DIAGENEZA A POROWATOŚĆ PIASKOWCÓW JURY DOLNEJ NA NIŻU POLSKIM

Abstract

DIAGENESIS AND POROSITY OF THE LOWER JURASSIC SANDSTONES IN THE POLISH LOWLANDSAbstract. Sandstones from the boreholes located in Szczecin–Radom area have been subjected to petrographic and petrophysical analysis. Sandstones are represented mostly by quartz arenites and wacks, from very fine- to middle-grained. The main type of cements are matrix and cements, among which quartz, carbonates, clay minerals and locally pyrite were distinguished. Quartz cement occurring as syntaxial overgrowths developed on the quartz grains displays the greatest significance in the Lower Jurassic sandstones. Authigenic clay minerals are represented mainly by vermiform kaolinite. Authigenic illite and Fe/Mg chlorites are locally observed. Carbonate cements are represented mainly by early and late generation of siderite (mineral of series of siderite–magnesite). The early diagenetic siderite represented by siderite and sideroplesite is predominant. The siderite δ18C values are in the range from –24.50 to –4.56‰PDB and δ13O values range from –14.37 to –0.33‰PDB . The late siderite of pistomesite and sideroplesite chemical composition also occurs, but occasionally. Furthermore, there are Fe-dolomite/ankerite and occasionally calcite. The Jurassic sandstones are characterized by very good and good filtration properties. The primary porosity is dominant, it often exceeds 20%, and permeability is in the range of 0.001 to 1930.756 mD. The most important diagenetic processes operating in the sandstones are: compaction, cementation and dissolution. Compaction reduced primary porosity on average by about 30%, and cementation by about 21%.