Characteristics and implications of carbon and oxygen isotope of lacustrine carbonate rocks from Es4s in Dongying Sag

This study investigates the characteristics and paleoenvironmental implications of carbon and oxygen isotopes in lacustrine carbonate rocks from the Es4s of the Paleogene Shahejie Formation in the Dongying Sag, Bohai Bay Basin, China. Although lacustrine carbonate reservoirs account for a small proportion globally, they are an important and underexplored reservoir type in continental rift basins like the Dongying Sag. Meanwhile, a systematic study of carbon and oxygen isotope geochemistry and paleoenvironmental controls of lacustrine carbonate reservoirs are still required for better understand the reservoir potential of carbonate rocks. Thirty-five carbonate rock samples were collected from 14 wells, and their major and trace elements, as well as carbon and oxygen isotopes, were analyzed. The carbon and oxygen isotope compositions suggest a stable and shallow-lake depositional environment during the Es4s period, with a relatively closed lake system. The paleosalinity index (Z) ranges from 117 to 136, consistent with a closed, evaporative lacustrine setting. The paleotemperature ranges from 12 to 39 °C, reflecting a relatively warm paleoclimate environment. Carbon (δ13C: −2.281‰ to + 5.300‰) and oxygen (δ18O: −11.367‰ to − 5.018‰) isotope ranges align with typical lacustrine carbonates. The negative δ18O trend suggests episodes of freshwater influx, while the overall positive and clustered δ13C values predominantly reflect carbon cycling within an evaporative, closed lake basin. This research offers new perspectives on the paleoenvironmental conditions and depositional processes of lacustrine carbonate rocks in the Dongying Sag and has important implications for understanding the formation and distribution of carbonate reservoirs in this region.