Oil source and charge in the Wuerxun Depression, Hailar Basin, northeast China: A chemometric study

One-hundred-fifty-four samples collected from the Wuerxun Depression, Hailar Basin, northeast China were analyzed using GC-MS. The K1n Formation (including K1n2 and K1n1 members) consists mainly of argillaceous rock with relatively abundant organic matter and is considered to be the best source rock in the study area, whereas the K1t Formation and K1d1 member (Damoguaihe Formation) provide a limited contribution to the Wuerxun oils. Chemometric methods, i.e., principal component analysis (PCA) and multidimensional scaling (MDS), were used for oil-oil and oil-source rock correlations in this study. Based on PCA and geological evidence, the oils from northern Wuerxun originated primarily from source rocks of the K1n1 member that were charged ca. 100 Ma, whereas those in the southern Wuerxun form separate genetic groups I and II. When combined with trend surface analysis, the MDS technique also revealed variations in the depositional conditions and the maturity of the source rocks and oils. Group I oils, which have relatively higher maturity, are characterized by relatively high saturate/aromatic hydrocarbon ratios, high Pr/Ph ratios (>1), high Ts/(Ts + Tm) ratios, a slight predominance of C29 regular steranes, and more positive δ13C values of saturated and aromatic hydrocarbon fractions, whereas, the lower maturity group II oils have relatively low saturate/aromatic hydrocarbon ratios, low Pr/Ph ratios (<1), low Ts/(Ts + Tm) ratios, high C29 regular steranes and more negative δ13C values of saturated and aromatic hydrocarbon fractions. Analysis indicates that group I oils represent mixtures generated from the K1n1 and K1n2 members during the late hydrocarbon generation stage (ca. 25 Ma), whereas group II oils were primarily generated from the K1n1 member during the early oil generation stage (ca. 93 Ma). The distributions of group I and group II oils are mostly controlled by tectonic development in the southern Wuerxun Depression.