| Abstract [eng] |
GEOCHEMISTRY OF ROCKS AND BRACHIOPODS OF MINIJA REGIONAL STAGE SUMMARY The so-called closeness/openness of carbonate system is still a big problem in predicting reservoir properties. Only when the systems are closed (or closed with respect to major elements) adequate prediction of diagenesis (e.g. dissolution, cementation) and the effect on porosity and permeability can be made. When systems would be closed, dissolution because of unstable or metastable primary mineralogical composition of grains (aragonite, high-Mg calcite) or because of grain size and texture (reactive surface area versus volume) would deliver the material for cementation (in fact cementation is thus merely the result of redistribution of carbonate within the system). Brachiopods precipitate low – Mg calcite shells and have a high potencial for retaining their primary chemical composition of palaeoseawater. The sample material in this study was cleaned brachiopod shells (Isorthis ovalis) and the associated whole rock carbonates. For the analysis we took 18 samples of rocks and brachiopods, from 11 different boreholes, which are located from West – Central to South part of Lithuania. Both, brachiopods and associated rocks samples were analysed for Ca, Mg, Fe, Mn and Sr and the results were compared in order to assess in how far carbonate material was added to the Silurian system during burial. The study of Ca, Mg, Fe, Mn and Sr chemical element contents suggests that in Central and South part of Lithuania (Pamituvys – 98, Sutkai – 87 and Vilkaviškis – 126, 128 and 131 boreholes) rocks are more diagenetically altered when compared to other boreholes. The variation of iron contents in brachiopod shells and rocks are positively high correlated, which is interpreted that the content of iron in brachiopods shells could depend on environmental condition (facies). The values of Ca, Mg, Fe, Mn and Sr in all but one (Vilkaviškis – 131 borehole, 778.9 m) brachiopod sample varies slightly. It means that they are unaltered and retains primary composition of palaeoseawater. |
