Coupled effects of temperature and solution compositions on metasomatic dolomitization: Significance and implication for the formation mechanism of carbonate reservoir

Abstract

Dolomitization, as one of the most important diagenetic processes in carbonate reservoirs, is essential for reservoir porosity development and evolution. However, the geochemical mechanism of dolomitization is unclear. In this study, theoretical analysis, laboratory physical experiments, thermodynamic calculation and computer numerical simulation were combined to explore the coupled effects of temperature and solution compositions on dolomitization. A series of water–rock reaction experiments at various temperature were firstly carried out. X-ray diffraction (XRD), scanning electron microscope (SEM), ICP-MASS and other analytical methods were used to monitor ion concentration, relative content and surface morphology of all minerals. Secondly, molecular dynamics (MD) simulation and thermo-hydro-chemical (THC) coupling simulation were employed to analyze various processes of dissolution, precipitation and transformation of carbonate rock. Finally, combining thermodynamic calculation, coupled effects of temperature and solution compositions on dolomitization were determined and the optimum formation conditions of dolomite were confirmed. Dolomitization experienced a series of transitional processes from calcite → Mg-bearing calcite → high-calcium dolomite → low-calcium dolomite. At 60 ℃, 80 ℃ and 95 ℃, the equilibrium constants of dolomitization are 1.67, 1.75 and 1.80, respectively. Dolomite is formed when Mg/Ca ratio equals 5.3 (at 95 ℃) and the scope activity of Mg2+ is 32.91–54.52%. It would need a much higher activated Mg2+ or it may be impossible to form dolomite if the Mg/Ca ratio is less than 3. This study has revealed coupled effects of temperature and solution compositions on metasomatism dolomitization, which could provide a basis for evaluating and predicting high-quality carbonate reservoirs.