Abstract:

The Late Cretaceous Qingshankou and Nenjiang Formations in the Songliao Basin, extensively deposited with dark mudstone, and interspersed with layers of dolomite and dolomite nodules, are regarded as important source rock strata.Studying important oil-bearing dolomite can help to understand the terrestrial paleoenvironment evolution and source rock reservoir characteristics.In this study, carbon and oxygen isotopes of dolomites from the Qingshankou Formation (the first member) in Niaohe section and the Nenjiang Formation (the first member) in Lijiatuozi section in the southeastern margin of Songliao Basin are analyzed to explore paleoenvironmental significance.The results show that δ¹³C values of dolomites in both sections are relatively high (δ¹³C_Niaohe =-5.89‰~0.78‰, δ¹³C_Lijiatuozi =-8.36‰~6.70‰), while δ¹⁸O values are relatively low (δ¹⁸O_Niaohe=-10.03‰~-7.84‰, δ¹⁸O_Lijiatuozi=-11.30‰~-3.82‰), and the correlation between δ¹³C and δ¹⁸O is high (R²>0.7). This demonstrates that the late Cretaceous dolomites in the Songliao Basin were the product of closed (semi-)brackish lacustrine system with a long retention time.According to the empirical equation Z=2.048×(δ¹³C+50)+1.498×(δ¹⁸O+50), the average values of dolomites in Qingshankou and Nenjiang Formations are 124 and 126.5, respectively, indicating that the dolomites may be affected by seawater intrusion during the formation process.Furthermore, we estimated the scale of transgression using a mixing model of seawater and freshwater oxygen isotopes.In conclusion, we consider that the Late Cretaceous lacustrine dolomite in the Songliao Basin was deposited in closed (semi-)brackish lacustrine system with a long retention time, under a transgressive background.

Key words: Songliao Basin, Cretaceous, dolomite, carbon and oxygen isotope, paleoenvironment