Procesy diagenetyczne kształtujące przestrzeń porową piaskowców karbonu w rejonie Lublina

Abstract

Diagenetic processes affecting pore space in Carboniferous sandstones of the Lublin region.A b s t r a c t . In the Lublin region Carboniferous sandstones are represented by subarkosic, quartz and sublithic, occasionally arkosic arenites and wackes. These rocks are characterized by porosity (primary and secondary) ranging from 0 to 22.43% and permeability from 0 to over 1000 mD. Two diagenetic processes, that is compaction and cementation, were primarily responsible for reduction of porosity in these sandstones. The compaction decreased porosity at about 50%, while the cementation at 30%. Quartz, kaolinite and carbonates and locally fibrous illite are the most common cements here. Dissolution is also important as a diagenetic process responsible for development of secondary porosity. Good porosity of the bulk of Carboniferous sandstones is due to precipitation of early overgrowth cements (quartz, Fe-chlorite and siderite), which stopped the mechanical compaction, and dissolution of feldspar grains and authigenic quartz leading to origin of secondary porosity. The processes of mechanical compaction and advanced quartz and carbonate cementation (mainly of the ankerite and Fe-calcite type) as well as crystallization of fibrous illite were the major factors responsible for reduction of porosity in some sandstones. The results of the studies on pore space indicate good reservoir properties of sandstones of the Lublin and Dęblin formations as well as some parts of the Terebin formation. The sandstones from the Huczwa formation and a part of the Terebin formation belong to rocks characterized by low petrophysical parameters. Comparisons of sandstones formed in different environments have shown that best reservoir conditions are displayed by these formed in river channel and delta bottom ennvironments. During diagenesis, the Carboniferous deposits remained under influence of the maximum temperature of about 120oC, but locally the temperatures could have been even higher. Maximum temperatures were reached by the Carboniferous deposits at the end of Carboniferous whereas diagenetic proccesses were active until early Permian. The results of studies on diagenesis of these rocks, projected onto the thermal-erosional model, point to the Variscan overheating.