Paleoenvironmental reconstruction of the Middle Ordovician Majiagou Formation in the northeastern Ordos Basin and its effect on organic matter enrichment

The change of paleoenvironment is an important factor affecting the change of hydrocarbon-forming organisms and the preservation of organic matter. Combined with global environmental changes, the study of Ordovician paleoenvironment in Ordos Basin is of great significance to guide oil and gas exploration. Taking the marine carbonate rocks of the Middle Ordovician Majiagou Formation in the Daniudi area of the northeastern Ordos Basin as an example, according to the contents of major, trace and rare earth elements, as well as the characteristics of carbon and oxygen isotope composition, the paleoenvironment was reconstructed and its influence on the enrichment of organic matter was analyzed. The results show that carbonate minerals are mainly developed in the Majiagou Formation, detrital minerals are mainly developed in the first member of Majiagou Formation, the Majiagou Formation is in a suboxic-anoxic environment as a whole, and the seawater paleotemperature shows a gradual upward trend, which corresponds to the global seawater temperature change trend from the Darriwilian to Dapingian 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.