Abstract:

To constrain the diagenesis and property evolution of the Ordovician carbonate reservoir in southern Ordos basin, it is important to understand the origin and mechanism of dedolomitization there. Based on core and thin-section observations, and geochemical analysis, the dedolomitization mechanism and diagenetic evolution were determined by oxygen isotopes and trace elements. Subsequently, combined with the property data, the influence of different dedolomitization styles on the reservoirs was analyzed. Finally, the dedolomitization model of the study area was constructed. Combined with the carbon-oxygen isotopes and trace element data of the samples, we show that the dolomite with gypsum core in M5₁₊₂ is generally affected by freshwater leaching, yet some of them also underwent dedolomitization caused by freshwater leaching. The Th/U, Th/Cr, Cr/Zr, and Y/Ho ratios indicate that meteoric water may have dissolved granitoids and carried terrestrial argillaceous sediments into the strata, again due to freshwater leaching. Ca²⁺ source for the freshwater-leaching-related dedolomitization was likely from the dissolved evaporite (gypsum) and gypsum cores, whilst the {{\mathrm{CO}}_3}^{2-} may have sourced from calcite decomposition during the supergene dolomitization. And the main fluid migration pathways may have been dissolution fractures and suture zones. This process can be divided into 2 stages: replacement (supergene to shallow-burial stage) and cementation (shallow-burial stage). In this process, freshwater rich in Ca²⁺ and {{\mathrm{CO}}_3}^{2-} may have flowed along the migration pathways, and infiltrated along intra-crystal cracks, resulting in the selective dissolution and dolomite replacement with equal volume of calcite, together with the infilling of intra-crystal pores, cracks and fractures by calcite cement, which is destructive to the reservoir.

Key words: dedolomitization, fresh water leaching, diagenetic model, Ordos Basin, Majiagou Formation