Abstract
The alteration of paleoenvironment is a crucial factor influencing the variations in hydrocarbon-forming organisms and the preservation of organic matter. This study focuses on the marine carbonate rocks of the Middle Ordovician Majiagou Formation located in the Daniudi area of the northeastern Ordos Basin. By analyzing major, trace, and rare earth element concentrations alongside carbon and oxygen isotope composition characteristics, we reconstructed the paleoenvironment and examined its impact on organic matter enrichment. The findings indicate that carbonate minerals are predominantly developed within the Majiagou Formation, which exists primarily in a suboxic-anoxic environment. Furthermore, paleotemperature data for seawater reveals a gradual upward trend that aligns with global seawater temperature changes from the Darriwilian to Dapingian stages of the Middle Ordovician. There are differences in the paleoenvironment of the Majiagou Formation in the northeastern part of the basin in different periods, which is an important factor affecting the enrichment of organic matter. The paleoenvironment during the deposition of the first member of the Majiagou Formation is more conducive to the enrichment of organic matter and is an effective source rock development interval.