Paleoenvironment of the Fifth Member of the Ordovician Majiagou Formation, Central Ordos Basin: Evidences From Trace Elements and C–O Isotopes

ABSTRACT Carbonate rocks hold significant importance in investigating depositional environments. This paper reconstructs the paleoenvironment of carbonate rocks of the Majiagou Formation in the Ordos Basin through the analysis of trace elements and carbon–oxygen isotopes. The results indicate that high Sr/Cu ratios, low Rb/Sr ratios and the high paleotemperature (28.3°C) all reveal that carbonate rocks of the Fifth member of Majiagou Formation (the Ma5 member) were formed under arid and hot climatic conditions. V/(V + Ni), V/Cr, Ni/Co and U/Th values indicate that the sedimentary environment of the Ma5 member was oxic–suboxic, likely due to the water‐retreat period during which the sedimentary water was relatively shallow and less reducing. High Sr/Ba ratios, high paleosalinity Z values and salinity S values all suggest that the carbonate rocks formed in a high‐salinity marine environment. Comprehensive studies show that during the deposition of the Ma5 stratum, the basement experienced uplift coupled with regressive sea‐level conditions. Meanwhile, the arid and hot climate, coupled with an oxic water environment, led to increased evaporation, elevated salinity and reduced sedimentation rates, making it unfavourable for the enrichment and preservation of organic matter. However, this evaporative environment promoted early penecontemporaneous dolomitization, enhancing intercrystalline porosity and permeability to develop high‐quality reservoirs in the Ma5 member. These findings provide a geochemical framework for identifying high‐quality carbonate reservoirs. These reservoirs offer storage space for natural gas, contributing to the formation of large‐scale gas deposits in the region.