Dedolomityzacja w anhydrytach cechsztynu monokliny przedsudeckiej

Abstract

DEDOLOMITIZATION IN ZECHSTEIN ANHYDRITES FROM THE FORE-SUDETIC MONOCLINE Summary The paper deals with dedolomitization the effects of which were found in anhydrite rocks of the Werra and Stassfurt cyclothems of the Zechstein from the Fore-Sudetic Monocline. The studies covered rocks of "Basal Limestone", "Lower Anhydrite", "Top Anhydrite", "Main Dolomite" and "Basal Anhydrite" horizons. These horizons representing lithostratigraphic units are loosing their stratigraphic meaning in the light -of the studies revealing the presence of complexes -of carbonate rocks in anhydrite horizons. Limestone and dolomite intercalations are fairly common in anhydrites or rock salts and boundaries between different lithological types are highly blurred by continuous transitions and interfingings. Dedolomitization was found in anhydrites yielding carbonate accumulations of various types. Carbonate grains clearly marked in anhydrite groundmass exclusively represent secondary calcite formed of primary dolomite in result of dedolomitization processes. The alteration of dolomite into calcite is evidenced by some relics of dolomite in calcite grains or marked amounts of post-dolomite dust remaining in calcite (Figs. 2-6). It is stated that anhydrite is not plastic but rigid and characterized by excellent cube-like cleavage according to three dihedrons. A tendency to cleavage results in the fact that in anhydrite rock mass there found very numerous microfissures and structural loosenings inside and between well-crystallized grains (Fig. 7). That is why anhydrites are not very compact but highly permeable and provide efficient routes for migration of various solutions. The flow of reactive solutions results in dedolomitization of carbonates dispersed in anhydrite rocks and makes possible removal of liquid products of this process outside the rock. The migration of reactive solutions also results in origin of sulphate solutions which represent the product of dedolomitization. Anhydrite from carbonates cooccurring with anhydrite rocks has crystallized from these sulphate solutions. Nests of secondary anhydrite originating in dolomites (Fig. 8) clog the pores in these rocks and thus make them less and less permeable and they impede free migration through them. It appears that dedolomitization is more common and advanced in carbonate accumulations from anhydrites than in separate dolomitic layers. This evidences more favourable conditions of migration of reactive solutions through anhydrites than dolomites. According to the present author this fact makes it possible to draw conclusions important for the practice in oil and gas prospecting. Under favourable geological conditions oil and gas may accumulate in anhydrites. Therefore, anhydrites of the Zechstein series should be treated as potential good collectors for oil and gas and, thus, previous views that carbonates are good collectors and anhydrites are bad and not perspective should be rejected. Migration of solutions and, eventually, oil and gas through anhydrite complexes is possible because of commonness of above described microfissures and infrastructural loosenings as well as macroporosity of these rocks which may be sometimes fairly high (Fig. 9).