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Oil sources and accumulations in complex oil reservoirs of superimposed basins are difficult to identify and recover because of multiple kitchens and multistage charging processes. Constraining the oil sources and characterizing their accumulation patterns is a critical challenge. In this study, 46 crude oils from the Upper Wuerhe Formation, a widely distributed oil reservoir in the Mahu Sag of the Junggar Basin, NW China, are geochemically characterized to assess their sources and spatial distribution. These oils are divided into four groups: group I oils (characterized by high β-carotane and gammacerane concentrations, low C21/C23 and (C19 + C20)/C23 tricyclic terpane ratios, and low Pr/Ph ratios); group II oils (characterized by low β-carotane and gammacerane concentrations, high C21/C23 and (C19 + C20)/C23 tricyclic terpane ratios, and moderate Pr/Ph ratios); group III oils (very high Pr/Ph ratio (>3), the lowest β-carotane and gammacerane concentrations, and low tricyclic terpane concentration, but relatively high C19 and C20 tricyclic terpane abundance); and group IV oils (refered to here as geochemically hybrid, and inferred to be sourced from mixed P1f and C/P1j source rocks). The integration of oil-source correlation and geological framework indicates that oils generated from multiple kitchens are more likely to be near-sourced accumulations, providing a plausible example for oil accumulation in a complex oil reservoir of superimposed basin with several sets of kitchens and multistage oil charging history. The results show that multisource and multistage hydrocarbon accumulation is a common characteristic of prolific superimposed basins. The oil accumulation pattern must be established by carefully characterizing sources and charging processes to reduce the risk of exploration activities. [ABSTRACT FROM AUTHOR]

The Upper Triassic Xujiahe Formation (T3x) is the most important terrigenous stratum in the Western Sichuan Depression; however, it is still insufficient for the understanding of molecular compositional characteristics of its source rocks. Based on the analysis of both saturated and aromatic fractions of extracts of source rocks from different members of the Xujiahe Formation, the indication is then discussed for sedimentary environment and thermal evolution of source rocks. The distribution of tricyclic terpanes (TTs) and dibenzothiophene/phenanthrene (DBT/P) ratio of the argillaceous source rocks in 2nd to 4th members of the Xujiahe Formation mainly display the signatures of typical lacustrine source rocks, and the relatively high gammacerane/C30 hopane (0.16-0.23), as well as 9-/1-methylphenanthrene (1.16-1.41) ratios suggest relatively high salinity of water body. The parameters such as methylphenanthrene index (MPI1), methylphenanthrene distribution fraction (F1), and methyldibenzothiophene distribution index (MDBI) are effective for the evaluation of thermal maturity, and they show positively and negatively linear correlation with maturity degree before and after Ro=1.35%. The distribution pattern of C27-C29 regular steranes, relative composition of three fluorine series, and methyldibenzothiophene/methyldibenzofuran ratios for high to over-maturity T3x source rocks are distorted due to the effect of thermal maturation.