Composition and geologic significance of carbon and oxygen isotopes in hydrocarbon source rocks, Dongying Sag, Bohai Bay Basin

Lacustrine hydrocarbon source rocks of the Shahejie Formation in the Dongying Sag were sampled, and the carbon and oxygen isotope composition of carbonates were analyzed. Combined with previous references and precise stratigraphic division, the isotope composition in different stratigraphic units and the evolution of ancient lacustrine environment were discussed. The δ¹³C and δ¹⁸O of carbonates in the lower section of the fourth member of Shahejie Formation are positively correlated, which indicated that the basin was hydrologically closed during the initial period of rifting. But the carbon and oxygen isotopes are relatively light on the whole, which indicated that the water body of the salt lake was not stable, and was frequently influenced by fresh water. The δ¹³C and δ¹⁸O of carbonates in the upper section of the fourth member of Shahejie Formation are relatively heavy and positively correlated, which indicated that the salt lake was closed and stable during the acceleration phase of rifting. The carbon and oxygen isotopes of carbonate rocks with different lithologies in the lower section of the third member of Shahejie Formation varied. The δ¹³C and δ¹⁸O of non-calcium shale vary very little and are highly concentrated, which implied that the brackish lacustrine environment during peak rifting period was very stable and hydrologically closed. The middle and lower sections of the third member of Shahejie Formation have similar properties to the non-calcium shale, which suggested that the former may have inherited the isotope features of the latter.