Isotope characteristics of dolomite from the fifth member of the Ordo vician Majiagou Formation, the Ordos Basin

The dolomite from the fifth member of the Majiagou Formation in the Ordovician is a significant reservoir in the central gas field of the Ordos Basi n. The origin of the Majiagou Formation in the Ordovician has been a hot topic f or many years. The carbon, oxygen, strontium isotope compositions of dolomite de veloped in this deposit were determined in this study. The results show that the values of δ13C and δ18O of secondary void-filling dolomite (including saddle dolomite) are much similar to those of the dolomite. The δ18O value of the dol omite is about 1.752‰-4.395‰(averaged 2.911‰) lower than that of the Ordovician seawater. The averaged value of diagenetic fluid (water) was +8‰with respe ct to SMOW, higher than that of the Ordovician seawater. The 87Sr/86Sr ratio was higher than that of the Ordovician seawater. According to analysis, the fluid t hat generated dolomite and void-filling dolomite (including saddle dolomite) ma y come from the same fluid sources. The negative value of δ18O of dolomite was likely due to temperature effectof buried process. The high value of 87Sr/86 Sr may be explained by metasomatic fluids came from or flew though aluminosilica te basal rock or siliciclastic rock, which is rich in radioactive strontium. Tho se characteristics of dolomite from the fifth member of the Majiagou Formation w ere likely formed in the buried environment.