Application of X-Ray diffraction analysis for sedimentological investigation of Middle Devonian dolomites from Northeastern Bulgaria

The studied Middle Devonian (Givetian) dolomites occur in three well sections: OP-2 Mihalich, R-119 Kardam and R-1 Vaklino (Northeastern Bulgaria). Two general genetic dolomite groups are distinguished and interpreted on the basis of performed XRD analyses and petrographic observations. The first one is represented by early diagenetic crypto- to microcrystalline dolostones that are characterized by nearly stoichiometric composition (from 51.0 to 51.7 mole % CaCO₃) and degree of order ranging from 0.50 to 0.91. They are interpreted as products of rapid precipitation in an arid peritidal (sabkha) environment with hypersalinity of the water milieu and elevated Mg/Ca ratio of the dolomitizing solutions. The second group includes late diagenetic medium crystalline dolostones that have almost stoichiometric composition (from 50.0 to 51.3 mole % CaCO₃) and higher degree of order (from 0.77 to 1.18) indicating a possible slow crystallization process and precipitation from dilute solutions at elevated temperatures in an open diagenetic system. A part of them might have also resulted from neomorphic alteration of precursor metastable dolomite phases.