**2.4 Calcitization**

Sulphates, as well as gypsum and anhydrite can undergo calcification by:


Dissolution of sulphates in presence of hydrocarbons leads to biogenic SO4 reductions and calcite precipitation according to reactions:

$$\text{CH}\_4 + \text{CaSO}\_4 \rightarrow \text{CaCO}\_3 + \text{H}\_2\text{S} + \text{H}\_2\text{O}$$

Calcitization of the sulphates can be a multi-stage process (Scholle et al., 1992), which begins with (1) dissolution (or at least corrosion) of anhydrite, (2) hydration of anhydrite and gypsum formation, (3) dissolution of gypsum (this process can be accompanied by the formation of collapse breccia), and afterwards (4) precipitation of calcite inside free spaces and pores arisen after leached sulphates. Sometimes sulphur is the secondary product of calcitization of sulphates (see fig. 18.).

Generally, the gypsum – more easily than the anhydrite – can be substituted by calcite. In case where this process occurs in bigger scale, the post-gypsum limestones form. They can occur in the highest parts of the cap-rock, covering the upper parts of some diapirs - upon the area of Costal Gulf the shallowest subsurface cap-rock levels are usually formed as the calcitic deposits and therefore named as calcitic cap-rocks. However, the microscopic analyses of the cap-rock deposits demonstrated that among the secondary coarse-grained gypsum with the anhydrite remains, the calcification process starts exactly with these anhydrite inclusions, not with the gypsum.

### **2.5 Polyhalitization**

The sulphate rocks can also undergo the polyhalitization process. It proceeds during the early stages of the diagenesis of evaporites as a result of infiltration of hot brines into the sulphate deposits (in the peripheral zones of the evaporite basins): halite saturated, with high contents of Mg2+ and K+ (originating from the dissolution of the potassium salts in the local salt pans), sulphate-rich (Peryt, 1995 and 1996). This process starts from the edges of the grain/crystal and proceeds with deep embayments into the core – the anhydrite/gypsum grain disintegrates into smaller parts that undergo polyhalitization more easily (Stańczyk, 1970).
