Lithologic Determination of Devonian Dolomitic Carbonate-Siliciclastic Rocks from Estonia by Physical Parameters

Devonian fully dolomitized sedimentary succession occurring in the southern Estonia is represented by carbonate, siliciclastic and mixed-carbonate siliciclastic rock types. These three lithologic rock types were subdivided using 50 and 70% borders of insoluble residue (IR). Carbonate rock type includes dolostones (IR<25%) and dolomitic marlstones (IR>25% and IR<50%). Mixed rock type is represented by primary dolomitic marlstones and siltstones and diagenetic sandstones and sandy siltstones. Siliciclstic rock type is represented mainly by sandstones, siltstones and silty claystones. Porosity-density crossplots together with gamma-ray logs permit determination of clean lithologies, while discrimination of mixed rocks usually causes difficulties. The aim of the work is lithologic determination of mixed rocks. Density, porosity and magnetic susceptibility measured on 150 core samples from 8 boreholes accompanied with gamma-ray logs were analysed together with thin-sections and geochemical data. As a result mixed rocks were distinguished from carbonate and siliciclastic. Primary mixed rocks were discriminated from diagenetic. Density, porosity, magnetic susceptibility and gamma-ray spectrometry logging tools are recommended for lithologic determination of the rocks.