Geochemical characteristics and origin of dolomite: A case study from the middle assemblage of Ordovician Majiagou Formation Member 5 of the west of Jingbian Gas Field, Ordos Basin, North China

The trace elements, rare earth elements (REE), carbon and oxygen isotopic compositions and strontium isotopes were analyzed to study systematically the geochemical characteristics and origin of the middle assemblage dolomites from the Ordovician Majiagou Formation Member 5 in the Jingxi area (the west of Jingbian Gas Field), Ordos Basin. The analytical results of samples demonstrate that the MgO and CaO contents of the middle assemblage dolomite are negatively correlated, and the Mg/Ca values are high; the dolomites are characterized by high Fe content, relatively high Mn content and low Sr and Na content, while part dolomites have low Fe and Mn content but high Na content; the dolomites have very low REE content and similar REE distribution patterns, which are generally characterized by enrichment of light REE, losses of heavy REE, obviously negative anomaly of Eu and weak negative anomaly of Ce; The δ13C values of dolomites are consistent with those of the coeval seawater, while most of the δ18O values of dolomites are more negative than those of the coeval seawater; most of the 87Sr/86Sr values of dolomites are significantly higher than those of the coeval seawater while only a few are close to those of the coeval seawater. Combined with the geological setting and geochemical characteristics of the middle assemblage dolomites, the middle assemblage dolomites of the Majiagou Member 5 are probably the origin of seepage reflux dolomitization, and were superimposed by burial dolomitization during the burial period. The eastern part of the Jingxi area is favorable reservoir development area, the favorable area can extend eastward.