with different carbon numbers in the source rocks of the Lower Wuerhe Formation have a large span, reaching 4.6‰, while the carbon isotope values of n-alkanes with different carbon numbers of crude oil in the Mosuowan area have a smaller span, between 2.2‰ and 3.0‰. It is mainly caused by the different mixing ratio of crude oil. The quantitative calculation results of biomarkers in mixed-source oil showed that crude oil in the Mosuowan area was mainly derived from the late high-maturity source rocks in the Lower Wuerhe Formation, mixed with those from the early mature source rocks in the Fengcheng Formation. Among them, the high-maturity source rocks in the Lower Wuerhe Formation have a relatively greater contribution to crude oil in the Pen 5 well block, around 80%, and contribute about 60% to crude oil in the Pencan 2 well block. As a result, it is concluded that different proportions of mixed sources are the main reason which caused differences in the geochemical characteristics of crude oil in the Mosuowan area.