Mg isotopic geochemistry and origin of Early Ordovician dolomite and implications for the formation of high-quality reservoir in the Tabei area, Tarim Basin, NW China

Abstract

The source and driving mechanism of Mg-rich fluids in the dolomitization process of the Lower Ordovician Penglaiba Formation in the Tarim Basin have not been effectively traced, limiting the accuracy of the prediction of high-quality dolomite reservoirs. In this study, based on Mg isotopes combined with petrology, trace elements, and C-O-Sr isotopes, the powder crystalline dolomite (D1) and fine crystalline dolomite (D2) of the Penglaiba Formation were found to have high Na and Sr contents and low Fe and Mn contents. The REE, δ13C, δ18O and 87Sr/86Sr ratios ranges were similar to those of seawater, and δ26Mg shows a steady downward trend. The dolomitization fluids of D1 and D2 were derived from seawater and migrated vertically downward. Penecontemporaneous dolomitization is beneficial for the inheritance and preservation of original rock pores. The Na and Sr contents of medium crystalline dolomite (D3) and coarse rystalline dolomite (D4) were low, whereas the Fe and Mn contents were high. The REE, δ18O and 87Sr/86Sr ratios differed from those of seawater, and δ26Mg shows a fluctuating trend. The fluids for the precipitation of D3 and D4 were sourced from concentrated marine pore water during burial, , with the dolomitization fluid migrating through intergranular pores and structural fractures. Burial dolomitization has preservation, adjustment, and transformation effects on dolomite reservoirs. Deep high-quality dolomite reservoirs are not only controlled by the original sedimentary environment, but is also affected by late-stage or subsequent burial transformation.