Geochemical characteristics of lacustrine carbonate rocks in lower part of first member of Shahejie Formation in Qijiawu area of Huanghua Depression, Bohai Bay Basin

Carbonate reservoirs dominate in the lower part of the first member of the Shahejie Formation in Qijiawu area of the Huanghua Depression, the Bohai Bay Basin. Through analyzing the chemical composition of rock, carbon and oxygen isotope and trace element of the core samples from over 10 wells, the results show that the contents of SiO₂, Al₂O₃ and CaO are relatively higher, while that of MgO varies with the differences of lithology. Meanwhile, SiO₂ is positively correlated with Al₂O₃ and K₂O + Na₂O. SiO₂ is negatively associated with MgO. The enrichment of SiO₂ shows that the sedimentary environment in the study area is lake carbonate facies. Carbon and oxygen isotope tests show that δ¹⁸O_PDB varies from -4.73‰ to -9.46‰ and δ¹³C_PDB varies from -0.08‰ to 7.71‰, which is in line with the carbon and oxygen isotope characteristics of limited lake, reflecting the salinization and dilution effects happened in the study area. Trace tests show that the composition and content of trace element are so complex, that is to say, the content of Sr is higher than 1 000×10^-6 in the limestone-based rock, but the content of Sr is less than 1 000×10^-6 in the limestone with higher argillaceous. The content of B mostly distributes in (7.77-122)×10^-6. For mud samples, the B/Ga ratio is greater than 4, and the Sr/Ba ratio is greater than 1. For carbonate samples, the Sr/Ba ratio ranges from 2.3 to 20.33, up to 33.98, and the B/Ga ratio is less than 10, mainly from 2 to 5, reflecting the alteration between freshwater and brackish sedimentary environments.