MICRO- AND NANOPORES IN TIGHT ZECHSTEIN 2 CARBONATE FACIES FROM THE SOUTHERN PERMIAN BASIN, NW EUROPE

The study evaluates pore systems in samples of tight Upper Permian Zechstein 2 Carbonate (Z2C) facies from widely dispersed locations in the eastern and western parts of the Southern Permian Basin, NW Europe. Samples of Z2C drill cores comprising platform to shallow-basin deposits were examined by petrographic techniques, porosity measurement and SEM analysis, in order to acquire a better understanding of porosity development and pore microstructure. Four carbonate lithofacies were identified by core-scale and thin-section analyses: subtidal / intertidal planar stromatolites and thrombolites; slope facies (laminated dolo- and lime-mudstones and grain-dominated turbidites); toe-of-slope / lower slope facies (laminated lime mudstones with turbidite beds); and shallow-basin (embayment) facies (laminated lime mudstones and hemipelagic deposits). Porosity ranges from 0.08% to 9.6% and is lowest in the subtidal / intertidal planar and columnar stromatolites. The highest porosities, in the slope and shallow-basin (embayment) lithofacies, probably resulted from catagenetic processes and microbial activity. High porosity in subtidal thrombolites is related to the original pore network. SEM analyses showed that pores are present within organic matter, pyrite framboids and microfossils, and between crystals. Subtidal / intertidal planar and columnar stromatolites and thrombolites have a high proportion of nanopores, probably resulting from microbial activity. Although porosity development is a combined function of the presence of organic matter and mineral components, together with sediment fabric and fractures, it is mostly a result of the early diagenetic transformation of mineral phases, microbial activity, and evaporite and carbonate dissolution.